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Lebw4993 generator systems


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Lebw4993 generator systems

  2. 2. ContentsGenerator Systems................................................................. 1 Generation System Basics................................................... 2 Main Generator Components ........................................... 2 Magnetic Field and Voltage ............................................. 4 Phase and Voltage ......................................................... 5 Generator Design ............................................................... 6 Rotor ............................................................................ 6 Amortisseur Windings................................................. 6 Salient Pole Rotor....................................................... 6 Non-Salient Pole Rotor ................................................ 6 Permanent Magnet Rotor ............................................ 7 Stator ........................................................................... 7 Windings ................................................................... 9 Insulation Systems ....................................................... 12 Rotor Insulation........................................................ 12 Stator Insulation Combinations .................................. 13 Insulation Life .......................................................... 14 Coil Connections .......................................................... 17 Generator Features and Attachments ............................. 19 Bearings .................................................................. 20 Space Heaters ......................................................... 20 Ingress Protection (IP)............................................... 20 Physical Data............................................................... 21
  3. 3. Nameplate ............................................................... 22 NEMA & IEC Design Considerations ............................... 23 Temperature Rise ..................................................... 23 Maximum Momentary Overloads................................ 25 Maximum Deviation Factor........................................ 25 Telephone Influence Factor (TIF) ................................ 25 Single Frequency Weighting Short Circuit Requirements........................................................... 26 Overspeed ............................................................... 26Generator Performance Characteristics ............................... 27 Rated and Per Unit ....................................................... 27 Efficiency ................................................................ 27 Fault Current/Short Circuits ........................................... 28 Reactance ............................................................... 31 Transient Reactance (X’d).......................................... 32 Sub-Transient Reactance (X”d)................................... 34 Synchronous Reactance (Xd) ..................................... 36 Negative Sequence Reactance (X2) ............................ 37 Zero Sequence Reactance (Xo) .................................. 37Harmonics & Distortion..................................................... 39Miscellaneous Terms ........................................................ 43 Overspeed Capability ................................................ 43 Heat Dissipation ....................................................... 43 Derating .................................................................. 43Generator Limits .............................................................. 44
  4. 4. Exciter & Regulator Characteristics and Performance ........... 45 Excitation System ........................................................ 45 Self-Excitation ......................................................... 46 Permanent Magnet Excitation .................................... 46 Voltage Regulation ....................................................... 46 Performance ............................................................ 49 Specifications .......................................................... 50 Frequency Sensing ................................................... 51 Type ....................................................................... 52
  5. 5. Foreword This section of the Application and Installation Guide generally describesGenerator Systems for Caterpillar® engines listed on the cover of thissection. Additional engine systems, components and dynamics are addressedin other sections of this Application and Installation Guide. Engine-specific information and data is available from a variety of sources.Refer to the Introduction section of this guide for additional references. Systems and components described in this guide may not be available orapplicable for every engine. Information contained in this publication may be considered confidential. Discretion is recommended when distributing. Materials and specifications are subject to change without notice. CAT, CATERPILLAR, their respective logos, “Caterpillar Yellow” and the POWER EDGE trade dress, as well as corporate and product identity used herein, are trademarks of Caterpillar and may not be used without permission.©2008 Caterpillar®All rights reserved.
  6. 6. Application and Installation Guide Generator SystemsGenerator Systems Electricity is a useful source of energy because it is versatile; much moreversatile than mechanical energy. It can be used for a variety of tasks, suchas lighting, heating and rotating electrical machinery and it can be used in avariety of locations, such as offshore oilrigs, natural gas fields, remote areasand urban confines. Caterpillar generators convert the mechanical energy of an engine, orprime mover, to electricity. The well-proven, innovative designs of Caterpillargenerators have lead to several highly reliable lines of generators used inelectric power generation applications worldwide. All Caterpillar electricsets use AC generators. The AC generator, also called an alternator,converts mechanical energy to electrical energy and acts as a voltagesource for the load. This module describes basic concepts involved in the various Caterpillargenerator designs. It addresses basic electrical generation concepts,generator design and performance as well as voltage regulation. It is important to note that the information in this section applies, primarily,to synchronous generators. The term synchronous describes the relationshipbetween the engine rpm and the generator output frequency; they areexactly proportional. SECTION CONTENTS Generator System Basics ...... 2 Generator Performance • Main Generator Components Characteristics................... 28 • Magnetic Field & Voltage • Rated & Per Unit • Phase & Voltage • Fault Current / Short Circuits Generator Design ................ 6 Harmonics & Distortion ....... 40 • Rotor Miscellaneous Terms .......... 44 • Overspeed Capability • Stator • Heat Dissipation • Insulation Systems • Derating • Coil Connections Generator Limits ................ 45 • Generator Features & • Rated & Per Unit Attachments • Fault Current / Short Circuits • Physical Data Exciter & Regulator • NEMA & IEC Design Characteristics................... 46 Considerations • Excitation System • Voltage Regulation©2008 Caterpillar®All rights reserved. Page 1
  7. 7. Generator Systems Application and Installation GuideGeneration System Basics There are three basic requirements Essentially the process offor the generation of voltage. They generating voltage goes in theare magnetism, motion and following order. The exciter providesconductors. DC current to the rotor windings. DC When a coil moves relative to current through these wires createsa magnetic field, a voltage is magnetic flux. Magnetic fluxproduced; generation systems are generates an AC voltage in thebased on this concept. When a nearby stator windings when thereconductor cuts through a magnetic is relative motion between the two.field, a current is produced in that The regulator then senses thisconductor. These two concepts are output and controls the excitervery closely connected. Keep in current.mind that it makes no difference if In Caterpillar generators, the rotorthe magnetic field is stationary and (the source of the magnetic field)the conductor moves or whether rotates inside a stationary armaturethe conductor is stationary and the called a stator. One reason for usingmagnetic field moves. The important a stationary armature and a rotatingaspect is that there is relative magnetic field is the difficulty ofmotion. taking 3-phase current from a The simplest generator consists rotating armature. The rotor isof a loop of wire rotating between rotated by a prime mover. In thetwo permanent magnet poles. case of Caterpillar generator sets, the prime mover is usually anNote: In any generator set engine.installation, the frame of thegenerator must be positively The rotor contains magnetic polesconnected to an earth ground with windings wrapped around themor to the hull of the vessel. to form coils. These coils are called field coils or field windings becauseMain Generator Components they create a magnetic field when An AC synchronous generator excited with a DC current. Typically,is significantly more complex than the generator field windings containthe simple generator of a wire loop many turns.rotating between two permanent A magnetic field radiates out frommagnets. An AC synchronous the rotor as lines of magnetic flux.generator consists of four main As the rotor rotates, so does thecomponents and/or systems: magnetic field. When this moving • Field (rotor) magnetic field comes across a stator • Armature (stator) winding, an AC voltage is produced. The magnetic field is strongest at • Exciter the center of the north and south • Automatic Voltage Regulator poles where the lines of magnetic ©2008 Caterpillar®Page 2 All rights reserved.
