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  • 1. Please purchase PDFcamp Printer on to remove this watermark. (ii) The specific clearance between power and telcom lines, earth wires and earth-structures are to be adhered to. The minimum clearance between lines of various voltages to be maintained are as follows : L.T. lines (400/230 V) 1.22 Mtr 220 KV lines 4.58 Mtr 11 KV lines 1.83 Mtr 400 KV line 5.49 Mtr 33/66 KV lines 2.44 Mtr 800 KV lines 7.94 Mtr 132 KV lines 3.05 Mtr (iii) Guardians are to be provided at crossings of telecom lines with power lines upto 33 KV. (iv) Maximum value of induced electromagnetic volt - 250 volts (faults duration equal to or less than 200 ms.) (v) Maximum value of induced noise (noise interference) -200 microvolts. (to be taken congnizance if noise is persistent) EHV - SUB-STATION v SUB-STATION PLANNING CRITERIA v The maximum fault level on any new Sub-Station Bus should not exceed 80% of the rated rupturing capacity of the Circuit Breaker. The 20% margin is intended to take care of the increase in short circuit level as the system grows. The rate of breaking current and making currnt including fault clearing time capability of Switch-gear at different voltage levels may be taken as :- Fault cleaning Voltage Operating Breaking Acking Time level Time current current 150ms 33kV 60-80ms 25KA 62.5KA 120ms 132kV 50m s 25/31.5KA 70KA 100 ms 220kV 50m s 31.5/40KA 100KA 100ms 400kV 40m s 40KA 100KA 765kV 40KA v The capacity of any single sub-station at different voltage levels shall not normally exceed. 765 KV.- 2500 MVA. 400 KV.- 1000 MVA. 220 KV.- 320 MVA. 132 KV.- 150 MVA. v Size and Number of inter-connecting Transformer (ICTs.) shall be planned in such a way that outage of any single unit would not over load the remaining ICT (s) or the underlying system. v A stuck breaker condition shall not cause disruption of more-than four feeders for 220 KV. system and Two Feeders for 400 KV. system and one Feeder for 765 KV. system. 65
  • 2. Please purchase PDFcamp Printer on to remove this watermark. EHV SUB-STATION SYSTEM REQUIREMENT Sl No. Description of Technical Parameter Unit System 1. Nominal system voltage kVrms 400kV 220kV 132kV 33kV 2. Maximum system voltage kVrms 420kV 245kV 145kV 36kV 3. Power frequency with stand voltage kVrms 630kV 460kV 275kV 70kV 520 kV 4. Switching surge withstand voltage kVp (for 250/2500ms) 1. Line to earth 1050kVp Not Not Not 2. Accross isolating gap 900kVp+345kVrms applicable applicable applicable 5. Lightinging impluse withstand voltage kvp for 1.2/50(ms) 1. Line to earth 1425 kvp 1050kvp 650kvp 170kvp. 2. Across isolating gap 1425kvp+ 1200kvp 750kvp 195kvp. 240kvms 6. One minute power frequency withstand value Dry kVrms 520 460 275 70 Wet kVrms 610 530 315 80 7. System frequency : Hz 50 8. Variation in frequency % +2.5 9. Corona extiniction voltage 320kV 156kV 84kV 10. Radion interference voltage 1000 mV at 1000 mV 1000 mV at 266kv at 167 kv 93kv 11. System neutral rating Solidly earthed 12. Continous current rating 1600A or 2000A 1600A 800A 600A 13. Symmetincal short circuit fault current kA 40 40 31.5 25 14. Duration of short circuit fault current Second 1 1 1 3 15. Dynamic short circuit current rating kAp 100 100 79 62.5kA 16. Conductor spacing for AIS layouts meters Phase to ground meters 6.5 4.5 3 1.5 Phase to phase meters 7.0 4.5 3 1.5 17. Design ambient tempertures oC 50 18. Pollution level as per IEC-815 and 71 III 19. Creepage distance mm 10500 6125 3625 900 20. Maximum fault clearing time ms <100 <100ms <150ms 21. Bay width meter 27 16.4-18 5.5 22. Height of bus equipment interconnection meter 8 5.5 5 4 from ground 23. Height of strung busbar meter >15 10 8 5.5 66
  • 3. Please purchase PDFcamp Printer on to remove this watermark. STUDY ON SUB-STATIONS 1. Nomenclature 2. General Reliability : Substations or switching stations are integral part of The reliability of a power system means supply of the transmission system, and function as a connection or uninterrupted power at the specified voltage and frequency. switching point for transmission lines, sub-transmission feeders, generating circuits and step-up and step -down The reliability of a substation depends on the reliability transformers Substations of voltages 66 KV to 40KV are of associated equipment such as busbars, circuit breakers, termed as EHV sub-stations. Above 500KV, they come