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Information Circular 9258 Mine Power Systems By Lloyd A. Morley UNITED STATES DEPARTMENT OF THE INTERIOR Manuel Lujan, Jr., Secretary BUREAU OF MINES T S Ary, Director
Library of Congress Cataloging in Publication Data: Morley, Lloyd A. Mine power systems. (Information circular: 9258) Includes bibliographies. Includes index. Supt. of Docs. no.: 128.27:9258. 1. Electricity in mining. I. Title. 11. Series: Informalion circular (United States. Bureau of Mines); 9258. TN295.U4 [TN343] 622 s [622'.48] 87-600213 -- For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC 20402
The author is grateful to the several individuals and companies that supplied noncopyrighted material for use in this publication. This material is noted by the vari- ous courtesies given throughout the text. Its incorporation does not constitute an en- dorsement by the author, The Pennsylvania State University, the University of Ala- bama, or the Bureau of Mines. Reference to specificproducts, equipment, or manufacturers does not imply endorsement by the Bureau of Mines.
The application of electricity to the mining industry is a distinctive area of both mining engineering and electrical engineering. The diEcult environment, the dynamic power loads, the cyclic and mobile operation and stringent safety requirements that characterize mining, all place unique demands on the mine power system. No other industry makes such extensive use of portable extensible equipment or has such com- plex grounding problems. Mine power systems can range from relatively simple in- stallations for small surface mines to complex underground systems where the harsh environment of dust, humidity, and cramped spaces stretches the ingenuity and crea- tivity of the engineer to provide reliable service. At the present time there is no up-to-date engineering text available that deals specifically with mine power systems. This has created extensive difficulties for edu- cators, industry engineers, and regulatory agency personnel. The need for a suitable reference for students in mining engineering provided the main impetus for this book, since the technician-level material that was in existence proved unsuitable for teach- ing young engineers who have little practical experience. The objective in preparing this manuscript was to assemble a single engineering reference on mine electrical power systems that is as comprehensive as possible. Ear- lier drafts of this material have been used successfullyto instruct university students in courses ranging from basic electrical engineering through power-system design. It is felt, however, that the usefulness of this material extends beyond that of a student text. While not intended to replace other electrical or mining references, this publica- tion is also an indexed, reasonably comprehensive reference handbook for industry engineers and training personnel, and a sourceof material for electrical engineers who wish to expand their education into industrial power-system applications. Obviously, there will be some omissions; to include all aspects of mine electrical systems in one volume would approach an impossibility, but an attempt has been made to collect together the most significant information, thereby providing the tools needed to con- tinue a knowledgeable involvement in mine electricity. This reference work is divided into three general content areas. Chapters 1through 5 contain information considered elementary, chapters 6 through 1 1deal with power- system components, and chapters 12 through 17 contain specifics on mine power sys- tems. A person familiar with electrical principals can use the earlier chapters as re- view material, but all chapters contain material relevant to mining and discuss the necessary combinations of equipment and components that should be contained in the mine power system. Emphasis throughout is placed on coal mining systems, although much of the material pertains to all mining operations. Both surface and underground power systems are discussed, the latter in more detail since these are the more com- plex systems and encounter the most problems. This publication is a thoroughly upgraded and extensively revised edition of Bureau of Mines Open File Reports 178(1)-82and 178(2)-82,prepared under Bureau contract 50155009by The Pennsylvania State University. It contains new chapters,new illustra- tions, and example problems that were not included in the original report. The assembly of this material has been a major undertaking. Many industry, academic, and Government agency personnel helped to review and critique practically every stage of draft preparation. The original report version was made available to students taking the mine power-systems courses at The Pennsylvania State Univer- sity, and their involvement was critical input to manuscript -- preparation. -- The author is grateful to all the companies and individuals who contributed or cooperated in this effort; so much information could not have been gathered without their help. A specialthanks is owed to the late Robert Stefanko. He originally perceived the need for this text and provided guidance and encouragement throughout the proj- ect that produced the original report version. Others deserving special mention are A. M. Christman, R. H. King, J. A. Kohler, G. W. Luxbacher, T. Novak, J. N. Tomlinson, F. C. Trutt and D. J. Tylavsky. Each contributed directly to the text while on the fac- ulty or staff at The Pennsylvania State University; acknowledgements for their con- tributions are made in the individual chapters.
CONTENTS Page Page Preface ............................. Abstract ............................. Part l : Fundamentals Chapter l.--Electrical power in mining ...... Mine electrical history ................. Underground mine history ............ Surface mine history ................ Mine power equipment ................ ....................... Substations Switchhouses ...................... Power centers ..................... Distribution equipment .............. Basic distribution arrangements .......... Radial system ..................... Primary-selective system ............. Primary-loop system ................ Secondary-selective system ............ Secondary-spot network .............. Utility company power ................ ...................... Surface mining Power systems in surface mines .......... Main substations and subtransmission ... Surface mine distribution ............. Underground coal mining .............. Room-and-pillar mining .............. Longwall mining ................... Power systems in underground mines ...... Regulations ....................... Underground mine distribution ........ Surface facility power requirements ....... Basic design considerations ............. References ......................... Chapter 2.--Electrical fundamentals I . . . . . . . Basic electrical phenomena ............. .................... Coulomb's law Voltage and current . . . . . . . . . . . . . . . . System of units ...................... Experimental laws and paramcters ........ ....................... Ohm's law Kirchhoff's voltage law .............. KirchofPs current law ............... Series circuits ..................... Parallel circuits .................... The magnctic field ................. Inductance ....................... ...................... Capacitance Electric field ...................... Instantaneous power ................ Idealization and concentration ......... ................. Direct current circuits Direct current and circuit elements ..... Series and parallel resistance .......... ............ Wye-delta transformations Circuit and loop equations ............ Node equations .................... Network theorems .................. Time-varying voltages and currents ....... .............. Steady alternating current ........... Effective alternating current Phasors .......................... ........ Phasors and complex quantities Impedance transforms ............... ................ Steady-state analysis ....... Chapter 3.-Electrical fundamentals I1 ......... Average power and power factor ........... Complex and apparent power Resonance ......................... Series resonance ................... Parallel resonance .................. Transformers ....................... Ideal transformer .................... Actual transformers .................. Conductor loss .................... Leakage reactance .................. ........ Core losses and exciting current ........ Power-transformer construction Transformer models ................ Determination of transformer ..................... parameters ... Transformer efficiency and regulation Autotransformers .................... .............. Multivoltage transformers ....... Current and potential transformers Chapter 4.-Power.systcm concepts .... Basic power circuit .............. Three-phase circuits ............. Balanced three-phase circuits ...... Three-phase system voltages ..... Load connections ............. Line and phase currents ........ Equivalent delta and wye loads ... Three-phase power ............ Three-phase transformers ......... Balanced three-phase circuit analysis . One-line and three-line diagrams ..
Page Page ........ Circuits containing transformers Per-unit system ...................... Transformer impcdancc .............. ........... Three-winding transformers ...... Per-unit method in system analysis ......... Unbalanced three-phase circuits Fault types ....................... Fault analysis ..................... ............... Symmetrical componcnts Sequence components ............... ........ Sequence-quantity combinations .... Symmetrical-componentrelationship Symmetrical-component impedance ..... .................. Fault calculations ................... Power terminology References ......................... Chapter 5.-Basic solid-state devices and instrumentation ...................... 104 Semiconductors ...................... 104 Diodes and rectifiers .................. 104 Diodc equations ................... 105 Rectifier circuits ................... 105 Cooling .......................... 106 Overloads ........................ 107 Three-phase rectification ............... 107 Rectifier circuits ................... 108 Parallel rectifier operation ............ 109 Transistors ......................... 109 Transistor operation ................ 109 Bipolar-transistor amplifiers ........... 110 Field-effect transistors ............... 112 Silicon-controllcdrcctifiers ............. 113 Integrated circuits .................... 114 Basic instrumentation ................. 114 Basic meter movements ............... 115 Meter-movement applications ......... 116 Wattmeters ....................... 117 Varmeters ........................ 118 Power-factor meters ................ 118 Power-system instrumentation ........... 118 Instrument transformers ............. 118 Single-phase connections ............. 119 Three-phase connections ............. 120 Special instruments ................... 122 Watthour meters ................... 122 Demand meters ................... 122 Bridges .......................... 122 Megohmmeters .................... 123 Phase-sequence indicators . . . . . . . . . . . . 124 Recording instruments ................ 124 Electronic instruments ................ 125 Electronic meters .................. 125 ..................... Oscilloswpes 125 .................... Tape recorders 126 Transducers ...................... 126 ................ Instrument installations 127 Part 11: Power-System Components Chapter 6.--Motors and motor control ...... 129 Alternating current generation ........... 129 Principle of generator operation ........ 129 .............. Generator construction 129 Three-phase generation .............. 131 Direct current generators .............. 131 Motor basics ........................ 133 Torque .......................... 133 ............ Speed-torque relationships 133 .................... Standardization 134 Motor type ....................... 135 Three-phase squirrel-cage induction motors .......................... 136 ......... Elementary three-phase motor 136 ................. Motor conslruction 138 Motor behavior .................... 138 Insulation ........................ 139 ............... Design characteristics 139 ............. Induction-motor starting 141 .......... Wound-rotor induction motors 142 Three-phase synchronous motors ......... 143 ........... Synchronous-motor starting 144 ............ Synchronous-motor torque 145 Generated voltage .................. 146 Power factor ...................... 146 Applications ...................... 147 Direct current motors ................. 147 Elementary motor .................. 147 ............ Actual motor construction 148 Torque .......................... 148 .... Motor connections and performance 148 ............... Ward-Leonard system 152 Mine motors ........................ 153 Applications ...................... 153 ........... Actual equipment operation 153 Single-phase motors .................. 156 ................ Rotating stator lield 156 ................. Split-phase starting 157 ............... Capacitor-start motors 157 References ......................... 158 Chapter 7..-Grounding .................. 159 Grounding systems ................... 160 Ungrounded neutral .................. 160
............... Solidly grounded neutral ....... Low-resistance grounded neutral High-resistance grounded neutral ....... Electric shock ....................... Characteristics of mine grounding systems .. Ground beds ...................... ...... Grounding in underground mining ........... Grounding in surface mines ............... Ground-bed construction .................. Ground resistance ....... Electrode co~gwation formulas ............ Two-layer earth structures ................. Soil-heating effects ......... Control of potential gradients ...... Ground-bed resistance measurement Measurement method ............... Ground test instruments ............. ................. Ground-bed resistivity Factors affecting resistivity ............ Resistivity measurements ............. Effect of chemical treatment of soils .... ................. Ground-bed corrosion General ground-bed guidelines .......... Grounding equipment ................. ................. Grounding resistor Grounding transformers ............. .......................... Summary References ......................... Chapter 8.--Distribution ................. Nature of cable distribution ............. ................... Cable components ....................... Conductors Insulation ........................ ...................... Cable jacket .................... Cable shielding Cable types ......................... Cable terminations ................... Cable couplers ...................... Coupler contacts ................... .................. Coupler insulation ................... Coupler housing High-voltage couplers ............... ................ Low-voltage couplers ...................... Cable selection Cable length ...................... ................. Conductor selection Cable installation and handling .......... .................... Borehole cables Feeder cable installation ............. Recommended handling practices ...... Cable failures and repairs .............. Cable testing ...................... .................... Failure location Splicing .......................... Trolley systems ...................... Trolley wire ...................... Trolley feeder ..................... ..... Supports. lubrications. and turnouts ................... Rails and bonds Overhead lines ...................... ................ Overhead-line design Overhead-line electrocutions .......... References ......................... Chapter 9.-Protective equipment and relaying ........................... Switching apparatus .................. ............ Arcs and circuit interruption Switches ........................... Circuit breakers ..................... Circuit breakers for low and medium voltage .......................... Molded case circuit breakers .......... ............... Power circuit breakers High-voltage circuit breakers ............ Typical ratings ..................... Oil circuit breakers ................. Minimum-oil circuit breakers .......... Vacuum circuit breakers ............. Fuses ............................. Low-voltage fuses .................... Non-time-delay fuses ................ Time-delay fuses ................... Dual-element fuse .................. Current-limiting fuses ............... Standard fuses ..................... Nonstandard fuses .................. High-voltagefuses .................... Expulsion types .................... Current-limiting high-voltage fuses ...... Load-break switches ................ Relays ............................ Relay terminology and types .......... Thermal relays .................... Electromagnetic-attraction relays ....... Electromagnetic-induction relays ....... Basic relay connections ................ Alternating current direct relaying ...... Alternating current potential relaying .... Alternating current differential relaying .. Direct current connections ............ Kinds of protection ................... ..................... Control wiring Page 206 207 207 211 211 211 211 215 216 217 218 222 224 224 225 226 226 227 228 232 232 232 232 233 234 235 235 236 236 236 236 236 237 237 237 238 239 240 240 240 241 242 244 244 246 247 247 248 248
Page Page Phase protection ................... Ground overcurrent ................. Ground-check monitoring ............ ......... Advantages and disadvantages Arrangements for mining .............. Zones of protection ................. ...................... Coordination .............. Ground-fault protection Overloads and short circuits ........... Surface mines ..................... Underground mines ................. ......................... References ....... Chapter 10.Suing protective devices ....................... Fault current Fault-current sources ................ ............. Source equivalent circuit Fault calculations for three-phase systems .. Short-circuit calculation procedures ..... Three-phase calculation example ....... Computer fault analysis .............. Ground-fault current calculations ....... Direct current system faults ............. Device settings ...................... Relay pickup settings ................. Short-circuit protection .............. Overload protection ................. Ground-fault protection .............. Current transformer matching ........... Current transformer accuracy ......... Accuracy calculations................ Low-voltage circuit breaker trips ......... Overload protection ................. Short-circuit protection .............. Low-voltage power circuit breakers ..... Fuses ............................. ....................... Coordination ......................... References Chapter ll.--Transients and overvoltages ..... Transient sources .................... Lightning phenomena ................. Switching transients ................... ............... Capacitance switching .................. Current chopping Prestrike ......................... ........... Direct current interruption General switching transients .......... Other transient phenomena ............. Traveling waves ..................... Electromagnetic phenomena ............ Transient-induced failures .............. Winding response .................. ........ Coupling through transformers Transient protection .................. Surge arresters .................... ............ Surge arrester applications Capacitors and system capacitance ...... ............ Other suppression devices Faraday shields .................... ................ Circuit arrangements ........... Protection of overhead lines Impulse performance of ground beds .... References ......................... Part 111: Mine Power Systems .......... Chapter 12.-Mine power centers ............... Equipment specifications Mine power centers .................. High-voltage cable coupler ............. Interlock switches .................... Disconnect switch .................... High-voltage fuses .................... Surge arrestors ...................... Transformers ....................... SpeciF~cations ..................... Transformer construction ............. Faraday shields .................... Grounding resistor ................... Busway ............................ ............... Outgoing circuit breaker Ground-fault protection ............... .............. Single-phase transformers Metering circuits ..................... ............... Outgoing cable couplers ................ Ground-check monitors ................ Power-factor correction ............... Direct current utilization Rectifier transformer ................. ........................... Rectifier Direct current ground-fault protection schemes ......................... .......... Direct current control circuitry ........ Direct current interrupting devices References ......................... Chapter 13.Switchhouses and substations ... Switchhouses ....................... Switchhouse internal components ......... Switchhouse protective relaying .......... Power circuit breakers ................ Switchhouse control circuits ............. Switchhouse design ...................