  8. 8. Application and Installation Guide Generator Systemsflux are concentrated. Therefore, synchronous or no-load speed inthe closer a pole is to a stator revolutions per minute (rpm).winding, the higher the voltage 120 x fproduced in that stator winding. rpm = Number of polesIt is important to note voltage isa function of flux change per time,not only the proximity to the field. If 50 Hz is desired from a four-pole generator, the generator must be The symmetrical design of the driven at 1500 rpm. A six-polegenerator ensures that the rotor generator is driven at 1000 rpm.poles extend over equal arcs andthat the magnetic flux density The generated frequency of 50 Hzdistribution is similar across all is entirely a function of the drivenstator windings. speed. Magnetic poles refer to the 120 x f 1000 rpm =magnetic north and south and are 6the points where the magnetic field f = 50is strongest on the rotor. The relationship between the number of poles and the synchronous speed is shown in Table 1. These calculations are figured by taking the fundamental frequency of 50 or 60 Hz and dividing it by the number of pole pairs. It is then multiplied by 2π to get the synchronous speed into rad/s, that is then converted into rpm. Synchronous Speeds 60 Hz 50 Hz Poles rpm Poles rpm 2 3600 2 3000 Figure 1 4 1800 4 1500 6 1200 6 1000 A pole relates to the number of 8 900 8 750magnetic poles developed in the 12 600 12 500rotating field. Magnetic poles ina four-pole generator are arranged Table 1north-south-north-south around thecircumference of the rotor, as Regardless of the number ofshown in Figure 1. The number of pole pairs, the rotor moves 360poles (north-south-north-south) and mechanical degrees in onethe desired frequency (cycles per revolution. In electrical degrees,second or hertz) determine the however, each pole pair rotates 360 mechanical degrees. In other©2008 Caterpillar®All rights reserved. Page 3
  9. 9. Generator Systems Application and Installation Guidewords, electrical degrees are the as high as 250V is often used inmechanical degrees times the larger generators, while smaller andnumber of pole pairs: medium sized generators seldom use Mechanical Degrees x Number of Pole Pairs voltages higher than 125V. In a four-pole generator (two pole Because it is so difficult to extractpairs), each pole pair moves 360 high voltage from a rotatingmechanical degrees, so the total armature, the magnetic field of theelectrical degrees moved is: main generator is rotated rather than the armature. A rotating armature 360 x 2 = 720 generator will be used as an exciter The following illustrates electrical in Caterpillar generators.degrees in terms of number of poles: Generator output voltage depends 2-poles = 360° electrical on the following: 4-pole = 720° electrical • Speed of relative motion 6-pole = 1080° electrical between magnetic field and 8-pole = 1440° electrical stator conductors The main armature, or the stator, • Strength of magnetic fieldremains stationary. The stator • Number of series turns in theconsists of the stator core, and stator windingsits own windings called stator The strength of the magnetic fieldwindings, or armature windings. The is proportional to the current flowingstator may also include exciter field through the field coils. As thecoils when used with a self-excited current rises, the magnetic fieldfield arrangement. The stator grows stronger.windings are placed in slots along The speed of relative motionthe inside of the stator. The stator between the magnetic field and theusually contains a large number of stator windings depends on theslots. The rotor magnetic field cuts rotational speed of the rotor (engineacross the stator windings as it rpm). As the rpm rises, so does therotates inside the stator. As a result, speed (V) of relative motion. Thevoltage is produced in these higher the speed of relative motion,windings. the greater the generators output The stator voltage is the generator capabilities.output that is supplied to the load. Voltage can be adjusted byMagnetic Field and Voltage arranging the stator windings in The magnetic field is induced in coils and varying the number ofthe main generator by a DC current turns, or times the windings arefrom the exciter through low voltage wound around the stator. Morefield or rotor windings. These voltage can be induced by increasingwindings are low voltage compared the number of turns and less voltageto the stator windings. DC voltage is induced by using fewer turns. Consequently, stator windings can ©2008 Caterpillar®Page 4 All rights reserved.
  10. 10. Application and Installation Guide Generator Systemsbe arranged with the optimum Three-Phase Generatornumber of coil turns to producethe required output voltage.Phase and Voltage Phase, voltage and the statorcore are all inter-dependent. Thecalculated design of the stator coreand winding distribution enables agenerator to provide the appropriate Figure 3output voltage. In three-phase generation, three The phase voltage of a generator phases of voltage are producedis directly linked to the voltage 120° apart. However, to make theoutput of that generator. The connection a symmetrical 3-phasetype of voltage induced is partially connection, a phase coil must havedependent on the number of phases an equal and opposite winding 180°in a generator. away. Figure 4 illustrates this A single-phase generator will relationship; Phase coil B has angenerate a voltage sine wave when equal and opposite winding at phasethe rotor completes one cycle (one coil –B. The result is that the three360° revolution). Refer to Figure 2. phases and their opposing windings are actually 60° apart. Single Phase Generator Figure 2 A three-phase generator consistsof three coils equally spaced aroundthe stator and connected in a wye Figure 4(Y) or Delta (∆) configuration.Therefore, three voltages can beproduced consecutively with a 120°phase difference. Refer to Figure 3.©2008 Caterpillar®All rights reserved. Page 5
  11. 11. Generator Systems Application and Installation GuideGenerator Design Generators are constructed in Amortisseur Windingsvarious ways to satisfy different Amortisseur windings, also knownload and customer requirements. as damper windings, are special At Prime power ratings, SR4B conducting bars laid into a squirrelgenerators are limited to 105°C cage configuration. They are set in(221°F) temperature rise over an the notches in the rotor and thenassumed 40°C (104°F) ambient and are shorted on the end by either amay be used on a continuous duty shorting ring or shorting laminations.(24 hours per day) basis. They are used to increase machine stability. This is due to the windings SR5 generators are limited to a and rotors relationship to the125°C (257°F) temperature rise synchronous speed. If the rotor isover the same conditions. at synchronous speed, there is no At Standby power ratings, SR4B induced voltage through thegenerators are limited to 130°C amortisseur bars, hence there is no(266°F) temperature rise over an interaction. Conversely, if there isassumed 40°C (104°F) ambient a differential between the rotor andand are used many times for back-up synchronous speed, a voltage willpower needs, and are rated on a be induced in the windings. Thistemporary use basis. voltage produces a current flow and SR5 generators are limited to a in turn creates a magnetic field.150°C (302°F) temperature rise The interaction of the two magneticover the same conditions. fields in turn cause a torque that will The rating is expressed in kilowatts correct this speed difference(kW) at 0.8 power factor. A resulting in speed and torquecommon ratio, kW divided by stability.0.8 permits calculation of kilovolt- Salient Pole Rotoramperes (kVA) on all generators. A rotor configured with the individual rotor poles protruding fromRotor the center of the rotor is known as a The rotor is defined as any rotating salient pole rotor.winding or element of a generator.It can be described as an assembly The rotating salient pole fieldof thin magnetic steel laminations, arrangement is affected bytightly compressed and then riveted, reluctance torque and is used forbolted or welded together to form a engine-driven generators on mostmagnetic path. It is around this core machines in the 20 kVA (16 kW)that the field windings, or coils of and larger sizes.conducting material, are wound Non-Salient Pole Rotoraround or inserted. A non-salient pole rotor, or cylindrical rotor, is typically made from solid cylindrical material and ©2008 Caterpillar®Page 6 All rights reserved.