under transformers, isolators and controlling devices. the terminology of UHV system. Failure Rate : The design considerations and procedures are almost It is the average number of failures per year. the same for the sub-stations in the EHV range except that Outage time : certain factors become predominant at different voltage levels. It is the time taken to repair the failed component or Switching surges are very important at 345 KV and above, restore supply from an alternative source by switching. whereas it can be safely neglected upto 220 KV level. Switching time 3. Design Criteria and Studies. It is the time taken from the initiation of outage to The following studies are to be performed to establish restoration of service by switching action. the design criteria for a substation. Switching scheme 1. Load flow studies It is the type of arrangement of bus bars and The purpose of a substation is to provide a path for equipments considering cost, flexibility of operation and reliable delivery of power to system loads. Load flow studies reliability of the system. establish the current carrying requirements of the new substation or switching station, when all lines are in and Phase to ground clearance when selected lines are out for maintenance. After studying The phase to ground clearance in a substation are, (a) a number of load flow cases, the continues and emergency distance between the conductor and the structures. (b) ratings required for various equipment can be determined. distance between the live parts of the equipment and structures 2. Short circuit studies. and (c) distance between the live conductor and ground. In addition to the continuous current ratings, the Phase to phase clearance substation equipment must have short time ratings,. These The phase to phase clearances in a substation are (a) must be adequate to permit the equipment to sustain, without distance between the live conductors (b) distance between damage, the severe thermal and mechanical stresses of short the live conductors and apparatus and (c) distance between circuit currents. In order to provide adequate interrupting the live terminals in equipment like, circuit breakers, isolators capability in the breakers, strength in post insulators and etc. appropriate setting for protective relays, which sense the Ground clearance fault, the maximum and minimum short circuit currents which will flow for various types and locations of short circuits and It is defined as the minimum clearance from any point, where a for different system configuration must be established. person may be required to stand, to the nearest part (which is 3. Transient Stability Studies. not at earth potential) of an insulator supporting the live conductor. Under normal conditions, the mechanical input to a generator will be equal to the electrical output plus generator Sectional Clearance losses. So long as this is continues, the system generators It is defined as the minimum clearance from any point, rotate at 50 Hz. If this balance is destroyed by upsetting either where a person may be required to stand, to the nearest the mechanical or the electrical flow, the generator speed unscreened live conductor. The basis for fixing the sectional deviates from 50Hz. and begins to oscillate about a new clearance is to take the height of a man with stretched hands equilibrium point. plus the phase to ground clearance. The most common disturbance is a short circuit. When Safety Clearance a short circuit occurs close to the generator, the terminal voltage drops and the machine accelerates. When the fault is This comprises of ground clearance and sectional cleared, the unit will try to revert to its original state by feeding clearance. the excess energy into the power system. If the electrical ties Electro static Field in Substations are strong, the machine will quickly decelerate and became An energised conductor or metallic part of the stable. If the ties are weak, the machine will become unstable. equipment produces electrostatic field. The magnitude of the The factors which affect the stability are electrostatic field varies at different points in an EHV sub- i. Severity of the fault. station (above 400 KV), depending on the geometry of ii. Speed with which the fault is cleared. energised conductor/metallic part and the nearby earthed iii. Ties between the machine and the system after the object or ground. fault is cleared. 67