Page Page Substations ......................... Basic substation arrangements ........... Single-ended substations ............. Double-ended substations ............ Substation transformers ................ Substation switching apparatus .......... ........................ Reclosers Disconnect switches and fuses ......... Protective relaying in substations ......... Lightning and surge protection in substations ....................... .................. Substation grounding Substation ground mat ............... .............. Ground-fault protection Additional mine substation loads ......... Portable substations .................. ....... Utility voltage as mine distribution Additional substation design ..................... considerations References ......................... Chapter 14.4olid-state control and relaying .. ....................... Motor control ................ Simple motor control Control systems .................... ..... Physical characteristics of thyristors Direct current applications ............. .......... Alternating current applications ............... Static protective relaying Operation of simpwed solid-state and hybrid relays ...................... Static and electromechanical relay ....................... comparison Static relay mining applications .......... Sensitive earth-leakage system ......... Phase-sensitiveshort-circuit protection ... Solid-state relays in the future ........... .......................... Summary References ......................... Chapter IS.--Batteries and battery charging ... ... Basic battery and battery-charging theory Battery maintenance .................. .......................... Chargers Charging stations .................... Battery-box ventilation ................ 1.1. Simple mine electrical system arrangement . 1.2. Simple radial distribution system ........ 1.3. Power-center type of radial distribution ... 332 Battery surface leakage and faults ........ 375 ............... 332 Battery-charginghazards 377 ......................... 333 References 381 Chapter 16.-Permissibility and hazard .......................... reduction ........................ Terminology ............. Hazard-reduction methods Explosion-proof enclosures ............. Explosion transmission .............. ................... Enclosure joints Enclosure mechanical strength and internal pressures ....................... Enclosure hazards .................. ................ Permissible equipment ........ Permissible equipment schedule ... Maintenance of permissible equipment .................... Coal dust hazards ........ Classifications of dust locations ............... Reducing dust hazards Hazardous locations in preparation plants ......................... ......................... References ................ Chapter 17.-Maintenance ............. Mine maintenance program ............... Economicjustification Preventive maintenance program .................. implementation ..... Techniques of preventive maintenance ......... Basic electrical measurements ............. Insulation measurements ................. Megohmmeter tests .............. Mechanical measurements .......... Continuous-monitoring systems ............................ Corona 406 ................... Corona behavior 408 ................... Corona detection 409 .... Partial-discharge problems in mining 410 .................. Intermachiue arcing 411 ........... Ground direct current offsets 412 .......................... Summary 413 ......................... References 414 .......................... Bibliography 415 ...... Appendix.-Abbreviations and symbols 416 ............................... Index 420 ILLUSTRATIONS
Page .................................................. Primary-selective distribution system ........................................................... Primary-loop distribution .......................................................... Secondary-selectivesystem .................................................... Secondary-spot network technique ........................................ Representative utility transmission and distribution ..................................................... Subtransmission for surface mine Radial strip mine distribution system .................................................. ........................................... Secondary-selectivedistribution in strip mining Primary-loop design for strip mining .................................................. Radial distribution for strip mine with overhead poleline base line ............................. Radial distribution for strip mine with all-cable distribution .................................. Surface mine distribution system using two base lines ...................................... Openpitpowersystem ............................................................ ..................................................... Layout of underground coal mine ...................................................... Plan view of retreating longwall Subtransmission for underground mine ................................................. Radially distributed underground power system .......................................... Secondary-selective distribution in underground mines ..................................... Utilization in continuous mining section ................................................ Power-system segment with longwall equipment .......................................... ........................................ Diagram of electrical-systemsegment for longwall Parallel-feed haulage system ........................................................ Representative expanded radial distribution for preparation plant ............................. Representative secondary-selectivedistribution for preparation plant ........................... Circuit element illustrating voltage polarity and current flow direction .......................... Simpleseriescircuit ............................................................... Ideal and actual voltage sources ...................................................... Circuit for example 2.1 ............................................................. Demonstration of Kirchhoffs current law ............................................... Simple parallel circuits ............................................................. Ideal and actual current sources ...................................................... Parallel circuit for example 2.2 ....................................................... Simple series circuit and equivalent ................................................... Simpleparallelcircuit ............................................................. Series-parallel circuit for example 2.3 .................................................. Series-parallel circuit for example 2.4 .................................................. Magnetic flux in a straight conductor and in a long coil ..................................... Demonstration of induced current .................................................... Two coils demonstrating mutual inductance ............................................. Long-coil inductance and inductor symbols .............................................. Toroidalcoil .................................................................... Charge. voltage. and current relationships of capacitor ..................................... Electric lines of force between two parallel charged plates .................................. Resistor used to demonstrate instantaneous power ........................................ Simple example of idealization and concentration ......................................... Modeling of load center. trailing cable. and shuttle car ..................................... Basic elements of resistance. inductance. and capacitance ................................... Sirnplilicalion of dc circuit .......................................................... 2.25. Simple circuit reduction ............................................................
Page ............................................................ Cicuitforexample2.5 ............................................................ Circuitforexample2.6 ............................................. Series-parallel conductancesfor example 2.7 Series-parallel circuit for example 2.8 .................................................. Two-terminal and three-terminal networks .............................................. ................................................... Wye and delta circuit configuration "T"and "nucircuit configurations ..................................................... ............................................................ Commonbridgecircuit ..................................................... Circuit reduction of bridge circuit ................................................................... Partsofcircuit Circuit demonstrating two independent loops ............................................ .................................................... Two-loop circuit for example 2.11 Bridge circuit demonstrating loop analysis .............................................. Three-loop circuit for example 2.12 ................................................... Simple two-node circuit ............................................................ Three-junction circuit ............................................................. Three-junction circuit with grounds ................................................... ........................................ Voltage-source circuit demonstrating node analysis Circuit for examples 2.13, 2.15, and 2.16 ................................................ Circuit for example 2.14 ............................................................ Circuit for demonstrating superposition theorem .......................................... Circuit in figure 2.44 with sources turned off ............................................. Demonstration of reciprocity theorem ................................................. Practical voltage-source model ....................................................... Practical current-source model ....................................................... Source transformation ............................................................. ....................... Circuit in figure 2.44 with current sources transformed to voltage sources ............................................................... Thevenin'stheorem ................................................................ Norton'stheorem Comparison of Thevenin's and Norton's circuits .......................................... Circuit for example 2.17 ............................................................ Active circuit for example 2.18 ....................................................... Circuits illustratingsolution steps to example 2.18 ......................................... Some time-varying electrical waves .................................................... Sinusoidal ac waveform ............................................................ Steady ac showing phase shift ........................................................ Steady ac through resistance ........................................................ ........................................................ Steady ac through inductance Steady ac through capacitance ....................................................... Simple series RL circuit ............................................................ Simple series RC circuit ............................................................ Simple series RLC circuit .......................................................... Graphical representation of complex number ............................................ Trigonometric or polar representation of complex number .................................. Sinusoid versus time and as phasor .................................................... ............................................ Phasor representation of current and voltage Other expressions for phasors ....................................................... .................................... Voltage-current phasor relationships for circuit elements ........................................ Steady sinusoid analysis of simple RL series circuit ........................................ Steady sinusoid analysis of simple RC series circuit
Page ....................................... 2.76. Steady sinusoid analysis of simple RLC series circuit 2.77. Circuit for example 2.21 ............................................................ 2.78. Circuit for example 2.22 ............................................................ .................................................... 2.79. Two-loop circuit for example 2.23 2.80. Activecircuitforexample2.24 ....................................................... ...................................... Power represented as real and imaginary components Illustration of leading and lagging power factors .......................................... ........................................... Circuit demonstrating sum of complex powers ................................................ Simple series RLC circuit for resonance Plot of impedance magnitude versus frequency for series RLC illustratingresonance ............... Circuits that exhibit parallel resonance ................................................. .............................................. Magnetic coupling between two conductors Magneticcouplingbetweentwocoils .................................................. Demonstration of coil winding sense .................................................. Dot convention for mutal inductance sign ............................................... Demonstration of impedance transfer in transformers ...................................... Ideal transformer with winding resistance included ........................................ Accounting for transformer leakage flux ................................................ Transformer magnetizing current ..................................................... Eddy current and magnetic hysteresis creating power loss in core ............................. .............................................. Equivalent circuit of practical transformer Common power-transformer construction techniques ...................................... Movement of exciting components to input .............................................. ........................................... Transferring secondary components to primary Final simplification of pratical circuit model ............................................. Transformer parameter test series .................................................... ............................................................ Circuit for example 3.8 Comparison of two-winding transformer and autotransformer ................................ Two-winding transformer as an autotransformer .......................................... Examples of transformers for multivoltage applications ..................................... TwotypesofCT's ................................................................ Examples of CT and PT placement in circuit ............................................ Basicpowercircuit ............................................................... Applications of basic power circuit .................................................... Elementary three-phase generation .................................................... Three-phase voltage sources ......................................................... Wye-connected source demonstrating line-to-line and line-to-neutral voltages ..................... ................................................. Balanced three-phase load connections Four-wire wye-to-delta system ....................................................... Balanced delta load illustrating phase and line currents ..................................... Comparison of equivalent delta and wye loads ........................................... Three-single-phasetransformers connected for three-phase operation .......................... ................................... Three-phase diagrams for the transformers of figure 4 . 1 0 ........................................... Open-delta three-phase transformer operation Per-phase reduction of wye-to-wye system .............................................. Per-phase reduction of delta-to-delta system ............................................. ............................................................... Three-linediagram One-line diagram of circuit shown in figure 4.15 ..........................................
Page .................................... Commonly used symbols for one-line electrical diagrams ......................................................... Symbolsforrelayfunctions ..................................................... One-line diagram for example 4.7 Three-phase diagram of figure 4.19 ................................................... ..................................................... Per-phase diagram of figure 4.19 One-line diagram with delta-delta transformer ........................................... ..................................................... Per-phase diagram of figure 4.22 ............................................ One-line diagram with delta-wye transformer ....................................... One leg of three-phase transformer from figure 4.24 Approximate per-phase equivalent circuit for 750-kVA load-center transformer; impedance referred ................................................................... tohighside Transformer of figure 4.26 with impedance referred to low side ............................... Simplified equivalent circuit of transformer expressed in per-unit .............................. Approximate equivalent circuit of three-winding transformer expressed in per-unit ................. ........................................... One-line diagram of small mine power system Impedance diagram of system in figure 4.30, expressed in per-unit on a 1.000.kVA base ............. ............................................................ Basicfaultdescriptions Positive.sequence. negative.sequence. and zero-sequence vector sets ........................... Symmetrical component addition to obtain unbalanced three-phase set ......................... ....................................... Equivalent delta-connected and wye-connected loads ............................................ Three-phase system with line-to-neutral fault ........................................... Symbol and operation of a p-n junction device ............................................. Bias conditions and current flow for a diode Diode or rectifier characteristic curve .................................................. ............................................... Half-wave rectifier circuit and waveforms ................................................ Single-way full-wave rectifier waveforms Bridge rectifier circuit and waveforms ................................................. Example of filtering a rectifier output .................................................. Heatsinkcooling ................................................................ Heat sink thermal relationships ...................................................... Three-phase half-wave rectifier circuit and output voltage waveform ........................... Three-phase full-wave rectifier circuit with input and output voltage waveforms ................... .................................... Parallel operation of rectifiers using paralleling reactors An n-pn junction transistor ......................................................... .......................................................... A p-n-p junction transistor ......................................... Current relationships for p-n-p and n-p-n devices Common-baseamplifiers ........................................................... .......................................................... Common-emitter amplifier ................................................. Common-emitter characteristic curves Bias techniques for common-emitter amplifiers ........................................... ................................................ Common-collector amplifier arrangment ................................................. Model and symbols for junction FET's ................................................. Example of a junction-FET application .............................................. Model and symbols for MOS-FET devices SCRmodelandsymbol ............................................................ .................................................... SCR equivalent model and circuit .................................................. General characteristic curve for SCR ............................................. Sketch of simple monolithic IC cross section TopviewofanactualIC ...........................................................