  12. 12. Application and Installation Guide Generator Systemsusually has grooves cut into the pole thick steel stampings/ laminationsfaces to place the windings. stacked together. Stacking machines A rotating non-salient pole field, use automatic welding to assureor cylindrical rotor, isn’t affected correct stator skew, stack pressure,by reluctance torque and it is mainly and slot alignment. Stator stampingsused for large, steam turbine-driven are usually stamped from specialgenerators. The non-salient silicone steel.configuration usually has grooves The stator core is subjected tocut into the pole faces to place the an alternating magnetic flux whichwindings. induces currents, called “eddy currents”, in the core that createPermanent Magnet Rotor losses. Creating the core from A permanent magnet (PM) field stacked stampings rather than fromeliminates the need for an exciter one solid piece of steel inhibits eddyand therefore is very cost effective currents, reducing losses. Thein smaller size generators. The silicone added to the steel stampingsdisadvantage to this is that its flux and an oxide coating help to inhibitdensity (field strength) is constant eddy currents.and voltage regulation is poor.However, if it is to be used in an In addition to eddy current lossesapplication with a steady load and in the stator core, there areminor fluctuations, a PM field can hysteresis losses. One definition ofbe very effective. hysteresis is the failure of a property that has been altered by an external A rotating armature, stationary agent to return to its original valuefield generator is used mainly in when the cause of the alteration issmall, low-voltage machines. A removed. Figure 5 illustratescommon use for this generator is as exciter for brushless generators.For additional discussion of PM The B-H curve in Figure 5 showsused in larger generator excitation that unmagnetized iron starts at zerosystems, refer to Exciter & Regulator and proceeds to saturation as theCharacteristics and Performance- magnetomotive force (MMF)Permanent Magnet. increases. In simpler terms, the unmagnetized iron becomesStator magnetized. As MMF is reduced to The stator is defined as any zero again (or as the iron loses itsstationary winding or element of a magnetism), a residual B’ results.generator. The stator core is where The residual B’ means that the ironthe usable electricity is generated. does not entirely lose its magnetismThe windings carrying the usable when MMF returns to zero. Theelectricity are placed in the stator same curve loops around as MMFcore slots. becomes negative then positive The stator core is made up of again from an AC exciting current.hundreds of 0.0185 in. to 0.025 in. Since the hysteresis loop results in©2008 Caterpillar®All rights reserved. Page 7
  13. 13. Generator Systems Application and Installation Guideinternal domain realignment, energy of attributes, one of which is theis expended. Losses are proportional physical size of the stator core. Theto the area of the loop created due inside diameter (called the air gapto the work expended in following diameter) and the inside core lengththrough the cycle of magnetization are among a number of attributesand demagnetization. related to the kVA output The inside stator diameter and core length are used to calculate the Dˆ2L value (which determines the kVA output of the generator). The air gap is the space between the pole face and stator lamination teeth. The air gap area is the ID of the stator minus the OD over the pole face divided by 2 multiply by the length of the stator. The flux density of the air gap (Bg) is then the total flux (ft) divided by theFigure 5 inside core area. The magnetic flux in the air gap is Figure 6 illustrates the core loss created by the field windings on thecurve for M-43, 24 gauge, hot rolled rotor. The rotor laminations are notsteel shows how core loss increases necessarily made of the samewith increasing induction at 50 Hz material as stator laminationsand 60 Hz. The core losses because the flux in rotor laminationsmeasured in this curve include both is unidirectional, always flowing outeddy currents and hysteresis. of the north and into the south magnetic poles. Core Loss Curve Different loads require different types of voltage. Generators are designed with various combinations of slot, conductor, and winding types to provide the specific kind of voltage required by the generators’ loads. Stator stampings contain one of two kinds of winding slots. The semi-closed slot accepts only a random winding, meaning one wireFigure 6 at a time is fed into the slot. The open slot accepts a random winding The output capability of a or a form-wound winding. The entiregenerator is measured by a number coil is placed into the slot, rather ©2008 Caterpillar®Page 8 All rights reserved.
  14. 14. Application and Installation Guide Generator Systemsthan being inserted wire by wire. called a phase sheet, whichDouble-layer winding consists of increases the creepage distance,two coil sides per slot, resulting in separates each coil group.the same number of coils as slots.Shown in Figure 7 are both a Lap Routed Windingrandom wound, semi-closed slotcross-section and a random wound,open slot cross-section. Stator Windings Figure 8 Lap winding is a double-layer winding. There are two coil sides per Figure 7 slot, resulting in the same number of coils as slots. Surge rope and tapeWindings are added to end turns to hold the Stator winding processes can be end turns stationary during surges,divided into two categories; the motor starting, and transients. Therouting of the coils and the type of surge rope, which is made with awinding. The routing of the coils strong fiberglass and epoxy, alsorefers to the pattern in which the reduces deformation of the endwindings are applied to the coil. turns when they are stressed. TheThe type of winding refers to the tape additionally serves to protectshape of the winding material. the coil end turns from abrasion between coils. Figure 8 shows aRouting Styles schematic of a lap wound coil. There are three coil routing styles.They are lap, wave, and concentric. Concentric windings are woundThe most common styles are lap into loops within larger loops. Thisand concentric. All SR4B and SR5 results in the simplest mechanicalgenerators are lap wound. assembly and the least amount of copper. This configuration however, Lap routed windings are loops experiences slightly higher levels ofrouted across one another and harmonics. This winding style is thewound into a double diamond shape. most economical and allows anAn entire coil group can be wound entire coil group to be wound atat one time, so connections are once and machine inserted. Like lapmade only at the coil group, not winding, connections are made atat each coil. An insulation material the coil groups, not at each coil.©2008 Caterpillar®All rights reserved. Page 9
  15. 15. Generator Systems Application and Installation GuideConcentric winding is a single-layer rectangular wires which can bewinding. There is one coil side per placed very close to one another,slot, resulting in twice as many slots resulting in the maximum amount ofas coils. Phase sheets are placed copper, least amount of spaces, and,between the phases to provide extra therefore, superior efficiency andinsulation between coil groups. durability. While concentric windingsFigure 9 shows a schematic of a and lap windings may be eitherconcentric wound coil. random or form wound, form windings are usually impractical for Concentric Winding generators under 250 kW because of inadequate spacing. The two winding types are shown in Figure 10 and Figure 11. Random WindingFigure 9Coil Pitch When a coil is wound 180electrical degrees, the voltages on Figure 10the conductors on either side areequal in magnitude and opposite in Random wound coils are wounddirection and are said to be a full with round copper magnet wire thatpitch coil. Sometimes a coil is is insulated with a moisture resistantwound less than 180° and is said to heavy polyester film and amide-imidebe a fractional pitch coil. The reason overcoat. Containing multiplefor this is to reduce certain harmonic conductors per turn, the coilsfactors that will be discussed later consist of one or many turns eachin this guide. SR5 generators are and are wound in a double diamond2/3-pitch. shape. Random wound coils are costWinding Types effective and are suited for low Winding types can be random or power ratings, standby applications,formed. Random windings utilize clean environments, and lowcoils of round wire. Material costs operating hours.are lower, but this method leaves The random wound stator isspaces between the individual wires. treated using a dip and bakeFormed windings utilize square or process. The stator is dipped and ©2008 Caterpillar®Page 10 All rights reserved.