  • 4. Please purchase PDFcamp Printer on to remove this watermark. The aspects of transient stability that are important in (iv) Ability to limit short circuit levels substation design are, (a) the type and speed of the line and Any arrangement which incorporates means of bus protection relaying, (b) the interrupting time of the breaker providing a substation into two separate sections either and (c) the bus configuration after the fault has been cleared. completely or through reactor coupling, is suitable for limiting The last point has a considerable bearing on the bus short circuit levels. By careful use of circuit breakers in ring arrangement. If a fault is cleared in the primary relaying time, system, a similar facility can be provided. only one line will be lost. If the fault is cleared in breaker failure (v) Maintenance facilities relaying time, owing to a stuck breaker, more than one line During the operation of the substation, maintenance may be lost which will weaken the tie to the system. will have to be carried out, either planned or emergency. The 4. Transient over-voltage studies. performance of the substation during maintenance is also Transient overvoltage may be due to lightning stroke dependent on the protection arrangements. or circuit switching . The most reliable means to establish (vi) Ease of extension switching over voltage is through the use of a Transient The substation arrangement shall be such that Network Analyser (TNA) study. extension of bays for new feeders are possible. As the system 4. Substation Arrangement expands, there may be need to convert a single bus The substation arrangement depends on physical and arrangement to double bus system, or to expand a mesh electrical aspects and is influenced by the following factors. station to a double bus station. There shall be space and (i) System Security. expansion facilities. The ideal sub-station is one were each circuit is (vii) Site considerations controlled by a separate breaker with facilities for replacement The availability of site plays an important role in of bus-bar or breaker in the event of a fault or during planning the substations. When the areas is limited, a station maintenance. System security may be specified, based on with less flexibility may have to be constructed. The substation whether complete reliance on the integrity of the substation which are simple in diagram and use least number of breakers or a percentage of outage due to periodic faults or maintenance occupy the least site. is permissible. (vii) Economy Double bus-bar system with double breaker A better switching arrangement on technical arrangement comes to near ideal, but the cost of such a requirements can be constructed, if the economics are substation is prohibitively high. reasonable. (ii) Operational flexibility 5. Substation Layout and Switching arrangement. For the efficient loading of the generators it is A number of factors are to be considered while necessary to control the MVA and MVAR loading under all finalizing the layout and switching arrangements of an EHV conditions of circuit connections. The grouping of load circuits substation. It must be reliable, safe and must provide a high requires to be capable of being arranged to give the best level of service continuity. control under normal and emergency conditions. Normally used substation schemes are detailed below. (iii) Simplicity of protection arrangements 1. Single Bus arrangement If more than one circuit is to be controlled from one This arrangement is a simple scheme adopted in less circuit breaker or greater number of circuit breakers are to be important substations. A breaker or bus failure can cause tripped during fault conditions, the protection arrangements total outage. By providing a bus sectionalism scheme, this are complex. The most advantageous arrangement is single can be overcome to some extent. Even though the protective bus-bar with no sectionalising. Ring bus arrangement where relaying is simple, single bus scheme is inflexible (Fig. 1) each circuit breaker can be in two zones of protection, causes for complex protection scheme. Sectionaliser LA Trans E.Sw PT Feeder Fig. 1. Single Bus Arrangement 68