Page ................................................ Examples of symbols employed for IC's .............................................. Permanent-magnet moving coil movements .......................................... Shunting d'Arsonval meter for high-current tests .......................................... D'Arsonval meter used to measure dc potentials ....................................... External shunts used for high-current measurements Simpleohmmetercircuit ........................................................... Rectifier ammeter ................................................................ ................................................. Dynamometer connected as wattmeter Power-factor movement ............................................................ .............................................. Simple instrument-transformer connections ........................... Voltmeter. ammeter. and wattmeter arranged as single-phase system .................................... Use of transducers with standard d'Arsonval movements ................................................... Three-phase wattmeter connections Two-wattmetermethod ............................................................ ......................................... Three-phase power measurement with transducer ...................... Balanced three-phase measurement of voltage. current. and average power Line current measurements with two or three CT's ........................................ ................................... Line-to-line voltage measurements with three or two PT's ................................... Simplified sketch of watthour meter induction mechanism Wheatstonebridgecircuits .......................................................... Kelvindoublebridge .............................................................. .............................................. Megohmmeter testing insulation resistance ................................................. Internal components of megohmmeter Phase-sequenceindicator ........................................................... ............................................................... Strip-chart recorder .................................................. Input circuits on electronic voltmeter Digitaldisplay ................................................................... Cathode-raytube ................................................................. .................................................. Semiconductor illustrating Hall effect ............................................... Production of voltage from magnetic field ...................................................... Demonstration of ac generation ................ Cross section of machine with salient poles on stator and nonsalient poles on rotor ............................ Cross section of machine with nonsalient poles on stator and rotor .................. Simplified sketch of electromechanical machine illustrating physical components .......................................... Elementary four.pole. single-phase ac generator ............................................. Elementary two.pole. three-phase generator ............................................ Elementary four.pole. three-phase generator ...................................................... Demonstration of dc generation .................................... Dc generator with two armature windings at right angles ...................................................... Separately excited dc generator Seriesdcgenerator ............................................................... Shuntdcgenerator ............................................................... Compounddcgenerator ........................................................... ........................................... Current-carrying conductor in a magnetic field .............................................. General speed-torque motor characteristic ............................................ Examples of three frame number dimensions ............................................ Demonstration of induction-motor operation ............................................... Elementary three-phase induction motor ......................................................... Squirrel-cage rotor winding
..................... Rotating magnetic field in elementary three.phase. two-pole induction motor ................................................ Induced rotor potential by rotating flux ............................................ Lapped windings of three-phase motor stator Characteristic curves of three-phase induction motor ....................................... ........................ Typical torque-speed characteristic for general-purpose induction motor ........................... Phasor diagrams of rotor and stator flux density for induction motor Typical torque-speed characteristics for NEMA-design three-phase squirrel-cage motors ............ Other rotor-conductor designs ....................................................... ...................................................... Across-the-line magnetic starter Starting methods for induction motors ................................................. .................. Schematic of wound-rotor induction motor showing external resistance controller .............. Torque-speed characteristics for wound-rotor motor with stepped-resistancecontroller ............................... Simplified step starter using individually timed magnetic relays ............................... Sketch showing construction of salient-pole synchronous motor ..................... Simplified diagram of synchronous motor using generator for field excitation ................................... External solid-state supply used to provide field excitation ................................. Schematic of low-speed cylindrical-rotor synchronous motor ................... Controller used to demonstrate general starting method for synchronous motor .................. Typical torque-speed characteristic for synchronous motor with damper winding Effect of load on rotor position ...................................................... Equivalent per-phase circuit of a synchronous motor and phasor diagrams for underexcited and overexcited field winding ......................................................... V-curves for synchronous motor ...................................................... ........................................ Plan view of typical mining shovel showing m-g set Elementary two-pole dc motor ....................................................... Elementary four-pole dc motor ...................................................... .................. Cross-sectional sketch of dc motor showing interpole and compensating windings ................. Interaction between armature and main-field flux to produce main-field distortion Four connections for dc motors ...................................................... Typical characteristics for shunt. series. and compound motors of equal horsepower and speed ....................................................................... ratings .................................... Simplified dc motor schematicswith starting resistances Faceplate manual starter ........................................................... Multiple-switch starting ............................................................ Drum-typestarter ................................................................ ............................... Simplified diagram of dynamic braking applied to shunt motor ........................................ Two-step resistance starting of series-wound motor ......................................... Forward-reverse switching of series-wound motor Dynamic braking applied to series-wound motor .......................................... ............................................ One-step starting of compound-wound motor Basic WardLeonard system ......................................................... ............................ Typical characteristic curves for each motor in traction locomotive Stator field of two.pole. single-phase induction motor ...................................... ............................. Rotor field of stationary two.pole. single-phase induction motor Phase relationshipsbetween stator and turning rotor ....................................... Starting and running stator windings ................................................... ............................................ Centrifugal switch to remove starting winding Capacitor-start motor ............................................................. Illustration of electrical shock hazard .................................................. ............................................. Capacitance coupling in ungrounded system
Page Solidlygroundedsystem ............................................................ 160 Resistance-grounded systcm ......................................................... 160 Effect of frequency on let-go current for men ............................................ 162 ............................................... Simplified one-line diagram of substation 163 ................................................ Step potentials near grounded structure 163 Touch potentials ncar grounded structure ............................................... 163 ~ine-to-earthfault resulting in current flow through safety ground bed ......................... 163 Lightning stroke to equipment causing current flow through safety ground bed ................... 164 Lightning stroke current through system ground bed causing elevation of safety ground bed .......... 164 One-line diagram of simplified mine power system ........................................ 164 Mied ac-dc mine power system; dc load energized from trolley system ......................... 165 System grounding with current-limiting resistors .......................................... 165 Diode grounding of machine frame ................................................... 165 Resistance of earth surrounding electrode ............................................... 166 Decrease in earth resistance as electrode penetrates deeper soil horizons ........................ 167 Calculated values of resistance and conductance for 314-in rod driven to depth of 25 it ............. 167 Calculated values of resistance and conductance for 314411 rod driven to depth of 100 ft ............ 167 Nomogram to provide resistance of driven rod ........................................... 168 ........................................... Resistance of one ground rod. 314411 diameter 168 Resistance of parallel rods when arranged in straight line or circle with spacing equal to rod length ....................................................................... 168 Variation of earth resistance as numbcr of ground rods is increased for various spacings between rods ........................................................................ 168 Values of coefficient kl as function of length-to-width ratio of area ............................ 169 Values of coefficient k2as function of length-to-widthratio of area ............................ 169 Influence of first-layer height of potentials .............................................. 171 Potential on ground surface due to rod 6 ft long and 1-in diameter buried vertically at various depths ....................................................................... 172 Potential on ground surface due to strips. 1in by 0.1 in. of various lengths buried horizontally at depthof2ft .................................................................. 172 Measuring resistance of grounding system .............................................. 173 Concentric earth shells around ground connection being tested and around current electrode ......... 173 Correct spacing of auxiliary electrodes to give true resistance within 2.0% ....................... 173 Resistivity range of some rocks. minerals. and metals ...................................... 174 Variation in soil resistivity with moisture content ......................................... 175 Typical resistivity curves of solutions ................................................... 175 Diagram for four-electrode resistivity survey showing lines of current flow in two-layer earth ......... 176 Connections for Wenner four-terminal resistivity test using megohmmeter ....................... 176 Typical curve of resistivity versus elcctrodc separation ...................................... 176 ~ . ...................................... Reduction in ground mat resistance by soil treatment 177 Seasonal resistance variations attenuated by soil treatment .................................. 177 Trench model of soil treatment ...................................................... 177 Voltage gradients in earth during ground-fault conditions ................................... 178 . . Delta secondary with rig-zag grounding ................................................ 180 ..................................... Delta secondary with wye-delta grounding transformer 180 Cable distribution in underground coal mines ............................................ 182 ................................................ Cable distribution in surface coal mines 183 Shieldtypes ..................................................................... 186 ......................................... Cross sections of round unshicldcd mining cablcs 188
Page Cross sections of flat unshielded mining cables ........................................... Cross sections of some shielded mining cables ........................................... Round unshielded mining cables ..................................................... Flat unshielded mining cables ........................................................ Round shielded mining cables ....................................................... Cable types for typical distribution systems in underground coal mines ......................... Cable types for typical distribution systems in surface coal mines .............................. Cable terminations for applications up to 15 kV .......................................... .............................................................. Couplercomponents Simplified one-line diagram for situation described in example 8.4 ............................. Allowable short-circuit currents for insulated copper conductors .............................. Representative end-suspension termination for borehole cable ................................ Messenger wire supports for mine power-feeder cable ..................................... Splice layout using template for staggered connections ..................................... Effective method for removing unwanted insulation ....................................... Staggering splice connections ........................................................ Examples of popular connectors and connections used in splices .............................. Reinsulating power conductors with soft rubber tape ....................................... Typical taped splice in high-voltage shielded cable ........................................ Trolley-wire cross sections .......................................................... Typical trolley-wire and feeder-cable supports ............................................ Trolley-wire semicatenary suspension .................................................. Trolley system accessories .......................................................... Theoretical resistance of bonded joint ................................................. Pole strength calculations ........................................................... Guy and log-anchor calculations ...................................................... ............................. Typical arrangements and pin-insulator spacings on wooded poles Typical system fault current ......................................................... Steps in circuit interruption ......................................................... Arc between two contacts .......................................................... Load-breakswitch ................................................................ Extinguishing arc by increasing the length ............................................... Metal-barrier arc chute assists in arc deionization ......................................... ....................................... Insulated-barrier arc chute used with mametic field - Molded-case circuit breaker components ............................................... Magnetic-trip relay ............................................................... Adjustable instantaneous setting ...................................................... Thermal-magnetic action of molded-case circuit brcakcr .................................... ............................. Time-current characteristics for thermal-magnetic circuit breakers Shunt-trip and undervoltage-release accessories .......................................... Construction and operation of dead-tank OCB ........................................... Turboaction are chamber for OCB's ................................................... Cross section of minimum-oil breaker ................................................. ............................................................. CrosssectionofVCB Operating mechanism for vacuum interrupter ............................................ VCB assembly incorporating a load-break switch ......................................... ........................................................... Common cartridge fuses Inside view of dual-element fuse ...................................................... Current-limiting action of fuses ......................................................
Energy-limitingactionoffuses ....................................................... High-voltage power fuse and support .................................................. Fusible element under spring tension in high-voltage fuse ................................... ............................................. Cross section of boric acid power fuse refill .............................................. Disassembled refill unit for boric acid fuse Load-break switch with interlocked high-voltage fuses ...................................... ............................................................. Relay contact symbols Temperature-monitoring protector .................................................... Electromechanical-thermal relays ..................................................... ......................................................... Solenoid and clapper relays Polarrelay ..................................................................... ........................................................ Common induction-disk relay ...................................... Front view of induction-disk relay removed from case ...................................... Inverse-time curve compared with definite-time curve Various time characteristics of induction units ........................................... Family of inverse-time characteristics .................................................. ........................................................... Cylinder directional relay Directional overcurrent relay using induction-diskrelay and cylinder relay ....................... ......................................................... Direct relaying in ac system ....................................................... Potential-relaying connections Differential-relayingconnections ..................................................... ....................................................... Dc dircct-relaying connections ...................................................... Typical control wiring for UVR Typical control wiriig for shunt-tripping element ......................................... ..................................... Three-phase overcurrent and short-circuit connections TwoCTapproaches .............................................................. Neutral-resistor current-relaying scheme ................................................ ............................................... Neutral-resistor potential-relaying scheme ................................................ Zero-sequence ground relay connections Ground relay in residual connection ................................................... ............................................................ Broken-delta protection .................................................... Series loop ground-check monitor Transmitter loop ground-check monitor ................................................ .................................................... Bridge-type ground-check monitor ....................................................... Pilotless ground-check monitor ........................... Some difficulties associated with ground-check monitoring in mining ...................................... Pilot interlocking circuit using ground-check monitor Simple surface mine power system illustrating protective relaying ............................. Typical schematic for three-phase molded-case circuit breaker with ground-overcurrent and .......................................................... ground-check protection One-line diagram of simple underground mine power system illustrating protective circuitry .......... ....................................... Diode-grounded system with possible fault indicated Basic grounding-conductor system .................................................... Relayed groundig-conductor system .................................................. Neutral-shiftsystem ............................................................... Current-balance dc ground-fault relaying using saturable reactor .............................. Current-balance dc ground-fault relaying using saturable transformer .......................... ........................................... Fault current waveform illustrating asymmetry ... Multiplying factors applied to three-phase faults to obtain momentary ratings for switching apparatus
Page Multiplying factors applied to three-phase faults to obtain close-and-latch ratings for switching apparatus .................................................................... ................................................. One-line diagram for fault calculations ..................................... Impedance diagram for one-line diagram of figure 10.4 ......................................................... Simplificationoffigure10.5 ......................................................... Simplificationoffigure10.6 ................................................. Further reduction of example network ...................................................... Equivalentcircuitoffigure10.6 ....................................... Example problem with motor contribution neglected Network to calculate momentary or close-and-latchcurrent duties ............................. Fault current in dc system .......................................................... .............. Available fault current versus distance of fault from rectifier on typical trolley systems ............................................ One-line diagram for pickup setting example ........................................................ Model of CT and its burden .......................... Typical set of saturation curves for 600/5 multiratio bushing-type CT ................. Example of one-line diagram for preparing a coordination curve plot for one path ............ Coordination curve plot for figure 10.17 showing various protective-device characteristics ..................................... Schematic representation of lightning stroke discharge Distribution of crest currents in lightning strokes ......................................... .................... Map showing average number of thunderstorm days per year in United States Striking distances for negative and positive strokes ........................................ ............................... Crest voltages induced on transmission lines by nearby strokes ............................. Simple circuit to illustrate capacitance-switching voltage transients .......................... Voltage and current waveforms before and after current interruption Voltage and current transient waveforms occurring with capacitance switching and restrike .......... Per-phase diagram of 4.16O . V pump-motor circuit ........................................ ........ Voltages and current wavesforms resulting from multiple restrikes after capacitance switching Graphic example of current chopping by breaker interruption ................................ Equivalent circuit of power-system segment with lumped components per phase. neglecting resistance .. Graphic example of chopping voltage transients .......................................... Segmentofminepowersystem ...................................................... Circuit to demonstrate voltage transients in dc system ...................................... Transient overvoltage resulting from current interruption on dc system ......................... An undergrounded system. showing capacitive-current flow .................................. An undergrounded system. with fault on phase A ......................................... The distributed inductance and capacitance of two-wire l i e shown as incremental sections .......... Demonstration of traveling wave on overhead line ........................................ ................................. Incident waves being reflected and refracted at discontinuity Electric field between conductors ..................................................... A 1.2 x 50 wave test used for BIL measurement .......................................... Equivalent circuit of multiturn winding showing distribution inductance and capacitance ............. .......................... Initial voltage distribution across uniform winding from step function ....................... Capacitive coupling of transient voltage through two-windiig transformer Basic valve surge arrester ........................................................... ................... Surge arrester with nonlinear resistance grading to equalize each gap structure Surge approaching surge-arrester-protectedequipment ..................................... .................... Typical surge protection of rotating machinery and dry-insulated transformers Simplified sketch of mine power-system segment .........................................