  16. 16. Application and Installation Guide Generator Systemstotally submerged into a polyester retarding moisture and contaminantresin. Polyester resin is used for its buildup. Finally, along with thesuperior heat dissipation, flexibility, normal dip and bake process, anvoltage breakdown strength, and extra epoxy dip and bake is done.moisture resistance. The resinpenetrates the spaces between the Formed Windingconductors and leaves a uniformresin build. The resin is then curedby a bake cycle. By holding theconductors together, the resinprevents vibration and prematurefailure. Once the polyester resin iscured, the stator is dipped andbaked using an epoxy resin. Theepoxy adds extra protection againstmoisture, chemicals, and otherenvironmental factors. If requested Figure 11and/or necessary, additional dips and Form wound coils are pre-formedbakes maximize resin buildup and with rectangular copper magnet wireenvironmental protection. As many and taped with multiple layers ofas 2 to 4 dips and bakes are fused double daglass over heavypossible. To further protect against film polyester with an amide-imideabrasion and moisture, asphalt overcoat. Daglass is a polyesterepoxy can also be applied to the and glass insulation material. Thelead end. All random wound stators coils are taped to add mechanicalare sprayed with red sealer to help strength. The number of tape layersretard voltage tracking and to seal varies depending on the voltageall parts from rust and corrosion. applied to the coil and the dielectricTracking is caused by contaminants strength (ground insulation) salt water, which can get Additional protection can be addedtrapped on the coil end turns. if required because of environmentalSurface currents then develop and factors. Mica tape is used for groundcarbonize or “track” the resin insulation because of its insulatingsurface. properties, such as: high dielectric Coastal insulation protection is strength, high breakdown voltage,sometimes added to random high insulation resistance, andwound windings for increased excellent protection againstenvironmental protection. The end moisture, chemicals, and otherturns are the most susceptible to environmental factors.moisture and contaminant buildup, Open stator slots allow the coilsso glass tape is added to the end to be placed in the slots withoutturns to aid in extra uniform resin separation of the wires within theretention capability. The tape also coils. Coils are connected either bymakes coil end turns smoother,©2008 Caterpillar®All rights reserved. Page 11
  17. 17. Generator Systems Application and Installation Guidegroups or by individual coils with Insulation Systemsboth crimp connections and The insulation system is a complexsoldering. Connections are taped combination of insulation materialsand sleeved to assure a good seal that are carefully selected by theagainst environmental factors. generator designer to prevent Because form wound coils are pre- undesirable electrical flow.formed, air space exists between Generators consist of three typeseach coil in the coil head, allowing of material; copper wire, iron, andbetter cooling. Each coil head is also insulation. To function properly,braced with double surge ropes and the individual copper conductors inother blocking between coils to stator windings must be completelyprevent coil movement during surges insulated from neighboringand other transients such as sudden conductors in the same coil andload changes and switching. from the surrounding iron (referred Form wound stators are Vacuum to as ground).Pressure Impregnated (VPI) for Insulation must prevent electricaladded protection from the breakdown between components,environment. VPI is a process where but it must also be used as sparinglyair is removed causing the resin and as possible. Less insulation meansother coatings to adhere more tightly more space available for copper andto the windings. This process was better heat dissipation. More copperdeveloped specifically for form and better heat dissipation improveswound windings. maximum output from the generator. After the connecting and brazing Due to the added stress put on thestages of the insulation process, material in higher voltage machines,the entire stator winding and core higher quality insulation or moreassembly is vacuum pressure insulation must be used.impregnated with a special polyester Rotor Insulationresin varnish. Each Caterpillar rotor is precision Manufacturing costs for layer wet wound. This means theconcentric/random windings are conductors are hand-placed preciselylowest, but the more expensive in rows, layering one row on top oflapped/formed windings provide the other. Each layer of conductorsincreased durability and electricity is brushed with high bond strengthquality. Caterpillar generators have epoxy before the next layer is addedbeen designed in accordance with to assure a good seal betweenthe expected application; Medium conductors.output prime and high output Since the field coils employ a lowstandby use lapped/random. High voltage, field coil insulation problemsoutput prime power generators use are mostly mechanical. As the rotorlapped/formed windings. rotates, centrifugal force is applied to the field coils causing the coils to ©2008 Caterpillar®Page 12 All rights reserved.
  18. 18. Application and Installation Guide Generator Systemsbow out. When centrifugal force is Windings are coated with variousbroken down into its components, layers of insulation. Insulationthere is a vertical force (FV) and a materials can be used individuallyside force (FS). The side force or in combinations and may becauses the coils to bow out. comprised of acrylics, asphalts, To prevent field coils from bowing epoxies, melamines, phenolics,out due to centrifugal force, wedges polyamides, polyimides, polyesters,are placed in-between the poles with polyethylenes and silicones. Possibletheir two sides holding the coils insulation systems include:tightly against the poles. Extra • Magnet wire with a polyesterwedges and bracing are sometimes and amide-imide overcoat.added for extra assurance that rotor • Nomex™-mylar™-nomex™materials do not move. ground insulation. (Mylar The purpose of the Amortisseur or is a DuPont polyester film.)damper windings (discussed earlier) Table 2 shows examples of systemis to prevent rotational oscillations voltages that may be the rotor. These windings provide However, the list is by no meansa motoring effect and produce a complete. In the US, systemtorque in the rotor working against voltages are based on 120V withsuch oscillations, thereby providing multiples of that voltage.superior voltage stability. This is Nominal System Identical Systemparticularly important during parallel Voltages* Voltageoperation where generators carrying 120 110, 115, 125uneven loads can result in voltage 120/240 110/220, 115/230and operational instability whichtranslates to added stress on the 208Y/120 199Y/115insulation. Damper windings will 240 220, 230minimize these oscillations. Damper 480 440, 460windings also dampen oscillations 600 550, 575occurring from short-circuit and 2400 2200, 2300, 2500engine pulses. 4160Y/2400 3810, 4000 Tests are performed on the rotorto assure that there is no material Table 2movement in the rotor. A prototypespin test runs at 125% rated speed Caterpillar system voltagesfor two hours at 170°C (338°F). are divided into three classes to differentiate between types ofStator Insulation Combinations generators. The classes are Low Stator insulation is categorized into Voltage, Medium Voltage and Highclasses based on their ability to Voltage.withstand heat for a specified period • Low Voltage — A class ofof time. Class H is temperature rated nominal system voltages ofat 180°C (356°F) and Class F at 1 KV or less155°C (311°F).©2008 Caterpillar®All rights reserved. Page 13
  19. 19. Generator Systems Application and Installation Guide • Medium Voltage — A class exceed the damage curve (like a of nominal system voltages fuse), currents above the damage greater than 1 KV, but less curve will cause insulation to than 100 KV become brittle, carbonized, and • High Voltage — A class of cracked. Another threat is the nominal system voltages equal differential expansion of materials. to or greater than 100 KV and Copper expands significantly and equal to or less than 230 KV iron expands, relatively, little with high current. These materials, Caterpillar may supply generators expanding at different rates, causeup to 15 KV in some applications. stress and stress cracking in theApplications above 15 KV are used insulation. Conductors may alsofor transmission voltages and are lose strength, fracture, and/or melt.used in vary large power plant However, insulation can safelygenerators or they are specifically exceed its rated temperature bydeveloped for step up transformers. 30° to 50°C (86° to 122°F) if the Generator armature windings overheating does not last too long.must withstand a test voltageof 1000 volts plus two times the Typical Damage Curverated voltage of the machine.The field windings must withstand1500 volts rms. A Megger test is used to measureinsulation resistance with respectto ground. A measurement of100 meg-ohms is common at thetime of manufacture. Meggerreadings of one meg-ohm per1000 volts of generator rating aregenerally considered acceptable. Figure 12 Larger generators are sometimesrequired to pass tests associated Insulation Lifewith insulation coordination for The more current the statorlightening and voltage surges. windings can accommodate, theThis requirement falls under Basic greater the generator output. “Q”Impulse Insulation Levels (BIL) and represents ampere conductors perthe generator must be tested under inch of armature circumference.surge conditions. There are, however, limitations to ampere conductors per inch of Figure 12 shows a typical damage armature circumference, temperaturecurve. Any generator operation with rise being the most important.currents above this curve should beavoided. Although a generator may Temperature rise is the limitationnot suddenly fail when currents of the insulation to withstand the ©2008 Caterpillar®Page 14 All rights reserved.
  20. 20. Application and Installation Guide Generator Systemsheat produced when current runs Maximum Class Operating Materialthrough the stator windings. The Temperaturethermal expansion of the wires A 105°C (221°F) Cotton, Silk, Paper, Suitably Impregnatedfrom this heat plays a part in this Glass, Asbestos,limitation. B 130°C (266°F) Mica. Suitably Bonded Since the late 1950’s, plastics Glass, Asbestos, Mica. Suitablyhave been used as the primary F 155°C (311°F) Bonded with materialsinsulation for generators. As that permit 155°C operation.late as 1960, British standards Silicone Elastomer,specifications referred to insulation H 180°C (356°F) Mica, Glass, Asbestos. Bondedclasses by material. Since the with silicone resins1960’s, however, insulation Table 3classes have been referred to bythe maximum temperature limit of The bar graphs in Figure 13 showthe materials in each class. Refer insulation class temperature limitsto Table 3. measured by resistance or thermometer and by embedded detector. Notice the inclusion of the hot spot as a separate factor in the resistance graphs. Figure 13 Ambient temperature and method 40°C (104°F), although marine andof measurement further subdivide some others prefer between 45°Cthe maximum temperature limits (113°F) and 50°C (122°F).shown in the table. Ambient When measuring stator windingstemperature is usually considered by resistance, a hot-spot allowance©2008 Caterpillar®All rights reserved. Page 15
  21. 21. Generator Systems Application and Installation Guideis added. The hot-spot is the and prevent excess bearing heatlocation in stator windings with the effects.highest temperature. A hot-spot is Thermal endurance is the abilitylocated approximately in the center of the insulation to withstand heat.of each stator slot and Figure 14 shows typical thermalapproximately in the center of each endurance curve bands for thefield winding. different insulation classes. The An embedded detector measures curve bands predict insulation life inthe temperature limit of the stator hours versus the temperature of thewindings. Because the embedded windings. There are many testdetector is located at the hot spot, points outside these bands; thethe measured temperature already points shown are merelyincludes the hot spot. Note that an representative. Note that this type ofembedded detector can measure the graph is used to compare insulationtemperature limit of the stator systems. These curve bands do notwindings and is usually a standard predict actual design life of afactory test. Because the rotor is machine. For example, one CANNOTrotated, however, an embedded assume that the design life of adetector cannot test the field machine is any given number ofwindings; they must be tested by hours because the insulation life isresistance. a given number of hours. The curve There are two basic types of bands only COMPARE insulationembedded detectors; Resistive systems.Temperature Devices (RTD) and Note: For every 10°C (18°F)thermocouples. increase in temperature, insulation An RTD is an electronic sensing life is halved.device that varies resistance with achange in temperature. This change Sample Thermal Endurancein resistance is then measured and Curve Bandconverted into a temperaturereading. The RTD delivers moreof an average reading than thethermocouple. The thermocouple is another typeof temperature sensing device. It istypically used in random woundmachines due to its sturdier design.The thermocouple delivers a readingat a precise point, usually the hot- Figure 14spot. Both devices can be placed inwindings and the machine’s There are several indications thatbearings. They can show increases thermal deterioration is occurring inin winding insulation temperature winding insulation. ©2008 Caterpillar®Page 16 All rights reserved.
  22. 22. Application and Installation Guide Generator Systems • Loss in weight and thickness • Moisture on creepage surfaces • Increase in stiffness or • Blocked ventilation passages brittleness • Abrasives in the cooling air • Increase in density Coil Connections • Non-uniform shrinkage with The design of the coil connections cracks penetrating from the in a generator and the way a load is surface connected to a generator determine • Reduced tensile strength the level and type of output voltage • Less resistance to moisture from that generator. penetration Figure 15 shows different types • Reduced dielectric strength of connections for one phase of a • Serious decrease in resistance 4-pole, three-phase winding. The phase consists of four pole-phase Insulation tends to breakdown at groups; one for each pole in thehigher voltages due to the added phase. Each coil could have anyelectromagnetic field force. Silicon number of turns, limited only byControlled Rectifier (SCR) loads that the space within the slot. Theof high harmonic content can also pole-phase groups can be connectedseriously affect insulation. in series, 4-parallel, or 2-series, 2- Heat is not the only cause of parallel.winding insulation deterioration.Damage to insulation can occurduring winding due to improperhandling, careless winding,insufficient resin coverage, orinsufficient cure time. Therefore,tests are conducted after everywinding process is completed. Additional causes of insulation Figure 15failure include the following. • Conducting contaminants; Different types of connections like dirt and chemicals induce different voltages. Therefore, the type of connections used in a • Mechanical damage from generator depends on the voltage shock, vibration, foreign required by the load. Series objects and stress connections produce high voltage • Surge voltages generated in and parallel connections produce the load or in the line low voltage. The series and series- • Operation at abnormal parallel connections are often voltage, current or power combined into the same generator factor so the generator can be connected to produce high or low voltage. This • Loosened coils and wedges©2008 Caterpillar®All rights reserved. Page 17
  23. 23. Generator Systems Application and Installation Guidedual voltage capability is usually Some applications demand aidentified; for example, a generator wye connection rather than a deltalabeled “120/240-277/480V” can connection and vice-versa. Forbe connected for either 120/240V example, the delta connection isor 277/480V. 120V and 277V sometimes used to obtain single-are line-to-neutral voltages (single- phase 120-240 volts, 3-wire, alongphase), while 240V and 480V are with three-phase, 3-wire on theline-to-line voltages (single-phase same generator. Figure 16 showsor three-phase). an example of a wye and a delta Caterpillar generators have 4, 6, connection.10, or 12 line leads or outputs.These can be connected into a lowvoltage wye configuration (coilsparallel) or into a high voltagewye configuration (coils in series).Generators with 12 leads can beconnected in delta configuration.Larger generators use more than onewire per line lead. This feature easesthe problem of forming very heavyconductors inside a limited spacefor terminal connections. All leads are identified. If morethan one wire is used per line lead,a line lead number identifies each Figure 16of these wires. Thus, on smaller In a wye connection, the terminalgenerators, there would be only one voltage is 1.73 times the terminal-wire marked T1. However, on larger to-neutral voltage (represented bygenerators, there will be two or the letter “V” in Figure 16). In themore wires marked T1. These are same generator, a delta connectionto be connected together to form would have the same terminal-to-one lead. neutral voltage as the wye The first voltage listed in a rating connection for its terminal 220/440 volts is the line-to-line However, the delta line current (I)voltage at the specified frequency would now be 1.73 times the wyewhen the generator leads are line current (I).connected for the low voltage. A wye connection can beThe second voltage listed is the structured in the two ways shown inline-to-line voltage at the specified Figure 17; a fixed neutral connectionfrequency when the re-connectable or a broken neutral connection.line leads are series connected for The broken neutral allows you tohigh voltage. reconfigure from a wye to delta and vice-versa. It also provides ©2008 Caterpillar®Page 18 All rights reserved.