............................................... 11.32. Capacitance for 2,300.V induction motors ............................................ 11.33. Capacitance for 2,300.V synchronous motors 11.34. Overhead ground-wire shielding for low and high distribution towers ........................... 11.35. Static-wire-protection designs of wooded support structures using 30 protective angle ............... 11.36. Ratio of impulse to 60-Hz resistance as a function of peak impulse current. for driven rods .......... 11.37. Impulse breakdown of sand for two moisture conditions using spherical electrodes ................. 11.38. Impulse characteristics of spherical electrode, with seven attached pointed protrusions of various lengths ..................................................................... Typical power centers used in underground wal mines .................................... Schematic illustrating major components in power center .................................. Top view of mine power center showing placement of many internal components ................. Interconnections between input and feedthrough receptacles ................................ Graph illustrating transient crest voltage caused by ribbon-element current-limiting fuse operation .... Comparison of transformer withstand characteristic and surge arrester withstand characteristic ....... Typical primary winding taps on power cable transformer .................................. Zig-zag grounding transformer ........................................... ................................. Delta-wye connection for deriving a neutral Technique for measuring transformer impedance ............................. ............................... Typical X/R ratio versus transformer capacity Typical mine power-center transformer undcr construction . . . . . . . . . . . . . . . . . . . . . . ................................. Completed transformer prior to installation Typical bus work in powcr ccntcr undcr construction .......................... Typical conductor connection to molded-case circuit breaker ..................... Zero-sequence relaying on outgoing circuit with control connections to breaker ....... Zero-sequence relaying with jumpcr in relay case ............................. Neutral relaying applied to grounding-resistor current as backup protection .......... Backup protection devices associated with mine power cables .................... ................................ Typical test circuit for zero-sequence relaying Simple control circuit incorporating one ground-fault relay and one ground-check relay . Simple convenience-outlet circuit for 120- or 240-V single phase .................. .................................................................. Fusemountings .......................................... Typical metering circuit for line-to-line voltages ............................................... Typical metering circuit for line currents .................................................... Typical impedance monitor circuit Block diagram of continuity monitor connected in pilotless mode ............................. ................................ Block diagram of continuity monitor wired for pilot operation ................................. Application of power-factor correction in mine power center General arrangement of dc components for combination power center ......................... Full-wavebridgerectifier ........................................................... .................................... Series reactance to reduce available short-circuit current Separate transformer to increase impedance of dc circuit ................................... Typical full-wave bridge rectifier with two diodes in parallel per leg ............................ .................................................... Diode with RC snubber protection Diode-groundedsystem ............................................................ .................................................... Basic grounding-conductor system Relayed grounding-conductor system .................................................. ............................................................... Neutral-shift system ......................................................... Differential current scheme ............................................... Representative control circuit for rectifier ........................................................ Cross section of dc contactor
Page ................................................. Diagram for typical single switchhouse ................................ Control circuitry for single switchhouse using battery tripping Diagram for typical double switchhouse ................................................ .............................. Control circuitry for double switchhouse using capacitor tripping ............................................ Typical family of curves for inverse-time relay Illustration of fault location for adjusting selectivity ........................................ .......................... Typical control circuit for double switchhouse using capacitor tripping ............................ Typical control circuit for single switchhouse using battery tripping ............................................ Overall view of main substation serving mine Radial distribution applied to underground mine and its surface facilities ........................ .................... One-line diagram for single-ended substation with fuse-protected transformer ............ One-line diagram for single-ended substation with circuit-breaker-protected transformer Simplified one-line diagram for doubled-ended substation ................................... ........................................ Typical liquid-immersed transformer in substation ....................................................... Dead-tank OCB in substation Standard percentage-differential relaying system for transformer protection ...................... ........................... One-line diagram of substation with percentage-differential relaying Insulation characteristic of liquid-immersed transformer compared with the characteristic of valve ................................................................. surgearrester Plan view showing locations of system and safety ground beds ................................ Typical system ground bed for large substation ........................................... ........................................... Typical system ground bed for small substation ................. Substation feeding both surface and underground loads (no pounding conductor) ..................................... Substation feeding both surface and underground loads ......................................... Typical portable substation to service small mine .......................... Providing mine ground and protective relaying from utility substation ........................................ Use of isolation transformer with utility substation ................................................. Model and circuit symbol for thyristor ............................................... Typical characteristics curve for thyristor ......................................................... Thyristor half-wave rectifier ................................................... Alternating current thyristor control Three-phase control with bidirectional thyristor arrangement ................................. .................................................... Full-wave thyristor bridge rectifier ............................................... Three-phase thyristor-controlled rectifier Simplifiedchoppercontrol .......................................................... ................................................... Basic control-system block diagram Simplified block diagram of a motor controller ........................................... ..................................................... Common thyristor configurations .................................................... Heat sinking of disk-type thyristors Block diagram of ac-dc shuttle car .................................................... ............................................... Block diagram of ac-dc continuous miner ..................................................... Simple variable-frequency control .......................................................... Elementary inverter circuit ................................. Use of variable-frequencydrive on production mining shovel Simplified diagram of current-regulated static belt starter ................................... Simplified diagram of linear-acceleration static belt starter .................................. ....................................................... Types of thyristor firing pulses Thyristor protection for static belt starters .............................................. ......................................................... Protective-relay connections ....................................................... Simple electromechanical relay
................................................................ Simplestaticrelay Transistor used as relay ............................................................ .......................................................... Optical transistor as relay ............................................................. Thyristor used as relay Triacusedasrelay ............................................................... ............................................................... Hybrid static relays ....................................................... Simple overcurrent static relay ................................................... Simplified sketch of the SEL system Simplified sketch of the multipoint SEL system ........................................... ................................................ Diode-bridge phase-sensitive protection Equivalent model of figure 14.33 ..................................................... Electronic-comparator method of phase-sensitive protection ................................. Digital-controlled continuous static relay used for timed overcurrent ........................... Composition of lead-acid storage battery in various states of charge ........................... Voltage per cell of a typical lead-acid battery with varying continuous rates of discharge ............ Typical charging process of cell from 18.cell. 725-Ah battery ................................. Simplified schematic of saturable-reactor charger ......................................... Simplified schematic of single-phase thyristor charger ...................................... Two-winding transformer model ...................................................... Representation transformer magnetization curve .......................................... ..................................................... Ferroresonant transformer model .......................................................... Ferroresonant transformer ....................................................... Ferroresonant battery charger Plan of underground charging station .................................................. Circuit for detecting faults in batteries ................................................. Curve of relay current for various fault positions on battery .................................. ............................................ One-line diagram of desired charger features Cross-sectionalsketch of typical explosion-proof enclosure .................................. Typical plane-flange joint ........................................................... Typicalstep-flangejoint ............................................................ Threadedjoint ................................................................... Tongue-and-groovejoint ........................................................... Blindscrewhole ................................................................. .............................. Pressure vent limiting pressure buildup during internal explosion Pressure vent assembly using metal-foam material ........................................ Typical slip-fit straight stufting box and packaging-gland lead entrance .......................... ................ Typical slip-fit angle stuffing box and packing-gland lead entrance with hose clamp Typical slip-fit angle stuffing box and packing-gland lead entrance ............................. ............................................... Typical plug for spare lead-entrance hole Typical threaded straight stuffing box and packing-gland lead entrance with provision for hose ...................................................................... conduit Prototype trailing cable entry with polyurethane grommet ................................... ......................................................... Insulated-stud lead entrance Decision flow chart of class 1 1 . division 1and 2 hazardous locations ............................ 17.1. Circuit modeling a dielectric ........................................................ ............................................ 17.2. Current-voltage characteristics in a dielectric ..... 17.3. Graph relating approximate insulation resistance variation with temperature for rotating machines
Page Insulation resistance versus application time of test voltage .................................. Megohmmeter test connectionsfor checking cable insulation in line A .......................... ............................................ Megohrnmeter test connectionsfor ac motor Megohrnmeter test connectionsfor dc motor ............................................ ................................................ Spot resistance curve for normal motor Spot resistance curve showing effects of dust and moisture .................................. ............................................... Spot resistance c w e for detective motor .......................................... Megohrnmeter test connections for transformer Tie-resistance curve ............................................................. Three time-resistance curves for deteriorating motor ....................................... Time-resistance curves showing polarization for hypothetical motor ............................ Polarization factor curve for deteriorating motor .......................................... ........................................... Multiple voltage curves for deteriorating motor Circuit for harmonic tests .......................................................... Power-factor versus voltage curves showing tie-up ......................................... Mounting techniques for two vibration transducers ........................................ Four typical vibration measurement points .............................................. Typical vibration severity chart ....................................................... Comparison of acoustic-emission techniques for detecting failing roller bearings ................... Conceptual diagram of generalized mine monitoring and control system ........................ Conductioningas ................................................................ ................................................ Discharge sequence in an ionizing field ............................................................. High-stress geometrics Typical dielectric voids in cables ...................................................... Block diagram for corona-detection system .............................................. High-voltage cable terminations ...................................................... Major insulation void sometimes found in high-voltage coupler terminations ..................... ....................................... Possible stress site in high-voltage coupler insulators Power-conductor transposition on three-conductor type G cable .............................. Application of diode-suppressionbridges in power center ................................... Typical saturable-reactor characteristic ................................................. TABLES SIsymbolsandunits .............................................................. ........................................... Resistivity of some common materials at 20 C IEEE device numbers and functions ................................................... ........................................... Device numbers and letters common to mining ............................................... Motor voltage ratings common to mining Motor insulation classes ............................................................ NEMA class A standard starters for three-phase induction motors ............................ ................................................ Common motors for mining equipment .................................. Current range and effect on a typical man weighing 150 Ib Typical resistances for various contact situations .......................................... Approximate resistance formulas for various electrode configurations .......................... Comparison of grounding grids with other types of electrodes ................................ General resistivity classification ...................................................... Variations in resistivity with geologic age ............................................... ............................................... Typical values of resistivity of some soils ......................................... Variation in soil resistivity with moisture content .... Typical potentials of metals in soil measured from a copper and copper sulfate reference electrode
Page ............................................... Conductor sizes and cross-sectional areas .................................................. Letters used in alphabetic cable code ................................................ Codes for typical cablcs used in mining Typical diameters for round portable power cables ........................................ ............................................... Typical diameters for flat portable cables ........................................ Specifications for trailing cablcs longer than 500 ft .................................................. Ampacities for portable power cables ........................................ Ampacities for three-conductor mine power cables ............................. Correction factors for ampacitics at various ambient temperatures ...................... Ampacity derating factors for 60 C-rated trailing cables operated on drums .......... Australian specifications for ampacity derating factors for trailing cables operated on drums Some estimated power factors and load factors for various underground coal mining equipment in goodopcratingconditions ......................................................... .............................................. intermittent-duty ratings for trailing cables ......................................... Resistance and reactance of portable power cable Resistance and reactance of mine-power-feeder cable ...................................... Solid-wire breaking strength ......................................................... ....................... Recommended minimum bending radius. unshielded or unarmored cables ......................... Recommcndcd minimum bending radius. shielded and armored cables Trolley-wire specifications .......................................................... .......................................... Characteristic data for solid copper feeder cable Characteristic data for stranded copper feeder cable ....................................... ................................................ Trolley-wire support spacings on curves Resistance of steel rail at 20 C ....................................................... Data for rail-bond cable ............................................................ Minimum vertical conductor clearances as specified by the NESC. applicable to mining and mining-related operations ......................................................... ....................... Minimum distances from overhead lines for equipment booms and masts .................................... Ratings for mining-service molded-case circuit breakers Interrupting-current ratings vcrsus system voltage ......................................... Maximum instantaneous-trip settings .................................................. ................ Commonly available magnetic-trip ranges for mining-service molded-case breakers .................................. Some typical ratings for low-voltage power circuit breakers Typical minimum-oil circuit breaker ratings ............................................. ................................................... Ratings of high-voltage power fuses ................................. Common current ratings of induction-disk overcurrent relays ............................................... Standard burden for current transformers Standard ratings for potential transformers .............................................. ................................... Sample reactances for synchronous arid induction motors .................. Three-phase transformer per-unit impcdanccs for liquid-immersed transformers ........... Three-phase transformers impedances for distribution transformers. including load centers Sample applications of fault calculations ................................................ .................................................... Impedance of cables in figure 10.4 .................................. Burdens of relay elements and ammeter connected to CT's Recommended instantaneous trip settings for 480.. 600.. 1 . 0 4 0.V three-phase trailing-cable protection .. ............ Recommcndcd instantaneous trip settings for 300- and GOO-Vdc trailing-cable protection Recommended station and intermediate surge arresters for resistance-grounded mine power systems to protect oil-immersed transformers ...................................................
Page Recommended distribution.class. RM.type. surge arresters for resistance-grounded mine power systems ................................... to protect rotating machinery and dry-type transformers Commonly used surge capacitors for limiting voltage rate of rise on rotating machinery and dry-insulated transformers ......................................................... Typical capacitances per phase of power-system components. for shielded power cable SHD. SHD.GC. andSHD+GC .................................................................. ................................. Typical capacitances per phase of power-system components Protective angle versus structure height ................................................ ........................................ Typical current ratings of 400-A load-break switch Typical ratings for combination power centers ........................................... ............................ Standard impedance for liquid-immersed three-phase transformers ...................................... Standard BIL's for oil-immersed power transformers Typical electromechanical and static relay characteristics .................................... .......................... Time-margin comparison between electromechanical and static relays ...... Comparison of induction-disk and static time-overcurrent relay burdens to a current transformer ............................................... Formulas to estimate hydrogen evolution ................ Structural gap dimensions for explosion-proof enclosures as specified by 30 CFR 18 ................... Minimum autoignition tcmpcraturcs versus layer thickness for bituminous coals Common causes of vibration ........................................................
By Lloyd A. Morleyl ABSTRACT This Bureau of Mines publication presents a comprehensive review of mine elec- trical power-systemtheory and practice. It discusses fundamental theory and the vital aspects to be considered in planning and designing mine electrical power systems. The report is divided intothree major sections. The first presents the history of electricity in mining and the fundamentals of electrical phenomena and components.The second focuses on power-system components: motors, grounding systems, cables, and protec- tive equipment and devices. The final section includes mine power-center equipment, switchhouses and substations, batteries, and mine maintenance. p~ ~-~ - ~ p 'Professor of mining engineering, The Pennsylvania StateUniversity,University Park, PA (nowprofessor and department head, mineral enginwring. University of Alabama, Tuscalwsa, AL).
CHAPTER 1.-ELECTRICAL POWER IN MINING Probably no other mining area has grown so rapidly yet been as little understood by the average mine worker or operator as the mine electricalpower system. Traditionally, the field has held little interest for the mining engineer, who has tended to avoid it, or for the electrical engineer, who has given it scant attention. But today's mine power system is both complex and subject to numerous legal con- straints, and it is no longer possible to treat it with the indifference of the past. Underground mining machines are among the most compact and rugged equipment over designed, and individ- ual units can have up to 1,000 total horsepower. Mining equipment is usually mobile and self-propelled; most is powered electrically through portable cables and, for safety, must be part of an elaborate grounding system. The ma- chines and power-distribution equipment are seldom sta- tionary, must be adapted to continuous cyclic operation, and must resist daunting levels of dust and vibration. Surface mining can involve the largest earth-moving equipment built, where one piece can have 12,000or more connected horsepower-the largest today is over 30,000 hp. The electrical loads created by this machinery are cyclic and extremely dynamic: the largest excavator, for exam- ple, can require electrical loads that range from 200% motoring to 100% generating every 50 to 60 s, under the most exacting physical conditions. In the ever-movingmin- ing operation where distribution of power must be con- stantly extended and relocated, subjected to abuse by machine and worker alike, the potential for safety hazards is always present. Engineering and maintaining such an electrical system is demanding and challenging. It requires a specialist with knowledge of both mining and electrical engineering. Yet conversely, the effective management of a mine requires that anyone responsible for production and safety also be conversant with the mine electrical system. Management should understand the advantages and disadvantagesof one system over another, for if the power system is poorly de- signed, not only will safety be compromised but the mine operator will pay for the resulting conditions with high power bills, high-cost maintenance, and loss of production. Too often, a new mine is designed to use the type of power system employed in the preceding mine, without a comprehensive power study to determine the system needs and examine the alternatives available. Problems arise in existing mines when new mining equipment has been adopted without due regard for its impact on the operating power system; these problems haunt the mine electrical engineer who must frequently cope with a system that is a mongrel, bred from diverse inheritances from the past combined with recent changes and additions. New laws, standards, and safety requirements must frequently be accommodated by power systems not originally designed to meet their specifications;new and unfamiliar equipment must be grafted to the existing network, and the result can be a hybrid of considerable complexity. This text has been produced to assist the power engineer and the student in understanding these complexities and the principles that lie behind them. The material presented here is structured so an indi- vidual unfamiliar with electrical engineering can first developthe necessary fundamentals before embarking into mine electrical design. A basic physics and calculus knowl- edge is necessary to understand the content completely. The goal has been to assemble the most significant information required for comprehension of mine power systems so that the reader may then progress to more specializedtopics. But first, a brief review of the development of electrical usage in mines is given, in order that the reasons for some of the peculiarities of mine power systems can be appreciated. MINE ELECTRICAL HISTORY Electricity was first introduced into coal mines shortly before the beginning of the 20th century in the form of di- rect current (dc)for rail haulage. This form of current was used because at that time most systems were powered by dc generators. It had a number of advantages for haulage; the most outstanding was that the dc series-woundmotor had (and has) excellent traction characteristics. Speed con- trol was a simple matter of placing a resistance in series with the motor armature or field circuits. Batteries served as the first power source, and hence the vehicle was extremely mobile even though constrained on rails. However, keeping the batteries charged was both- ersome, so trolley wires were soon introduced in several mines. Allowingthe trolley wire to act asoneconductorand the rail as the other provided the simplest form of power distribution yet known to the mining industry. Available haulage machinery of that period was low in horsepower and the mines were relatively small sothe increased resis- tance that reduced voltage and power supplied to the motors was still acceptable. Thus, the dc system at a voltage of 250 or 550 V became firmly entrenched in coal mines. Underground Mine History Underground, the first electrically driven coal mining machine, the coal cutter, was installed in the early 1920's. Although dc offered no special advantage, it was readily available; hence, the machine was equipped with a dc motor and added to the system. The cutter was followed almost immediately by the loader, and it too was driven by dc motors.I f there was rail haulage in the mine, trailing cablea supplied power from the trolley wire and the rail to the machines. The next significant increase in power consumption came with the introduction of the shuttle car, almost 20yr after the coal cutter. Actually, when the shuttle car was first invented in 1937, it was battery powered. The addi- tion of an automatic reeling deviceto handle a trailingcable came later, in an attempt to overcomebattery deficiencies. These trailing cables were also connected to the haulage power system, and this equipment, when combined with the cutters and loaders, placed additional stress on the dc distribution system. At that time, the horsepower required to operate each piece of electrical mining equipment was quite small and no individual machine used a large amount of current. However, when all machines were combined, significant power was required, and because all the conductors offered resistance. voltaee d r o ~ s and transmission losses in the distribution system we; extensive. Alternatingcurrent (ac) would have been more practical because it could have been
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Electrical Engineering in Mining Industries.pdf

  • 1. Information Circular 9258 Mine Power Systems By Lloyd A. Morley UNITED STATES DEPARTMENT OF THE INTERIOR Manuel Lujan, Jr., Secretary BUREAU OF MINES T S Ary, Director
  • 2. Library of Congress Cataloging in Publication Data: Morley, Lloyd A. Mine power systems. (Information circular: 9258) Includes bibliographies. Includes index. Supt. of Docs. no.: 128.27:9258. 1. Electricity in mining. I. Title. 11. Series: Informalion circular (United States. Bureau of Mines); 9258. TN295.U4 [TN343] 622 s [622'.48] 87-600213 -- For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC 20402
  • 3. The author is grateful to the several individuals and companies that supplied noncopyrighted material for use in this publication. This material is noted by the vari- ous courtesies given throughout the text. Its incorporation does not constitute an en- dorsement by the author, The Pennsylvania State University, the University of Ala- bama, or the Bureau of Mines. Reference to specificproducts, equipment, or manufacturers does not imply endorsement by the Bureau of Mines.
  • 4. The application of electricity to the mining industry is a distinctive area of both mining engineering and electrical engineering. The diEcult environment, the dynamic power loads, the cyclic and mobile operation and stringent safety requirements that characterize mining, all place unique demands on the mine power system. No other industry makes such extensive use of portable extensible equipment or has such com- plex grounding problems. Mine power systems can range from relatively simple in- stallations for small surface mines to complex underground systems where the harsh environment of dust, humidity, and cramped spaces stretches the ingenuity and crea- tivity of the engineer to provide reliable service. At the present time there is no up-to-date engineering text available that deals specifically with mine power systems. This has created extensive difficulties for edu- cators, industry engineers, and regulatory agency personnel. The need for a suitable reference for students in mining engineering provided the main impetus for this book, since the technician-level material that was in existence proved unsuitable for teach- ing young engineers who have little practical experience. The objective in preparing this manuscript was to assemble a single engineering reference on mine electrical power systems that is as comprehensive as possible. Ear- lier drafts of this material have been used successfullyto instruct university students in courses ranging from basic electrical engineering through power-system design. It is felt, however, that the usefulness of this material extends beyond that of a student text. While not intended to replace other electrical or mining references, this publica- tion is also an indexed, reasonably comprehensive reference handbook for industry engineers and training personnel, and a sourceof material for electrical engineers who wish to expand their education into industrial power-system applications. Obviously, there will be some omissions; to include all aspects of mine electrical systems in one volume would approach an impossibility, but an attempt has been made to collect together the most significant information, thereby providing the tools needed to con- tinue a knowledgeable involvement in mine electricity. This reference work is divided into three general content areas. Chapters 1through 5 contain information considered elementary, chapters 6 through 1 1deal with power- system components, and chapters 12 through 17 contain specifics on mine power sys- tems. A person familiar with electrical principals can use the earlier chapters as re- view material, but all chapters contain material relevant to mining and discuss the necessary combinations of equipment and components that should be contained in the mine power system. Emphasis throughout is placed on coal mining systems, although much of the material pertains to all mining operations. Both surface and underground power systems are discussed, the latter in more detail since these are the more com- plex systems and encounter the most problems. This publication is a thoroughly upgraded and extensively revised edition of Bureau of Mines Open File Reports 178(1)-82and 178(2)-82,prepared under Bureau contract 50155009by The Pennsylvania State University. It contains new chapters,new illustra- tions, and example problems that were not included in the original report. The assembly of this material has been a major undertaking. Many industry, academic, and Government agency personnel helped to review and critique practically every stage of draft preparation. The original report version was made available to students taking the mine power-systems courses at The Pennsylvania State Univer- sity, and their involvement was critical input to manuscript -- preparation. -- The author is grateful to all the companies and individuals who contributed or cooperated in this effort; so much information could not have been gathered without their help. A specialthanks is owed to the late Robert Stefanko. He originally perceived the need for this text and provided guidance and encouragement throughout the proj- ect that produced the original report version. Others deserving special mention are A. M. Christman, R. H. King, J. A. Kohler, G. W. Luxbacher, T. Novak, J. N. Tomlinson, F. C. Trutt and D. J. Tylavsky. Each contributed directly to the text while on the fac- ulty or staff at The Pennsylvania State University; acknowledgements for their con- tributions are made in the individual chapters.
  • 5. CONTENTS Page Page Preface ............................. Abstract ............................. Part l : Fundamentals Chapter l.--Electrical power in mining ...... Mine electrical history ................. Underground mine history ............ Surface mine history ................ Mine power equipment ................ ....................... Substations Switchhouses ...................... Power centers ..................... Distribution equipment .............. Basic distribution arrangements .......... Radial system ..................... Primary-selective system ............. Primary-loop system ................ Secondary-selective system ............ Secondary-spot network .............. Utility company power ................ ...................... Surface mining Power systems in surface mines .......... Main substations and subtransmission ... Surface mine distribution ............. Underground coal mining .............. Room-and-pillar mining .............. Longwall mining ................... Power systems in underground mines ...... Regulations ....................... Underground mine distribution ........ Surface facility power requirements ....... Basic design considerations ............. References ......................... Chapter 2.--Electrical fundamentals I . . . . . . . Basic electrical phenomena ............. .................... Coulomb's law Voltage and current . . . . . . . . . . . . . . . . System of units ...................... Experimental laws and paramcters ........ ....................... Ohm's law Kirchhoff's voltage law .............. KirchofPs current law ............... Series circuits ..................... Parallel circuits .................... The magnctic field ................. Inductance ....................... ...................... Capacitance Electric field ...................... Instantaneous power ................ Idealization and concentration ......... ................. Direct current circuits Direct current and circuit elements ..... Series and parallel resistance .......... ............ Wye-delta transformations Circuit and loop equations ............ Node equations .................... Network theorems .................. Time-varying voltages and currents ....... .............. Steady alternating current ........... Effective alternating current Phasors .......................... ........ Phasors and complex quantities Impedance transforms ............... ................ Steady-state analysis ....... Chapter 3.-Electrical fundamentals I1 ......... Average power and power factor ........... Complex and apparent power Resonance ......................... Series resonance ................... Parallel resonance .................. Transformers ....................... Ideal transformer .................... Actual transformers .................. Conductor loss .................... Leakage reactance .................. ........ Core losses and exciting current ........ Power-transformer construction Transformer models ................ Determination of transformer ..................... parameters ... Transformer efficiency and regulation Autotransformers .................... .............. Multivoltage transformers ....... Current and potential transformers Chapter 4.-Power.systcm concepts .... Basic power circuit .............. Three-phase circuits ............. Balanced three-phase circuits ...... Three-phase system voltages ..... Load connections ............. Line and phase currents ........ Equivalent delta and wye loads ... Three-phase power ............ Three-phase transformers ......... Balanced three-phase circuit analysis . One-line and three-line diagrams ..
  • 6. Page Page ........ Circuits containing transformers Per-unit system ...................... Transformer impcdancc .............. ........... Three-winding transformers ...... Per-unit method in system analysis ......... Unbalanced three-phase circuits Fault types ....................... Fault analysis ..................... ............... Symmetrical componcnts Sequence components ............... ........ Sequence-quantity combinations .... Symmetrical-componentrelationship Symmetrical-component impedance ..... .................. Fault calculations ................... Power terminology References ......................... Chapter 5.-Basic solid-state devices and instrumentation ...................... 104 Semiconductors ...................... 104 Diodes and rectifiers .................. 104 Diodc equations ................... 105 Rectifier circuits ................... 105 Cooling .......................... 106 Overloads ........................ 107 Three-phase rectification ............... 107 Rectifier circuits ................... 108 Parallel rectifier operation ............ 109 Transistors ......................... 109 Transistor operation ................ 109 Bipolar-transistor amplifiers ........... 110 Field-effect transistors ............... 112 Silicon-controllcdrcctifiers ............. 113 Integrated circuits .................... 114 Basic instrumentation ................. 114 Basic meter movements ............... 115 Meter-movement applications ......... 116 Wattmeters ....................... 117 Varmeters ........................ 118 Power-factor meters ................ 118 Power-system instrumentation ........... 118 Instrument transformers ............. 118 Single-phase connections ............. 119 Three-phase connections ............. 120 Special instruments ................... 122 Watthour meters ................... 122 Demand meters ................... 122 Bridges .......................... 122 Megohmmeters .................... 123 Phase-sequence indicators . . . . . . . . . . . . 124 Recording instruments ................ 124 Electronic instruments ................ 125 Electronic meters .................. 125 ..................... Oscilloswpes 125 .................... Tape recorders 126 Transducers ...................... 126 ................ Instrument installations 127 Part 11: Power-System Components Chapter 6.--Motors and motor control ...... 129 Alternating current generation ........... 129 Principle of generator operation ........ 129 .............. Generator construction 129 Three-phase generation .............. 131 Direct current generators .............. 131 Motor basics ........................ 133 Torque .......................... 133 ............ Speed-torque relationships 133 .................... Standardization 134 Motor type ....................... 135 Three-phase squirrel-cage induction motors .......................... 136 ......... Elementary three-phase motor 136 ................. Motor conslruction 138 Motor behavior .................... 138 Insulation ........................ 139 ............... Design characteristics 139 ............. Induction-motor starting 141 .......... Wound-rotor induction motors 142 Three-phase synchronous motors ......... 143 ........... Synchronous-motor starting 144 ............ Synchronous-motor torque 145 Generated voltage .................. 146 Power factor ...................... 146 Applications ...................... 147 Direct current motors ................. 147 Elementary motor .................. 147 ............ Actual motor construction 148 Torque .......................... 148 .... Motor connections and performance 148 ............... Ward-Leonard system 152 Mine motors ........................ 153 Applications ...................... 153 ........... Actual equipment operation 153 Single-phase motors .................. 156 ................ Rotating stator lield 156 ................. Split-phase starting 157 ............... Capacitor-start motors 157 References ......................... 158 Chapter 7..-Grounding .................. 159 Grounding systems ................... 160 Ungrounded neutral .................. 160
  • 7. ............... Solidly grounded neutral ....... Low-resistance grounded neutral High-resistance grounded neutral ....... Electric shock ....................... Characteristics of mine grounding systems .. Ground beds ...................... ...... Grounding in underground mining ........... Grounding in surface mines ............... Ground-bed construction .................. Ground resistance ....... Electrode co~gwation formulas ............ Two-layer earth structures ................. Soil-heating effects ......... Control of potential gradients ...... Ground-bed resistance measurement Measurement method ............... Ground test instruments ............. ................. Ground-bed resistivity Factors affecting resistivity ............ Resistivity measurements ............. Effect of chemical treatment of soils .... ................. Ground-bed corrosion General ground-bed guidelines .......... Grounding equipment ................. ................. Grounding resistor Grounding transformers ............. .......................... Summary References ......................... Chapter 8.--Distribution ................. Nature of cable distribution ............. ................... Cable components ....................... Conductors Insulation ........................ ...................... Cable jacket .................... Cable shielding Cable types ......................... Cable terminations ................... Cable couplers ...................... Coupler contacts ................... .................. Coupler insulation ................... Coupler housing High-voltage couplers ............... ................ Low-voltage couplers ...................... Cable selection Cable length ...................... ................. Conductor selection Cable installation and handling .......... .................... Borehole cables Feeder cable installation ............. Recommended handling practices ...... Cable failures and repairs .............. Cable testing ...................... .................... Failure location Splicing .......................... Trolley systems ...................... Trolley wire ...................... Trolley feeder ..................... ..... Supports. lubrications. and turnouts ................... Rails and bonds Overhead lines ...................... ................ Overhead-line design Overhead-line electrocutions .......... References ......................... Chapter 9.-Protective equipment and relaying ........................... Switching apparatus .................. ............ Arcs and circuit interruption Switches ........................... Circuit breakers ..................... Circuit breakers for low and medium voltage .......................... Molded case circuit breakers .......... ............... Power circuit breakers High-voltage circuit breakers ............ Typical ratings ..................... Oil circuit breakers ................. Minimum-oil circuit breakers .......... Vacuum circuit breakers ............. Fuses ............................. Low-voltage fuses .................... Non-time-delay fuses ................ Time-delay fuses ................... Dual-element fuse .................. Current-limiting fuses ............... Standard fuses ..................... Nonstandard fuses .................. High-voltagefuses .................... Expulsion types .................... Current-limiting high-voltage fuses ...... Load-break switches ................ Relays ............................ Relay terminology and types .......... Thermal relays .................... Electromagnetic-attraction relays ....... Electromagnetic-induction relays ....... Basic relay connections ................ Alternating current direct relaying ...... Alternating current potential relaying .... Alternating current differential relaying .. Direct current connections ............ Kinds of protection ................... ..................... Control wiring Page 206 207 207 211 211 211 211 215 216 217 218 222 224 224 225 226 226 227 228 232 232 232 232 233 234 235 235 236 236 236 236 236 237 237 237 238 239 240 240 240 241 242 244 244 246 247 247 248 248
  • 8. Page Page Phase protection ................... Ground overcurrent ................. Ground-check monitoring ............ ......... Advantages and disadvantages Arrangements for mining .............. Zones of protection ................. ...................... Coordination .............. Ground-fault protection Overloads and short circuits ........... Surface mines ..................... Underground mines ................. ......................... References ....... Chapter 10.Suing protective devices ....................... Fault current Fault-current sources ................ ............. Source equivalent circuit Fault calculations for three-phase systems .. Short-circuit calculation procedures ..... Three-phase calculation example ....... Computer fault analysis .............. Ground-fault current calculations ....... Direct current system faults ............. Device settings ...................... Relay pickup settings ................. Short-circuit protection .............. Overload protection ................. Ground-fault protection .............. Current transformer matching ........... Current transformer accuracy ......... Accuracy calculations................ Low-voltage circuit breaker trips ......... Overload protection ................. Short-circuit protection .............. Low-voltage power circuit breakers ..... Fuses ............................. ....................... Coordination ......................... References Chapter ll.--Transients and overvoltages ..... Transient sources .................... Lightning phenomena ................. Switching transients ................... ............... Capacitance switching .................. Current chopping Prestrike ......................... ........... Direct current interruption General switching transients .......... Other transient phenomena ............. Traveling waves ..................... Electromagnetic phenomena ............ Transient-induced failures .............. Winding response .................. ........ Coupling through transformers Transient protection .................. Surge arresters .................... ............ Surge arrester applications Capacitors and system capacitance ...... ............ Other suppression devices Faraday shields .................... ................ Circuit arrangements ........... Protection of overhead lines Impulse performance of ground beds .... References ......................... Part 111: Mine Power Systems .......... Chapter 12.-Mine power centers ............... Equipment specifications Mine power centers .................. High-voltage cable coupler ............. Interlock switches .................... Disconnect switch .................... High-voltage fuses .................... Surge arrestors ...................... Transformers ....................... SpeciF~cations ..................... Transformer construction ............. Faraday shields .................... Grounding resistor ................... Busway ............................ ............... Outgoing circuit breaker Ground-fault protection ............... .............. Single-phase transformers Metering circuits ..................... ............... Outgoing cable couplers ................ Ground-check monitors ................ Power-factor correction ............... Direct current utilization Rectifier transformer ................. ........................... Rectifier Direct current ground-fault protection schemes ......................... .......... Direct current control circuitry ........ Direct current interrupting devices References ......................... Chapter 13.Switchhouses and substations ... Switchhouses ....................... Switchhouse internal components ......... Switchhouse protective relaying .......... Power circuit breakers ................ Switchhouse control circuits ............. Switchhouse design ...................