  24. 24. Application and Installation Guide Generator Systemsdifferential protection by allowing however, will only providethe machine to monitor the currents approximately 57% of the originalinto and out of individual phases. three-phase rating. Open Delta Figure 17 The zigzag connection issometimes used to get analternative voltage from a generator.For example, the connection Figure 19shown in Figure 18 can provide The lead terminal numbering120-208V from a generator wound system follows the pattern shownfor 120-240V. in Figure 20. This system is used Zigzag Connection for both three-phase generators and motors. Figure 18 Figure 20 Several single-phase connectionsare possible for 2-wire or 3-wire Generator Features andloads. These connections are Attachmentsnormally used only on smaller Several attachments and featuresgenerators at or below 250 kW. can also be configured to assistAn open delta, as shown in Figure in meeting generator requirements.19 is reconfigured from an original These include bearings,delta from three-phase to single- environmental protection devicesphase use; this type of connection, and space heaters.©2008 Caterpillar®All rights reserved. Page 19
  25. 25. Generator Systems Application and Installation GuideBearings protection from water. Table 4 lists Bearings are devices that permit the levels of protection for solidsmooth movement between objects coming in contact with thesurfaces. In a generator, they are machine. Table 5 lists the levels ofused to hold the rotor in place and protection from water. These tablesallow that smooth rotation. only offer brief descriptions; each A single bearing system is used in level of each category has specificthe smaller generators with lighter definitions and qualifications.weight rotors. It is necessary to A standard Caterpillar SR5have a two bearing system when generator is rated for IP23. The firstthe weight of the rotor exceeds digit, 2, indicates the machine isthe limits of the engine rear main protected against solid objectsbearing. In a traditional two bearing greater than 12 mm. The secondsystem, the generator is not digit, 3, indicates protection fromattached directly to the engine’s spraying water.flywheel housing. This brings aboutalignment difficulty due to the Solid Object Ingress Protectionseparation of the two machines. (first digit of IP designation)A close coupled two bearing system First Descriptionalleviates these original alignment Digitproblems by attaching the generator 0 Non-Protected Machineto the flywheel housing. Machine protected against solid objects greater thanSpace Heaters 1 50 mm (approximately the size Space heaters are electrical of a hand)resistance heaters that are often Machine protected againstused to help prevent condensation solid objects greater than 2 12mm (approximately the sizeand moisture absorption during of a finger)nonuse periods. They are application Machine protected againstspecific and their operation time can solid objects greater than 3vary due to this application. 2.5 mm (approximately the size of a hand tool)Ingress Protection (IP) Machine protected against Ingress Protection (IP) refers to solid objects greater than 4the degree of protection an 1 mm (approximately the sizeenclosure provides for a machine. of wires) Machine protected againstThis is protection both for persons 5 dustin contact with moving parts andfor the protection of machines Table 4against the harmful effects due tothe collection of water and dust. An IP designation is typically twodigits, where the first digit is thelevel of protection from solid objectsand the second digit is the level of ©2008 Caterpillar®Page 20 All rights reserved.
  26. 26. Application and Installation Guide Generator Systems Inertia Water Ingress Protection Inertia data is essential to studies (second digit of IP designation) of transient response, such as the Second Description effect of a larger generator with Digit more rotor weight and greater mass 0 Non-Protected Machine moment of inertia. When the Machine protected against 1 information is given to an engineer, dripping water Machine protected against it must be correctly identified. 2 dripping water when machine These figures list the mass is tilted up to 15° moment of inertia in System Machine protected against 3 International (SI) units and English spraying water Machine protected against units. Other symbols used to 4 designate inertia of rotating splashing water Machine protected against machinery are WK2, WR2, and GD2. 5 water jets The standard inertia presentation Machine protected against below is globally recognized and can 6 heavy seas be readily converted to other inertia Machine protected against 7 effects of immersion designations. Table 5 Force x Distance x Time2 N•m sec2 = Newton x meter x Second2 Two types of IP protection are lb•in. sec2 = lb x in. x Second2totally enclosed water cooled(TEWAC) and totally enclosed air To convert to moment of inertiacooled (TEAC). (WK2) in SI units: Multiply N•m sec2 by 9.803Physical Data m/sec2.Rotor Weight Multiply lb•in. sec2 by 2.683 to get Rotor weight indicates the total inertia (WK2) in lb•ft2, the commonweight of the generator rotor. When English unit designation.used with the formula for inertia, the Center of Gravityflywheel effect of the generator can Center of gravity is informationbe found. Engineers may use these that gives the location of thefigures when generator loads are generator center of weight in threesudden or cyclic. planes. With the total generatorStator Weight weight, the information can be used Stator weight is given in kilograms to determine the center of gravityand pounds and gives an indication of an assembly consisting of engine,of the capacity of a generator. generator, radiator, and base.Engineers generally assume that a Combined calculations requireheavier stator contains more reducing all values to momentsworking copper and iron and (lb•in. or kg•mm). These aretherefore has greater capabilities. summed algebraically in each plane of reference.©2008 Caterpillar®All rights reserved. Page 21
  27. 27. Generator Systems Application and Installation GuideNameplate • Maximum ambient The nameplate is an engraved temperature for which themetal plate or printed film on the generator is designedside of a motor/generator. The plate • Insulation system designationcontains the minimum required (if the armature and field useinformation needed to identify its different classes of insulationfunctionality. systems, both shall be given,National Electrical Manufacturers that for the armature beingAssociation (NEMA) Requirement given first) The following is required ** Applies to the exciter in thenameplate information as set by case of a brush-less machine.National Electrical ManufacturersAssociation (NEMA). International Standards Organization (ISO) 8528 Requirement a. Manufacturer’s type and Generating sets shall bear the frame designation following rating plates as set by the b. Kilovolt-ampere output International Standards Organization c. Power factor (ISO) 8528. The nameplate for the set must provide the following d. Time rating information. e. Temperature rise* a. The words “Generating set f. Rated speed in rpm ISO 8528” g. Voltage b. The manufacturer’s name or h. Rated current in amperes mark per terminal c. the serial number of the set i. Number of phases d. the year of manufacture of j. Frequency The set k. Rated field current** e. The rated power, in kilowatts, l. Rated excitation voltage with the prefixes COP, PRP or LTP, in accordance with ISO Additional information may be 8528-1:1993, clause 13included such as: f. The performance class in m. Enclosure or IP code accordance with ISO 8528- n. Manufacturers name, mark, 1:1993, clause 7 or logo g. The rated power factor o. Manufacturer’s plant location h. The maximum site altitude p. Serial number or date of above sea-level, in meters manufacture. i. The maximum ambient * As an alternate marking, temperature, in degreesTemperature Rise can be replaced Celsiusby the following information. j. The rated frequency, in hertz ©2008 Caterpillar®Page 22 All rights reserved.