  • 9. Page Page Substations ......................... Basic substation arrangements ........... Single-ended substations ............. Double-ended substations ............ Substation transformers ................ Substation switching apparatus .......... ........................ Reclosers Disconnect switches and fuses ......... Protective relaying in substations ......... Lightning and surge protection in substations ....................... .................. Substation grounding Substation ground mat ............... .............. Ground-fault protection Additional mine substation loads ......... Portable substations .................. ....... Utility voltage as mine distribution Additional substation design ..................... considerations References ......................... Chapter 14.4olid-state control and relaying .. ....................... Motor control ................ Simple motor control Control systems .................... ..... Physical characteristics of thyristors Direct current applications ............. .......... Alternating current applications ............... Static protective relaying Operation of simpwed solid-state and hybrid relays ...................... Static and electromechanical relay ....................... comparison Static relay mining applications .......... Sensitive earth-leakage system ......... Phase-sensitiveshort-circuit protection ... Solid-state relays in the future ........... .......................... Summary References ......................... Chapter IS.--Batteries and battery charging ... ... Basic battery and battery-charging theory Battery maintenance .................. .......................... Chargers Charging stations .................... Battery-box ventilation ................ 1.1. Simple mine electrical system arrangement . 1.2. Simple radial distribution system ........ 1.3. Power-center type of radial distribution ... 332 Battery surface leakage and faults ........ 375 ............... 332 Battery-charginghazards 377 ......................... 333 References 381 Chapter 16.-Permissibility and hazard .......................... reduction ........................ Terminology ............. Hazard-reduction methods Explosion-proof enclosures ............. Explosion transmission .............. ................... Enclosure joints Enclosure mechanical strength and internal pressures ....................... Enclosure hazards .................. ................ Permissible equipment ........ Permissible equipment schedule ... Maintenance of permissible equipment .................... Coal dust hazards ........ Classifications of dust locations ............... Reducing dust hazards Hazardous locations in preparation plants ......................... ......................... References ................ Chapter 17.-Maintenance ............. Mine maintenance program ............... Economicjustification Preventive maintenance program .................. implementation ..... Techniques of preventive maintenance ......... Basic electrical measurements ............. Insulation measurements ................. Megohmmeter tests .............. Mechanical measurements .......... Continuous-monitoring systems ............................ Corona 406 ................... Corona behavior 408 ................... Corona detection 409 .... Partial-discharge problems in mining 410 .................. Intermachiue arcing 411 ........... Ground direct current offsets 412 .......................... Summary 413 ......................... References 414 .......................... Bibliography 415 ...... Appendix.-Abbreviations and symbols 416 ............................... Index 420 ILLUSTRATIONS
  • 10. Page .................................................. Primary-selective distribution system ........................................................... Primary-loop distribution .......................................................... Secondary-selectivesystem .................................................... Secondary-spot network technique ........................................ Representative utility transmission and distribution ..................................................... Subtransmission for surface mine Radial strip mine distribution system .................................................. ........................................... Secondary-selectivedistribution in strip mining Primary-loop design for strip mining .................................................. Radial distribution for strip mine with overhead poleline base line ............................. Radial distribution for strip mine with all-cable distribution .................................. Surface mine distribution system using two base lines ...................................... Openpitpowersystem ............................................................ ..................................................... Layout of underground coal mine ...................................................... Plan view of retreating longwall Subtransmission for underground mine ................................................. Radially distributed underground power system .......................................... Secondary-selective distribution in underground mines ..................................... Utilization in continuous mining section ................................................ Power-system segment with longwall equipment .......................................... ........................................ Diagram of electrical-systemsegment for longwall Parallel-feed haulage system ........................................................ Representative expanded radial distribution for preparation plant ............................. Representative secondary-selectivedistribution for preparation plant ........................... Circuit element illustrating voltage polarity and current flow direction .......................... Simpleseriescircuit ............................................................... Ideal and actual voltage sources ...................................................... Circuit for example 2.1 ............................................................. Demonstration of Kirchhoffs current law ............................................... Simple parallel circuits ............................................................. Ideal and actual current sources ...................................................... Parallel circuit for example 2.2 ....................................................... Simple series circuit and equivalent ................................................... Simpleparallelcircuit ............................................................. Series-parallel circuit for example 2.3 .................................................. Series-parallel circuit for example 2.4 .................................................. Magnetic flux in a straight conductor and in a long coil ..................................... Demonstration of induced current .................................................... Two coils demonstrating mutual inductance ............................................. Long-coil inductance and inductor symbols .............................................. Toroidalcoil .................................................................... Charge. voltage. and current relationships of capacitor ..................................... Electric lines of force between two parallel charged plates .................................. Resistor used to demonstrate instantaneous power ........................................ Simple example of idealization and concentration ......................................... Modeling of load center. trailing cable. and shuttle car ..................................... Basic elements of resistance. inductance. and capacitance ................................... Sirnplilicalion of dc circuit .......................................................... 2.25. Simple circuit reduction ............................................................
  • 11. Page ............................................................ Cicuitforexample2.5 ............................................................ Circuitforexample2.6 ............................................. Series-parallel conductancesfor example 2.7 Series-parallel circuit for example 2.8 .................................................. Two-terminal and three-terminal networks .............................................. ................................................... Wye and delta circuit configuration "T"and "nucircuit configurations ..................................................... ............................................................ Commonbridgecircuit ..................................................... Circuit reduction of bridge circuit ................................................................... Partsofcircuit Circuit demonstrating two independent loops ............................................ .................................................... Two-loop circuit for example 2.11 Bridge circuit demonstrating loop analysis .............................................. Three-loop circuit for example 2.12 ................................................... Simple two-node circuit ............................................................ Three-junction circuit ............................................................. Three-junction circuit with grounds ................................................... ........................................ Voltage-source circuit demonstrating node analysis Circuit for examples 2.13, 2.15, and 2.16 ................................................ Circuit for example 2.14 ............................................................ Circuit for demonstrating superposition theorem .......................................... Circuit in figure 2.44 with sources turned off ............................................. Demonstration of reciprocity theorem ................................................. Practical voltage-source model ....................................................... Practical current-source model ....................................................... Source transformation ............................................................. ....................... Circuit in figure 2.44 with current sources transformed to voltage sources ............................................................... Thevenin'stheorem ................................................................ Norton'stheorem Comparison of Thevenin's and Norton's circuits .......................................... Circuit for example 2.17 ............................................................ Active circuit for example 2.18 ....................................................... Circuits illustratingsolution steps to example 2.18 ......................................... Some time-varying electrical waves .................................................... Sinusoidal ac waveform ............................................................ Steady ac showing phase shift ........................................................ Steady ac through resistance ........................................................ ........................................................ Steady ac through inductance Steady ac through capacitance ....................................................... Simple series RL circuit ............................................................ Simple series RC circuit ............................................................ Simple series RLC circuit .......................................................... Graphical representation of complex number ............................................ Trigonometric or polar representation of complex number .................................. Sinusoid versus time and as phasor .................................................... ............................................ Phasor representation of current and voltage Other expressions for phasors ....................................................... .................................... Voltage-current phasor relationships for circuit elements ........................................ Steady sinusoid analysis of simple RL series circuit ........................................ Steady sinusoid analysis of simple RC series circuit
  • 12. Page ....................................... 2.76. Steady sinusoid analysis of simple RLC series circuit 2.77. Circuit for example 2.21 ............................................................ 2.78. Circuit for example 2.22 ............................................................ .................................................... 2.79. Two-loop circuit for example 2.23 2.80. Activecircuitforexample2.24 ....................................................... ...................................... Power represented as real and imaginary components Illustration of leading and lagging power factors .......................................... ........................................... Circuit demonstrating sum of complex powers ................................................ Simple series RLC circuit for resonance Plot of impedance magnitude versus frequency for series RLC illustratingresonance ............... Circuits that exhibit parallel resonance ................................................. .............................................. Magnetic coupling between two conductors Magneticcouplingbetweentwocoils .................................................. Demonstration of coil winding sense .................................................. Dot convention for mutal inductance sign ............................................... Demonstration of impedance transfer in transformers ...................................... Ideal transformer with winding resistance included ........................................ Accounting for transformer leakage flux ................................................ Transformer magnetizing current ..................................................... Eddy current and magnetic hysteresis creating power loss in core ............................. .............................................. Equivalent circuit of practical transformer Common power-transformer construction techniques ...................................... Movement of exciting components to input .............................................. ........................................... Transferring secondary components to primary Final simplification of pratical circuit model ............................................. Transformer parameter test series .................................................... ............................................................ Circuit for example 3.8 Comparison of two-winding transformer and autotransformer ................................ Two-winding transformer as an autotransformer .......................................... Examples of transformers for multivoltage applications ..................................... TwotypesofCT's ................................................................ Examples of CT and PT placement in circuit ............................................ Basicpowercircuit ............................................................... Applications of basic power circuit .................................................... Elementary three-phase generation .................................................... Three-phase voltage sources ......................................................... Wye-connected source demonstrating line-to-line and line-to-neutral voltages ..................... ................................................. Balanced three-phase load connections Four-wire wye-to-delta system ....................................................... Balanced delta load illustrating phase and line currents ..................................... Comparison of equivalent delta and wye loads ........................................... Three-single-phasetransformers connected for three-phase operation .......................... ................................... Three-phase diagrams for the transformers of figure 4 . 1 0 ........................................... Open-delta three-phase transformer operation Per-phase reduction of wye-to-wye system .............................................. Per-phase reduction of delta-to-delta system ............................................. ............................................................... Three-linediagram One-line diagram of circuit shown in figure 4.15 ..........................................
  • 13. Page .................................... Commonly used symbols for one-line electrical diagrams ......................................................... Symbolsforrelayfunctions ..................................................... One-line diagram for example 4.7 Three-phase diagram of figure 4.19 ................................................... ..................................................... Per-phase diagram of figure 4.19 One-line diagram with delta-delta transformer ........................................... ..................................................... Per-phase diagram of figure 4.22 ............................................ One-line diagram with delta-wye transformer ....................................... One leg of three-phase transformer from figure 4.24 Approximate per-phase equivalent circuit for 750-kVA load-center transformer; impedance referred ................................................................... tohighside Transformer of figure 4.26 with impedance referred to low side ............................... Simplified equivalent circuit of transformer expressed in per-unit .............................. Approximate equivalent circuit of three-winding transformer expressed in per-unit ................. ........................................... One-line diagram of small mine power system Impedance diagram of system in figure 4.30, expressed in per-unit on a 1.000.kVA base ............. ............................................................ Basicfaultdescriptions Positive.sequence. negative.sequence. and zero-sequence vector sets ........................... Symmetrical component addition to obtain unbalanced three-phase set ......................... ....................................... Equivalent delta-connected and wye-connected loads ............................................ Three-phase system with line-to-neutral fault ........................................... Symbol and operation of a p-n junction device ............................................. Bias conditions and current flow for a diode Diode or rectifier characteristic curve .................................................. ............................................... Half-wave rectifier circuit and waveforms ................................................ Single-way full-wave rectifier waveforms Bridge rectifier circuit and waveforms ................................................. Example of filtering a rectifier output .................................................. Heatsinkcooling ................................................................ Heat sink thermal relationships ...................................................... Three-phase half-wave rectifier circuit and output voltage waveform ........................... Three-phase full-wave rectifier circuit with input and output voltage waveforms ................... .................................... Parallel operation of rectifiers using paralleling reactors An n-pn junction transistor ......................................................... .......................................................... A p-n-p junction transistor ......................................... Current relationships for p-n-p and n-p-n devices Common-baseamplifiers ........................................................... .......................................................... Common-emitter amplifier ................................................. Common-emitter characteristic curves Bias techniques for common-emitter amplifiers ........................................... ................................................ Common-collector amplifier arrangment ................................................. Model and symbols for junction FET's ................................................. Example of a junction-FET application .............................................. Model and symbols for MOS-FET devices SCRmodelandsymbol ............................................................ .................................................... SCR equivalent model and circuit .................................................. General characteristic curve for SCR ............................................. Sketch of simple monolithic IC cross section TopviewofanactualIC ...........................................................