  28. 28. Application and Installation Guide Generator Systems k. The rated voltage, in volts NEMA & IEC Design l. The rated current, in amperes Considerations m. The mass, in kilograms. The following are design The rating and class of output of considerations as set in the NEMAthe generator shall be combined as code book.follows. Temperature Rise a. Where a continuous rating The observable temperature rise based on duty type S1 is under rated load conditions of each stated, the rated output shall of the various parts of the be followed by the marking synchronous generator, above the “BR” (basic continuous temperature of the cooling air, shall rating), e.g. Si=22 kVA BR; not exceed the values in the table b. Where a rating with discrete below. constant loads based on duty The temperature of the cooling air type S10 is stated, the basic is the temperature of the external air continuous rating based on S1 as it enters the ventilating openings shall be marked as mentioned of the machine, and the temperature immediately above. In addition rises given in the Table 6 are based the peak rated output shall be on a maximum temperature of 40°C shown followed by the (104°F) for this external air. marking “PR” (peak continuous rating), the maximum running time of 500 h per year and the factor TL, e.g. Si=24 kVA PR 500 h per year, TL=0,9.©2008 Caterpillar®All rights reserved. Page 23
  29. 29. Generator Systems Application and Installation Guide Temperature Rise Method of Temperature Rise, Degrees C° Item Machine Part Temperature Class of Insulation System Determination A B F H a. Armature Windings 1. All kVA ratings Resistance 60 80 105 125 2. 1563 kVA and Embedded 70 90 115 140 less detector** 3. Over 1563 kVA a) 7000 volts and Embedded 65 85 110 135 less detector** Embedded b) Over 7000 volts 60 80 105 125 detector** b. Field Winding Resistance 60 80 105 125 c. The temperature attained by cores, amortisseur windings, collector rings, and miscellaneous parts (such as brushholders, brushes, pole tips, etc.) shall not injure the insulation or the machine in any respect. * For machines which operate under prevailing barometric pressure and which are designed not to exceed the specified temperature rise at altitudes from 3300 feet (1000 meters) to 13000 feet (4000 meters), the temperature rises, as checked by tests at low altitudes, shall be less than those listed in the foregoing table by 1 percent of the specified temperature rise for each 330 feet (100 meters) of altitude in excess of 3300 feet (1000 meters). ** Embedded detectors are located within the slot of the machine and can be either resistance elements or thermocouples. For machines equipped with embedded detectors, this method shall be used to demonstrate conformity with the standard (see 20.63). NOTES: 1—Temperature rises in the above table are based upon generators rated on a continuous duty basis. Synchronous generators may be rated on a stand-by duty basis (see 22.85). In such cases, it is recommended that temperature rises not exceed those in the foregoing table by more than 25°C under continuous operation at a stand-by rating. 2—Diesel engine specifications often call for machines which are suitable for 10-percent overload for 2 hours out of any 24 consecutive hours of operation. Generators having a corresponding overload capability are sometimes required. In such cases, it is recommended that the generators and their excitation systems be designed to deliver 110 percent of kVA at rated power factor, frequency, and voltage with temperature rises under rated load conditions not exceeding those given in the above table. 3—Temperature rises in the foregoing table are based upon a reference ambient temperature of 40°C. However, it is recognized that synchronous generators may be required to operate at an ambient temperature higher than 40°C. The temperature rises of the generators given in the foregoing table shall be reduced by the number of degrees that the ambient temperature exceeds 40°C. (Exception—for totally enclosed water-air-cooled machines, the temperature of the cooling air is the temperature of the air leaving the coolers. Totally enclosed water-air-cooled machines are normally designed for the maximum cooling water temperature encountered at the location where each machine is to be installed. With a cooling water temperature not exceeding that for which the machine is designed. a. On machines designed for cooling water temperatures from 5°C to 30°C — the temperature of the air leaving the coolers shall not exceed 40°C. b. On machines designed for higher cooling water temperatures — the temperature of the air leaving the coolers shall be permitted to exceed 40°C provided the temperature rises of the machine parts are then limited to values less than those given in the table by the number of degrees that the temperature of the air leaving the coolers exceeds 40°C.) Table 6 ©2008 Caterpillar®Page 24 All rights reserved.
  30. 30. Application and Installation Guide Generator SystemsMaximum Momentary Overloads TIF According to the 1960 Single Synchronous Generators shall be Frequency Weightingcapable of carrying a 1-minute Frequency TIF Frequency TIFoverload with the field set for normal 60 0.5 1860 7820rated load excitation in accordance 180 30 1980 8330with Table 7. 300 225 2100 8830 360 400 2160 9080 Armature Current, % of 420 650 2220 9330 Synchronous Speed rpm Normal Rated Current 540 1320 2340 9840 1801 and Over 130 660 2260 2460 10340 1800 and Below 150 720 2760 2580 10600Table 7 780 3360 2820 10210 900 4350 2940 9820 It is recognized that the voltage 1000 5000 3000 9670and power factor will differ from the 1020 5100 3180 8740rated load values when generators 1080 5400 3300 8090are subjected to this overload 1140 5630 3540 6730condition. Also, since the heating 1260 6050 3660 6130affect in machine winding varies 1380 6370 3900 4400approximately as the product of the 1440 6650 4020 3700 1500 6680 4260 2750square of the current and the time 1620 6970 4380 2190for which this current is being 1740 7320 5000 840carried, the overload condition will 1800 7570result in increased temperatures anda reduction in insulation life. The Table 8generator shall therefore not besubjected to this extreme condition Shall not exceed the values infor more than a few times in its life. Table 7.It is assumed that this excess kVA Rating of Generator TIFcapacity is required only to 125 to 4999 150coordinate the generator with the 5000 to 19999 75control and protective devices. 2000 and Above 70 Table 9Maximum Deviation Factor The deviation factor of the open When specified, the residualcircuit line-to-line terminal voltage component TIF based on theof synchronous generators shall weighting factors given shall notnot exceed 0.1. exceed the values in Table 10.Telephone Influence Factor (TIF) These residual components apply to Telephone Influence Factor (TIF) those voltage ratings 2000 volts orshall be measured at the generator higher.terminals on open circuit at ratedvoltage and frequency. Whenspecified, the balance TIF is basedon the weighting factors in Table 8.©2008 Caterpillar®All rights reserved. Page 25
  31. 31. Generator Systems Application and Installation Guide kVA Rating of Generator TIF (residual) values which give an 125 to 4999 150 integrated product, equal or 5000 to 19999 75 less than 40 for salient pole 2000 and above 70 machines and 30 for air Table 10 cooled cylindrical rotor machines.Single Frequency Weighting Short b. The maximum phase currentCircuit Requirements is limited by external means A synchronous generator shall be to a value which does notcapable of withstanding, without exceed the maximum phasedamage, a 30-second, three-phase current obtained from theshort circuit when operating at rated three-phase fault.kVA and power factor, at fivepercent over voltage, with fixed Overspeedexcitation. The generator shall also Synchronous generator shall be sobe capable of withstanding, without constructed that, in an emergencydamage, any other short circuit at its not to exceed two minutes, they willterminals of 30 seconds or less withstand without mechanical injuryprovided: overspeeds above synchronous speed in accordance with the values a. The machine phase currents in Table 11. under fault conditions are such that the negative phase Overspeed, Percent of Synchronous Speed rpm Synchronous Speed sequence current (I2), 1801 and Over 20 expressed in per unit of stator 1800 and Below 25 current at rated kVA, and the Table 11 duration of the fault in seconds (t) are limited to ©2008 Caterpillar®Page 26 All rights reserved.
  32. 32. Application and Installation Guide Generator SystemsGenerator Performance Characteristics 220 Vpu = = 0.917Rated and Per Unit 240 “Per unit” values are expressed asa decimal fraction of some whole 100 Spu = = 0.833value. Twenty percent, 20% or 1200.20 indicates part of some wholevalue. Per unit is a pure number. It And from these base numbers youhas no label such as volts, amperes can calculate current and impedanceor ohms. Per unit is often bases.abbreviated as P.U. or p.u. Sbase (240 V)2 Ibase = = = 50A Zbase = The per unit system eliminates the Vbase 240 Vuse of cube roots that are inherentto three-phase calculations. The Vbase (240 V)2 Vbase = = = 4.8Ω 12kVaper unit basis also provides a Ibase Sbasecomparable value system for allgenerators, transmission lines, Efficiencymotors and related measures so Efficiency is the percentage ofthey look, more or less, alike. engine flywheel horsepower that kVA, volts, amps, power factor, is converted into electrical output,resistance and reactance are all or in other words, power out of thefigures converted to per-unit values. generator divided by power in.Per-unit calculations are defined as 100% efficiency is a theoretical levelthe actual value of the figure divided that assumes no losses in heat,by the base value. It is customary to windage or voltage and apparent powerbase values; once these are done, all output powerother base values can be calculated. Efficiency = input powerRefer to “Reactance” for more perunit calculations. output Efficiency = To convert a circuit quantity to output + lossesa per unit value, divide by the basevalue, which is usually the rated input - losses Efficiency =value. For example, the following inputequations show the per unit valuesfor a 240/120-V, 120 kVA generator Generator efficiencies arethat is operated at 100 kVA with published at 100% of rated.220 volts. Percent generator efficiency is shown in the following formula. % Gen. kVA x PF x 100 = Efficiency (kVA x PF) + gen. losses + exciter losses©2008 Caterpillar®All rights reserved. Page 27
  33. 33. Generator Systems Application and Installation Guide In discussion of generator generator to provide adequate faultefficiency, generator losses must current to the short in order to givebe considered. These include the a protective device, such as breakersfollowing. and fuses, enough time to react. • Armature Winding — The loss Permanent Magnet (PM) generators based upon phase currents are able to sustain fault current due and phase resistances. to their excitation characteristics while self excited (SE) generator • Field — The loss due to field may have trouble providing this current and field resistance. current for enough cycles. Refer to • Core Loss — Iron losses, the section entitled “NEMA Design hysteresis, and eddy currents Considerations” for official standards due to the flux variation in the on short circuits. armature core. Caterpillar IE system (Independent • Stray Load Losses — Iron Excitation) also has short circuit losses and eddy current losses current capabilities. It is designed in the copper due to fluxes such that the short circuit current varying with load and capabilities can be disabled during saturation. parallel operation with the utility and • Friction and Windage Loss — fully implemented during stand alone Loss of the power used to operation for maximum performance. overcome bearing friction and The IE excitation system relies on windage. There is a power residual magnetism for voltage build- used against the friction up. The immunity to non linear between the bearings and the loads, excellent motor starting and rotor that creates a friction the short circuit capabilities, are loss. The power necessary to standard IE features not available on move the rotor poles through Self Excited generators. the air and to drive cooling air When a generator experiences a through the generator creates sudden load increase, such as a windage loss. starting a motor, output voltage and • Exciter Loss — All losses speed of the genset dip for a short similar to those experienced time. This is called a transient dip by the main generator. These and is illustrated in Figure 21. When include field, core, stray load, the load is thrown off, an overshoot friction and windage losses. occurs. Without proper generator design, this overshoot couldFault Current/Short Circuits potentially cause a short-circuit. Short circuits generally occur whenelectric current is diverted from itsintended path to ground or anotherline of the generator. When shortcircuits occur, it is necessary for a ©2008 Caterpillar®Page 28 All rights reserved.
  34. 34. Application and Installation Guide Generator Systems offset the short circuit current for a Transient Dip very short time.Figure 21 Each phase of a generator hasthe following impedances: Figure 22 • Equivalent Armature A three-phase fault is a short Reactance between all three phases of the • Armature Leakage Reactance output. A line-to-line fault is a short • Armature Resistance between two of the three phases, and the same concept goes for line- • Synchronous Reactance to-neutral and line-to-ground faults. • Synchronous Impedance A more detailed explanation of faults • Voltage Proportional to is covered under “Power Systems.” Excitation Requirements Figure 23 illustrates a short circuit; • Line Current Variable definitions can be found on Page 32 of this section. • Phase Voltage The vectors shown in Figure 22demonstrate that to maintainconstant voltage at the terminals,the excitation requirements willvary according to the power factor.Therefore, excitation requirementsare greatest at lagging power factorsand less at leading power factors. There may also be a direct currentcomponent to short circuits. In most Figure 23short circuits, the ratio of reactanceto resistance is high. If a fault Stator Resistanceoccurs at zero voltage on the sine Stator resistance refers to thewave in such a circuit, the current internal resistance of the generatorwill not be symmetrical. It will that is developed by the statorcontain a DC component that will copper and winding configuration.©2008 Caterpillar®All rights reserved. Page 29