  • 14. Page ................................................ Examples of symbols employed for IC's .............................................. Permanent-magnet moving coil movements .......................................... Shunting d'Arsonval meter for high-current tests .......................................... D'Arsonval meter used to measure dc potentials ....................................... External shunts used for high-current measurements Simpleohmmetercircuit ........................................................... Rectifier ammeter ................................................................ ................................................. Dynamometer connected as wattmeter Power-factor movement ............................................................ .............................................. Simple instrument-transformer connections ........................... Voltmeter. ammeter. and wattmeter arranged as single-phase system .................................... Use of transducers with standard d'Arsonval movements ................................................... Three-phase wattmeter connections Two-wattmetermethod ............................................................ ......................................... Three-phase power measurement with transducer ...................... Balanced three-phase measurement of voltage. current. and average power Line current measurements with two or three CT's ........................................ ................................... Line-to-line voltage measurements with three or two PT's ................................... Simplified sketch of watthour meter induction mechanism Wheatstonebridgecircuits .......................................................... Kelvindoublebridge .............................................................. .............................................. Megohmmeter testing insulation resistance ................................................. Internal components of megohmmeter Phase-sequenceindicator ........................................................... ............................................................... Strip-chart recorder .................................................. Input circuits on electronic voltmeter Digitaldisplay ................................................................... Cathode-raytube ................................................................. .................................................. Semiconductor illustrating Hall effect ............................................... Production of voltage from magnetic field ...................................................... Demonstration of ac generation ................ Cross section of machine with salient poles on stator and nonsalient poles on rotor ............................ Cross section of machine with nonsalient poles on stator and rotor .................. Simplified sketch of electromechanical machine illustrating physical components .......................................... Elementary four.pole. single-phase ac generator ............................................. Elementary two.pole. three-phase generator ............................................ Elementary four.pole. three-phase generator ...................................................... Demonstration of dc generation .................................... Dc generator with two armature windings at right angles ...................................................... Separately excited dc generator Seriesdcgenerator ............................................................... Shuntdcgenerator ............................................................... Compounddcgenerator ........................................................... ........................................... Current-carrying conductor in a magnetic field .............................................. General speed-torque motor characteristic ............................................ Examples of three frame number dimensions ............................................ Demonstration of induction-motor operation ............................................... Elementary three-phase induction motor ......................................................... Squirrel-cage rotor winding
  • 15. ..................... Rotating magnetic field in elementary three.phase. two-pole induction motor ................................................ Induced rotor potential by rotating flux ............................................ Lapped windings of three-phase motor stator Characteristic curves of three-phase induction motor ....................................... ........................ Typical torque-speed characteristic for general-purpose induction motor ........................... Phasor diagrams of rotor and stator flux density for induction motor Typical torque-speed characteristics for NEMA-design three-phase squirrel-cage motors ............ Other rotor-conductor designs ....................................................... ...................................................... Across-the-line magnetic starter Starting methods for induction motors ................................................. .................. Schematic of wound-rotor induction motor showing external resistance controller .............. Torque-speed characteristics for wound-rotor motor with stepped-resistancecontroller ............................... Simplified step starter using individually timed magnetic relays ............................... Sketch showing construction of salient-pole synchronous motor ..................... Simplified diagram of synchronous motor using generator for field excitation ................................... External solid-state supply used to provide field excitation ................................. Schematic of low-speed cylindrical-rotor synchronous motor ................... Controller used to demonstrate general starting method for synchronous motor .................. Typical torque-speed characteristic for synchronous motor with damper winding Effect of load on rotor position ...................................................... Equivalent per-phase circuit of a synchronous motor and phasor diagrams for underexcited and overexcited field winding ......................................................... V-curves for synchronous motor ...................................................... ........................................ Plan view of typical mining shovel showing m-g set Elementary two-pole dc motor ....................................................... Elementary four-pole dc motor ...................................................... .................. Cross-sectional sketch of dc motor showing interpole and compensating windings ................. Interaction between armature and main-field flux to produce main-field distortion Four connections for dc motors ...................................................... Typical characteristics for shunt. series. and compound motors of equal horsepower and speed ....................................................................... ratings .................................... Simplified dc motor schematicswith starting resistances Faceplate manual starter ........................................................... Multiple-switch starting ............................................................ Drum-typestarter ................................................................ ............................... Simplified diagram of dynamic braking applied to shunt motor ........................................ Two-step resistance starting of series-wound motor ......................................... Forward-reverse switching of series-wound motor Dynamic braking applied to series-wound motor .......................................... ............................................ One-step starting of compound-wound motor Basic WardLeonard system ......................................................... ............................ Typical characteristic curves for each motor in traction locomotive Stator field of two.pole. single-phase induction motor ...................................... ............................. Rotor field of stationary two.pole. single-phase induction motor Phase relationshipsbetween stator and turning rotor ....................................... Starting and running stator windings ................................................... ............................................ Centrifugal switch to remove starting winding Capacitor-start motor ............................................................. Illustration of electrical shock hazard .................................................. ............................................. Capacitance coupling in ungrounded system
  • 16. Page Solidlygroundedsystem ............................................................ 160 Resistance-grounded systcm ......................................................... 160 Effect of frequency on let-go current for men ............................................ 162 ............................................... Simplified one-line diagram of substation 163 ................................................ Step potentials near grounded structure 163 Touch potentials ncar grounded structure ............................................... 163 ~ine-to-earthfault resulting in current flow through safety ground bed ......................... 163 Lightning stroke to equipment causing current flow through safety ground bed ................... 164 Lightning stroke current through system ground bed causing elevation of safety ground bed .......... 164 One-line diagram of simplified mine power system ........................................ 164 Mied ac-dc mine power system; dc load energized from trolley system ......................... 165 System grounding with current-limiting resistors .......................................... 165 Diode grounding of machine frame ................................................... 165 Resistance of earth surrounding electrode ............................................... 166 Decrease in earth resistance as electrode penetrates deeper soil horizons ........................ 167 Calculated values of resistance and conductance for 314-in rod driven to depth of 25 it ............. 167 Calculated values of resistance and conductance for 314411 rod driven to depth of 100 ft ............ 167 Nomogram to provide resistance of driven rod ........................................... 168 ........................................... Resistance of one ground rod. 314411 diameter 168 Resistance of parallel rods when arranged in straight line or circle with spacing equal to rod length ....................................................................... 168 Variation of earth resistance as numbcr of ground rods is increased for various spacings between rods ........................................................................ 168 Values of coefficient kl as function of length-to-width ratio of area ............................ 169 Values of coefficient k2as function of length-to-widthratio of area ............................ 169 Influence of first-layer height of potentials .............................................. 171 Potential on ground surface due to rod 6 ft long and 1-in diameter buried vertically at various depths ....................................................................... 172 Potential on ground surface due to strips. 1in by 0.1 in. of various lengths buried horizontally at depthof2ft .................................................................. 172 Measuring resistance of grounding system .............................................. 173 Concentric earth shells around ground connection being tested and around current electrode ......... 173 Correct spacing of auxiliary electrodes to give true resistance within 2.0% ....................... 173 Resistivity range of some rocks. minerals. and metals ...................................... 174 Variation in soil resistivity with moisture content ......................................... 175 Typical resistivity curves of solutions ................................................... 175 Diagram for four-electrode resistivity survey showing lines of current flow in two-layer earth ......... 176 Connections for Wenner four-terminal resistivity test using megohmmeter ....................... 176 Typical curve of resistivity versus elcctrodc separation ...................................... 176 ~ . ...................................... Reduction in ground mat resistance by soil treatment 177 Seasonal resistance variations attenuated by soil treatment .................................. 177 Trench model of soil treatment ...................................................... 177 Voltage gradients in earth during ground-fault conditions ................................... 178 . . Delta secondary with rig-zag grounding ................................................ 180 ..................................... Delta secondary with wye-delta grounding transformer 180 Cable distribution in underground coal mines ............................................ 182 ................................................ Cable distribution in surface coal mines 183 Shieldtypes ..................................................................... 186 ......................................... Cross sections of round unshicldcd mining cablcs 188
  • 17. Page Cross sections of flat unshielded mining cables ........................................... Cross sections of some shielded mining cables ........................................... Round unshielded mining cables ..................................................... Flat unshielded mining cables ........................................................ Round shielded mining cables ....................................................... Cable types for typical distribution systems in underground coal mines ......................... Cable types for typical distribution systems in surface coal mines .............................. Cable terminations for applications up to 15 kV .......................................... .............................................................. Couplercomponents Simplified one-line diagram for situation described in example 8.4 ............................. Allowable short-circuit currents for insulated copper conductors .............................. Representative end-suspension termination for borehole cable ................................ Messenger wire supports for mine power-feeder cable ..................................... Splice layout using template for staggered connections ..................................... Effective method for removing unwanted insulation ....................................... Staggering splice connections ........................................................ Examples of popular connectors and connections used in splices .............................. Reinsulating power conductors with soft rubber tape ....................................... Typical taped splice in high-voltage shielded cable ........................................ Trolley-wire cross sections .......................................................... Typical trolley-wire and feeder-cable supports ............................................ Trolley-wire semicatenary suspension .................................................. Trolley system accessories .......................................................... Theoretical resistance of bonded joint ................................................. Pole strength calculations ........................................................... Guy and log-anchor calculations ...................................................... ............................. Typical arrangements and pin-insulator spacings on wooded poles Typical system fault current ......................................................... Steps in circuit interruption ......................................................... Arc between two contacts .......................................................... Load-breakswitch ................................................................ Extinguishing arc by increasing the length ............................................... Metal-barrier arc chute assists in arc deionization ......................................... ....................................... Insulated-barrier arc chute used with mametic field - Molded-case circuit breaker components ............................................... Magnetic-trip relay ............................................................... Adjustable instantaneous setting ...................................................... Thermal-magnetic action of molded-case circuit brcakcr .................................... ............................. Time-current characteristics for thermal-magnetic circuit breakers Shunt-trip and undervoltage-release accessories .......................................... Construction and operation of dead-tank OCB ........................................... Turboaction are chamber for OCB's ................................................... Cross section of minimum-oil breaker ................................................. ............................................................. CrosssectionofVCB Operating mechanism for vacuum interrupter ............................................ VCB assembly incorporating a load-break switch ......................................... ........................................................... Common cartridge fuses Inside view of dual-element fuse ...................................................... Current-limiting action of fuses ......................................................
  • 18. Energy-limitingactionoffuses ....................................................... High-voltage power fuse and support .................................................. Fusible element under spring tension in high-voltage fuse ................................... ............................................. Cross section of boric acid power fuse refill .............................................. Disassembled refill unit for boric acid fuse Load-break switch with interlocked high-voltage fuses ...................................... ............................................................. Relay contact symbols Temperature-monitoring protector .................................................... Electromechanical-thermal relays ..................................................... ......................................................... Solenoid and clapper relays Polarrelay ..................................................................... ........................................................ Common induction-disk relay ...................................... Front view of induction-disk relay removed from case ...................................... Inverse-time curve compared with definite-time curve Various time characteristics of induction units ........................................... Family of inverse-time characteristics .................................................. ........................................................... Cylinder directional relay Directional overcurrent relay using induction-diskrelay and cylinder relay ....................... ......................................................... Direct relaying in ac system ....................................................... Potential-relaying connections Differential-relayingconnections ..................................................... ....................................................... Dc dircct-relaying connections ...................................................... Typical control wiring for UVR Typical control wiriig for shunt-tripping element ......................................... ..................................... Three-phase overcurrent and short-circuit connections TwoCTapproaches .............................................................. Neutral-resistor current-relaying scheme ................................................ ............................................... Neutral-resistor potential-relaying scheme ................................................ Zero-sequence ground relay connections Ground relay in residual connection ................................................... ............................................................ Broken-delta protection .................................................... Series loop ground-check monitor Transmitter loop ground-check monitor ................................................ .................................................... Bridge-type ground-check monitor ....................................................... Pilotless ground-check monitor ........................... Some difficulties associated with ground-check monitoring in mining ...................................... Pilot interlocking circuit using ground-check monitor Simple surface mine power system illustrating protective relaying ............................. Typical schematic for three-phase molded-case circuit breaker with ground-overcurrent and .......................................................... ground-check protection One-line diagram of simple underground mine power system illustrating protective circuitry .......... ....................................... Diode-grounded system with possible fault indicated Basic grounding-conductor system .................................................... Relayed groundig-conductor system .................................................. Neutral-shiftsystem ............................................................... Current-balance dc ground-fault relaying using saturable reactor .............................. Current-balance dc ground-fault relaying using saturable transformer .......................... ........................................... Fault current waveform illustrating asymmetry ... Multiplying factors applied to three-phase faults to obtain momentary ratings for switching apparatus
  • 19. Page Multiplying factors applied to three-phase faults to obtain close-and-latch ratings for switching apparatus .................................................................... ................................................. One-line diagram for fault calculations ..................................... Impedance diagram for one-line diagram of figure 10.4 ......................................................... Simplificationoffigure10.5 ......................................................... Simplificationoffigure10.6 ................................................. Further reduction of example network ...................................................... Equivalentcircuitoffigure10.6 ....................................... Example problem with motor contribution neglected Network to calculate momentary or close-and-latchcurrent duties ............................. Fault current in dc system .......................................................... .............. Available fault current versus distance of fault from rectifier on typical trolley systems ............................................ One-line diagram for pickup setting example ........................................................ Model of CT and its burden .......................... Typical set of saturation curves for 600/5 multiratio bushing-type CT ................. Example of one-line diagram for preparing a coordination curve plot for one path ............ Coordination curve plot for figure 10.17 showing various protective-device characteristics ..................................... Schematic representation of lightning stroke discharge Distribution of crest currents in lightning strokes ......................................... .................... Map showing average number of thunderstorm days per year in United States Striking distances for negative and positive strokes ........................................ ............................... Crest voltages induced on transmission lines by nearby strokes ............................. Simple circuit to illustrate capacitance-switching voltage transients .......................... Voltage and current waveforms before and after current interruption Voltage and current transient waveforms occurring with capacitance switching and restrike .......... Per-phase diagram of 4.16O . V pump-motor circuit ........................................ ........ Voltages and current wavesforms resulting from multiple restrikes after capacitance switching Graphic example of current chopping by breaker interruption ................................ Equivalent circuit of power-system segment with lumped components per phase. neglecting resistance .. Graphic example of chopping voltage transients .......................................... Segmentofminepowersystem ...................................................... Circuit to demonstrate voltage transients in dc system ...................................... Transient overvoltage resulting from current interruption on dc system ......................... An undergrounded system. showing capacitive-current flow .................................. An undergrounded system. with fault on phase A ......................................... The distributed inductance and capacitance of two-wire l i e shown as incremental sections .......... Demonstration of traveling wave on overhead line ........................................ ................................. Incident waves being reflected and refracted at discontinuity Electric field between conductors ..................................................... A 1.2 x 50 wave test used for BIL measurement .......................................... Equivalent circuit of multiturn winding showing distribution inductance and capacitance ............. .......................... Initial voltage distribution across uniform winding from step function ....................... Capacitive coupling of transient voltage through two-windiig transformer Basic valve surge arrester ........................................................... ................... Surge arrester with nonlinear resistance grading to equalize each gap structure Surge approaching surge-arrester-protectedequipment ..................................... .................... Typical surge protection of rotating machinery and dry-insulated transformers Simplified sketch of mine power-system segment .........................................
  • 20. ............................................... 11.32. Capacitance for 2,300.V induction motors ............................................ 11.33. Capacitance for 2,300.V synchronous motors 11.34. Overhead ground-wire shielding for low and high distribution towers ........................... 11.35. Static-wire-protection designs of wooded support structures using 30 protective angle ............... 11.36. Ratio of impulse to 60-Hz resistance as a function of peak impulse current. for driven rods .......... 11.37. Impulse breakdown of sand for two moisture conditions using spherical electrodes ................. 11.38. Impulse characteristics of spherical electrode, with seven attached pointed protrusions of various lengths ..................................................................... Typical power centers used in underground wal mines .................................... Schematic illustrating major components in power center .................................. Top view of mine power center showing placement of many internal components ................. Interconnections between input and feedthrough receptacles ................................ Graph illustrating transient crest voltage caused by ribbon-element current-limiting fuse operation .... Comparison of transformer withstand characteristic and surge arrester withstand characteristic ....... Typical primary winding taps on power cable transformer .................................. Zig-zag grounding transformer ........................................... ................................. Delta-wye connection for deriving a neutral Technique for measuring transformer impedance ............................. ............................... Typical X/R ratio versus transformer capacity Typical mine power-center transformer undcr construction . . . . . . . . . . . . . . . . . . . . . . ................................. Completed transformer prior to installation Typical bus work in powcr ccntcr undcr construction .......................... Typical conductor connection to molded-case circuit breaker ..................... Zero-sequence relaying on outgoing circuit with control connections to breaker ....... Zero-sequence relaying with jumpcr in relay case ............................. Neutral relaying applied to grounding-resistor current as backup protection .......... Backup protection devices associated with mine power cables .................... ................................ Typical test circuit for zero-sequence relaying Simple control circuit incorporating one ground-fault relay and one ground-check relay . Simple convenience-outlet circuit for 120- or 240-V single phase .................. .................................................................. Fusemountings .......................................... Typical metering circuit for line-to-line voltages ............................................... Typical metering circuit for line currents .................................................... Typical impedance monitor circuit Block diagram of continuity monitor connected in pilotless mode ............................. ................................ Block diagram of continuity monitor wired for pilot operation ................................. Application of power-factor correction in mine power center General arrangement of dc components for combination power center ......................... Full-wavebridgerectifier ........................................................... .................................... Series reactance to reduce available short-circuit current Separate transformer to increase impedance of dc circuit ................................... Typical full-wave bridge rectifier with two diodes in parallel per leg ............................ .................................................... Diode with RC snubber protection Diode-groundedsystem ............................................................ .................................................... Basic grounding-conductor system Relayed grounding-conductor system .................................................. ............................................................... Neutral-shift system ......................................................... Differential current scheme ............................................... Representative control circuit for rectifier ........................................................ Cross section of dc contactor
  • 21. Page ................................................. Diagram for typical single switchhouse ................................ Control circuitry for single switchhouse using battery tripping Diagram for typical double switchhouse ................................................ .............................. Control circuitry for double switchhouse using capacitor tripping ............................................ Typical family of curves for inverse-time relay Illustration of fault location for adjusting selectivity ........................................ .......................... Typical control circuit for double switchhouse using capacitor tripping ............................ Typical control circuit for single switchhouse using battery tripping ............................................ Overall view of main substation serving mine Radial distribution applied to underground mine and its surface facilities ........................ .................... One-line diagram for single-ended substation with fuse-protected transformer ............ One-line diagram for single-ended substation with circuit-breaker-protected transformer Simplified one-line diagram for doubled-ended substation ................................... ........................................ Typical liquid-immersed transformer in substation ....................................................... Dead-tank OCB in substation Standard percentage-differential relaying system for transformer protection ...................... ........................... One-line diagram of substation with percentage-differential relaying Insulation characteristic of liquid-immersed transformer compared with the characteristic of valve ................................................................. surgearrester Plan view showing locations of system and safety ground beds ................................ Typical system ground bed for large substation ........................................... ........................................... Typical system ground bed for small substation ................. Substation feeding both surface and underground loads (no pounding conductor) ..................................... Substation feeding both surface and underground loads ......................................... Typical portable substation to service small mine .......................... Providing mine ground and protective relaying from utility substation ........................................ Use of isolation transformer with utility substation ................................................. Model and circuit symbol for thyristor ............................................... Typical characteristics curve for thyristor ......................................................... Thyristor half-wave rectifier ................................................... Alternating current thyristor control Three-phase control with bidirectional thyristor arrangement ................................. .................................................... Full-wave thyristor bridge rectifier ............................................... Three-phase thyristor-controlled rectifier Simplifiedchoppercontrol .......................................................... ................................................... Basic control-system block diagram Simplified block diagram of a motor controller ........................................... ..................................................... Common thyristor configurations .................................................... Heat sinking of disk-type thyristors Block diagram of ac-dc shuttle car .................................................... ............................................... Block diagram of ac-dc continuous miner ..................................................... Simple variable-frequency control .......................................................... Elementary inverter circuit ................................. Use of variable-frequencydrive on production mining shovel Simplified diagram of current-regulated static belt starter ................................... Simplified diagram of linear-acceleration static belt starter .................................. ....................................................... Types of thyristor firing pulses Thyristor protection for static belt starters .............................................. ......................................................... Protective-relay connections ....................................................... Simple electromechanical relay
  • 22. ................................................................ Simplestaticrelay Transistor used as relay ............................................................ .......................................................... Optical transistor as relay ............................................................. Thyristor used as relay Triacusedasrelay ............................................................... ............................................................... Hybrid static relays ....................................................... Simple overcurrent static relay ................................................... Simplified sketch of the SEL system Simplified sketch of the multipoint SEL system ........................................... ................................................ Diode-bridge phase-sensitive protection Equivalent model of figure 14.33 ..................................................... Electronic-comparator method of phase-sensitive protection ................................. Digital-controlled continuous static relay used for timed overcurrent ........................... Composition of lead-acid storage battery in various states of charge ........................... Voltage per cell of a typical lead-acid battery with varying continuous rates of discharge ............ Typical charging process of cell from 18.cell. 725-Ah battery ................................. Simplified schematic of saturable-reactor charger ......................................... Simplified schematic of single-phase thyristor charger ...................................... Two-winding transformer model ...................................................... Representation transformer magnetization curve .......................................... ..................................................... Ferroresonant transformer model .......................................................... Ferroresonant transformer ....................................................... Ferroresonant battery charger Plan of underground charging station .................................................. Circuit for detecting faults in batteries ................................................. Curve of relay current for various fault positions on battery .................................. ............................................ One-line diagram of desired charger features Cross-sectionalsketch of typical explosion-proof enclosure .................................. Typical plane-flange joint ........................................................... Typicalstep-flangejoint ............................................................ Threadedjoint ................................................................... Tongue-and-groovejoint ........................................................... Blindscrewhole ................................................................. .............................. Pressure vent limiting pressure buildup during internal explosion Pressure vent assembly using metal-foam material ........................................ Typical slip-fit straight stufting box and packaging-gland lead entrance .......................... ................ Typical slip-fit angle stuffing box and packing-gland lead entrance with hose clamp Typical slip-fit angle stuffing box and packing-gland lead entrance ............................. ............................................... Typical plug for spare lead-entrance hole Typical threaded straight stuffing box and packing-gland lead entrance with provision for hose ...................................................................... conduit Prototype trailing cable entry with polyurethane grommet ................................... ......................................................... Insulated-stud lead entrance Decision flow chart of class 1 1 . division 1and 2 hazardous locations ............................ 17.1. Circuit modeling a dielectric ........................................................ ............................................ 17.2. Current-voltage characteristics in a dielectric ..... 17.3. Graph relating approximate insulation resistance variation with temperature for rotating machines
  • 23. Page Insulation resistance versus application time of test voltage .................................. Megohmmeter test connectionsfor checking cable insulation in line A .......................... ............................................ Megohrnmeter test connectionsfor ac motor Megohrnmeter test connectionsfor dc motor ............................................ ................................................ Spot resistance curve for normal motor Spot resistance curve showing effects of dust and moisture .................................. ............................................... Spot resistance c w e for detective motor .......................................... Megohrnmeter test connections for transformer Tie-resistance curve ............................................................. Three time-resistance curves for deteriorating motor ....................................... Time-resistance curves showing polarization for hypothetical motor ............................ Polarization factor curve for deteriorating motor .......................................... ........................................... Multiple voltage curves for deteriorating motor Circuit for harmonic tests .......................................................... Power-factor versus voltage curves showing tie-up ......................................... Mounting techniques for two vibration transducers ........................................ Four typical vibration measurement points .............................................. Typical vibration severity chart ....................................................... Comparison of acoustic-emission techniques for detecting failing roller bearings ................... Conceptual diagram of generalized mine monitoring and control system ........................ Conductioningas ................................................................ ................................................ Discharge sequence in an ionizing field ............................................................. High-stress geometrics Typical dielectric voids in cables ...................................................... Block diagram for corona-detection system .............................................. High-voltage cable terminations ...................................................... Major insulation void sometimes found in high-voltage coupler terminations ..................... ....................................... Possible stress site in high-voltage coupler insulators Power-conductor transposition on three-conductor type G cable .............................. Application of diode-suppressionbridges in power center ................................... Typical saturable-reactor characteristic ................................................. TABLES SIsymbolsandunits .............................................................. ........................................... Resistivity of some common materials at 20 C IEEE device numbers and functions ................................................... ........................................... Device numbers and letters common to mining ............................................... Motor voltage ratings common to mining Motor insulation classes ............................................................ NEMA class A standard starters for three-phase induction motors ............................ ................................................ Common motors for mining equipment .................................. Current range and effect on a typical man weighing 150 Ib Typical resistances for various contact situations .......................................... Approximate resistance formulas for various electrode configurations .......................... Comparison of grounding grids with other types of electrodes ................................ General resistivity classification ...................................................... Variations in resistivity with geologic age ............................................... ............................................... Typical values of resistivity of some soils ......................................... Variation in soil resistivity with moisture content .... Typical potentials of metals in soil measured from a copper and copper sulfate reference electrode
  • 24. Page ............................................... Conductor sizes and cross-sectional areas .................................................. Letters used in alphabetic cable code ................................................ Codes for typical cablcs used in mining Typical diameters for round portable power cables ........................................ ............................................... Typical diameters for flat portable cables ........................................ Specifications for trailing cablcs longer than 500 ft .................................................. Ampacities for portable power cables ........................................ Ampacities for three-conductor mine power cables ............................. Correction factors for ampacitics at various ambient temperatures ...................... Ampacity derating factors for 60 C-rated trailing cables operated on drums .......... Australian specifications for ampacity derating factors for trailing cables operated on drums Some estimated power factors and load factors for various underground coal mining equipment in goodopcratingconditions ......................................................... .............................................. intermittent-duty ratings for trailing cables ......................................... Resistance and reactance of portable power cable Resistance and reactance of mine-power-feeder cable ...................................... Solid-wire breaking strength ......................................................... ....................... Recommended minimum bending radius. unshielded or unarmored cables ......................... Recommcndcd minimum bending radius. shielded and armored cables Trolley-wire specifications .......................................................... .......................................... Characteristic data for solid copper feeder cable Characteristic data for stranded copper feeder cable ....................................... ................................................ Trolley-wire support spacings on curves Resistance of steel rail at 20 C ....................................................... Data for rail-bond cable ............................................................ Minimum vertical conductor clearances as specified by the NESC. applicable to mining and mining-related operations ......................................................... ....................... Minimum distances from overhead lines for equipment booms and masts .................................... Ratings for mining-service molded-case circuit breakers Interrupting-current ratings vcrsus system voltage ......................................... Maximum instantaneous-trip settings .................................................. ................ Commonly available magnetic-trip ranges for mining-service molded-case breakers .................................. Some typical ratings for low-voltage power circuit breakers Typical minimum-oil circuit breaker ratings ............................................. ................................................... Ratings of high-voltage power fuses ................................. Common current ratings of induction-disk overcurrent relays ............................................... Standard burden for current transformers Standard ratings for potential transformers .............................................. ................................... Sample reactances for synchronous arid induction motors .................. Three-phase transformer per-unit impcdanccs for liquid-immersed transformers ........... Three-phase transformers impedances for distribution transformers. including load centers Sample applications of fault calculations ................................................ .................................................... Impedance of cables in figure 10.4 .................................. Burdens of relay elements and ammeter connected to CT's Recommended instantaneous trip settings for 480.. 600.. 1 . 0 4 0.V three-phase trailing-cable protection .. ............ Recommcndcd instantaneous trip settings for 300- and GOO-Vdc trailing-cable protection Recommended station and intermediate surge arresters for resistance-grounded mine power systems to protect oil-immersed transformers ...................................................
  • 25. Page Recommended distribution.class. RM.type. surge arresters for resistance-grounded mine power systems ................................... to protect rotating machinery and dry-type transformers Commonly used surge capacitors for limiting voltage rate of rise on rotating machinery and dry-insulated transformers ......................................................... Typical capacitances per phase of power-system components. for shielded power cable SHD. SHD.GC. andSHD+GC .................................................................. ................................. Typical capacitances per phase of power-system components Protective angle versus structure height ................................................ ........................................ Typical current ratings of 400-A load-break switch Typical ratings for combination power centers ........................................... ............................ Standard impedance for liquid-immersed three-phase transformers ...................................... Standard BIL's for oil-immersed power transformers Typical electromechanical and static relay characteristics .................................... .......................... Time-margin comparison between electromechanical and static relays ...... Comparison of induction-disk and static time-overcurrent relay burdens to a current transformer ............................................... Formulas to estimate hydrogen evolution ................ Structural gap dimensions for explosion-proof enclosures as specified by 30 CFR 18 ................... Minimum autoignition tcmpcraturcs versus layer thickness for bituminous coals Common causes of vibration ........................................................
  • 26. By Lloyd A. Morleyl ABSTRACT This Bureau of Mines publication presents a comprehensive review of mine elec- trical power-systemtheory and practice. It discusses fundamental theory and the vital aspects to be considered in planning and designing mine electrical power systems. The report is divided intothree major sections. The first presents the history of electricity in mining and the fundamentals of electrical phenomena and components.The second focuses on power-system components: motors, grounding systems, cables, and protec- tive equipment and devices. The final section includes mine power-center equipment, switchhouses and substations, batteries, and mine maintenance. p~ ~-~ - ~ p 'Professor of mining engineering, The Pennsylvania StateUniversity,University Park, PA (nowprofessor and department head, mineral enginwring. University of Alabama, Tuscalwsa, AL).
  • 27. CHAPTER 1.-ELECTRICAL POWER IN MINING Probably no other mining area has grown so rapidly yet been as little understood by the average mine worker or operator as the mine electricalpower system. Traditionally, the field has held little interest for the mining engineer, who has tended to avoid it, or for the electrical engineer, who has given it scant attention. But today's mine power system is both complex and subject to numerous legal con- straints, and it is no longer possible to treat it with the indifference of the past. Underground mining machines are among the most compact and rugged equipment over designed, and individ- ual units can have up to 1,000 total horsepower. Mining equipment is usually mobile and self-propelled; most is powered electrically through portable cables and, for safety, must be part of an elaborate grounding system. The ma- chines and power-distribution equipment are seldom sta- tionary, must be adapted to continuous cyclic operation, and must resist daunting levels of dust and vibration. Surface mining can involve the largest earth-moving equipment built, where one piece can have 12,000or more connected horsepower-the largest today is over 30,000 hp. The electrical loads created by this machinery are cyclic and extremely dynamic: the largest excavator, for exam- ple, can require electrical loads that range from 200% motoring to 100% generating every 50 to 60 s, under the most exacting physical conditions. In the ever-movingmin- ing operation where distribution of power must be con- stantly extended and relocated, subjected to abuse by machine and worker alike, the potential for safety hazards is always present. Engineering and maintaining such an electrical system is demanding and challenging. It requires a specialist with knowledge of both mining and electrical engineering. Yet conversely, the effective management of a mine requires that anyone responsible for production and safety also be conversant with the mine electrical system. Management should understand the advantages and disadvantagesof one system over another, for if the power system is poorly de- signed, not only will safety be compromised but the mine operator will pay for the resulting conditions with high power bills, high-cost maintenance, and loss of production. Too often, a new mine is designed to use the type of power system employed in the preceding mine, without a comprehensive power study to determine the system needs and examine the alternatives available. Problems arise in existing mines when new mining equipment has been adopted without due regard for its impact on the operating power system; these problems haunt the mine electrical engineer who must frequently cope with a system that is a mongrel, bred from diverse inheritances from the past combined with recent changes and additions. New laws, standards, and safety requirements must frequently be accommodated by power systems not originally designed to meet their specifications;new and unfamiliar equipment must be grafted to the existing network, and the result can be a hybrid of considerable complexity. This text has been produced to assist the power engineer and the student in understanding these complexities and the principles that lie behind them. The material presented here is structured so an indi- vidual unfamiliar with electrical engineering can first developthe necessary fundamentals before embarking into mine electrical design. A basic physics and calculus knowl- edge is necessary to understand the content completely. The goal has been to assemble the most significant information required for comprehension of mine power systems so that the reader may then progress to more specializedtopics. But first, a brief review of the development of electrical usage in mines is given, in order that the reasons for some of the peculiarities of mine power systems can be appreciated. MINE ELECTRICAL HISTORY Electricity was first introduced into coal mines shortly before the beginning of the 20th century in the form of di- rect current (dc)for rail haulage. This form of current was used because at that time most systems were powered by dc generators. It had a number of advantages for haulage; the most outstanding was that the dc series-woundmotor had (and has) excellent traction characteristics. Speed con- trol was a simple matter of placing a resistance in series with the motor armature or field circuits. Batteries served as the first power source, and hence the vehicle was extremely mobile even though constrained on rails. However, keeping the batteries charged was both- ersome, so trolley wires were soon introduced in several mines. Allowingthe trolley wire to act asoneconductorand the rail as the other provided the simplest form of power distribution yet known to the mining industry. Available haulage machinery of that period was low in horsepower and the mines were relatively small sothe increased resis- tance that reduced voltage and power supplied to the motors was still acceptable. Thus, the dc system at a voltage of 250 or 550 V became firmly entrenched in coal mines. Underground Mine History Underground, the first electrically driven coal mining machine, the coal cutter, was installed in the early 1920's. Although dc offered no special advantage, it was readily available; hence, the machine was equipped with a dc motor and added to the system. The cutter was followed almost immediately by the loader, and it too was driven by dc motors.I f there was rail haulage in the mine, trailing cablea supplied power from the trolley wire and the rail to the machines. The next significant increase in power consumption came with the introduction of the shuttle car, almost 20yr after the coal cutter. Actually, when the shuttle car was first invented in 1937, it was battery powered. The addi- tion of an automatic reeling deviceto handle a trailingcable came later, in an attempt to overcomebattery deficiencies. These trailing cables were also connected to the haulage power system, and this equipment, when combined with the cutters and loaders, placed additional stress on the dc distribution system. At that time, the horsepower required to operate each piece of electrical mining equipment was quite small and no individual machine used a large amount of current. However, when all machines were combined, significant power was required, and because all the conductors offered resistance. voltaee d r o ~ s and transmission losses in the distribution system we; extensive. Alternatingcurrent (ac) would have been more practical because it could have been