  • 5. 2. Main and Transfer Bus breaker relaying must be so arranged to protect the A transfer bus is added to the single bus scheme. transmission line or transformer, if the protective relaysAn extra bus-tie breaker is provided to tie the main and also are not transferred. As the relaying selectivity istransfer buses together. poor this scheme is considered as unsatisfactory. Failure of the main bus can cause for total outage of the When a circuit breaker is in maintenance, the bus- substation (Fig-2)tie breaker can be used for energising the circuit . Bus-tie Ttrans MAIN BUS BI CT CB LI LA E.Sw LA PT BI TRANSFER BUS Feeder Feeder Feeder Fig-2 Main and Transfer Bus Arrangement3. Double Bus, Single Breaker The circuit may operate all from one bus, of half of This is superior to the single bus and main and transfer the circuit connected in each bus. For a bus fault, only halfbus schemes. There are two main buses and each circuit can the no. of circuits will be lost. In some cases the tie breaker isbe connected to either of the buses by bus isolators. A bus-tie permanently closed and both the buses stand connected. Abreaker connects the two main buses when closed allows bus protection scheme will be necessary for opening the tiethe transfer of a circuit from one bus to the other without a breaker in the event of a bus fault.break in supply (Fig.3) Possibility of operator error is more as two bus isolators are involved for every circuit. BUS 1 BUS 2 BI CT CB LI Bus E.Sw Coupler PT Trans LA Feeder Fig. 3. Double Bus Arrangement 69
  • 6. 4. Double bus, Double breaker Arrangement breakers for every circuit. The use of two circuit breakers per This scheme involves two main buses and two circuit circuit makes the arrangement very expensive, but this provides a very high order of reliability (Fig.4) BUS 1 BI CT CB Feeder LA ESW PT BI Trans BUS 2 Fig. 4. Double Bus, Double Breaker Arrangement5. Breaker - and -a- half-scheme. In this scheme two main buses are there and three When a source and a line are connected in oppositebreakers are connected in series between them. Two circuits directions in a 3 breaker series, even when both the busesare connected between the three breakers. Hence this is fails, it is possible to operate and provide some service.called 11/2 breaker scheme (Fig-5) It is more expensive then other schemes, except the Normally all the 3 breakers are in closed position, and double bus-double breaker scheme. Protective relaying andboth the buses are energised. When a line trip involves, two automatic reclosing schemes are complex in 11/2 breaker busbreakers open. No additional feeder or source is lost when arrangement. But this arrangement is superior in flexibility,one circuit is tripped. Any bus or any breaker can be taken reliability and safety.out of service for maintenance without loss of service. BUS 1 BI Line CB LA CT PT ESW Trans Trans BUS 2 Fig. 5. Breaker and A Half Scheme 70
  • 7. 6. Ring Bus arrangement During a breaker maintenance, the ring is broken, In this scheme (Fig-7) the breakers are arranged in a but the service is fully maintained. The circuits are generallyring with circuits connected between the breakers. There arranged such that sources and loads are alternated.are some number of breakers as the number of circuits. For a Where five or six circuits are to be provided, ringcircuit fault, two breakers are tripped. In the event of a breaker bus arrangement is ideal. This scheme is economical andfailure during a line fault, an additional breaker trips as backup provide good reliability, safety and flexibility. Protectiveprotection. In that case an additional feeder will also be out of relaying and automatic reclosing schemes are complex inservice. the case of ring bus arrangement. Feeder LA Trans ESW PT BI CT Fig. 6. Ring Bus CB LI Trans ESW Feeder LA7. Other Layout Designs bus with bypass arrangement. These arrangements are In additional to the above mentioned common bus mostly used in gas filled substations where more flexibility isarrangements, some other layouts are also employed. They ensured.are (i) Double bus arrangement with transfer bus (ii) Triple Simple schematics are as given below. All equipmentbus arrangement (iii) Double bus with bypass and (iv) Triple are not shown. BUS 1 BUS 2 TRANSFER BUS CT CB Feeder Coupler Fig. 7. Double Bus with Transfer Bus 71
  • 8. 7. Switchyard Structures and (vii) Impact load, if any, during operation of equipment. Structures are required to support and install buses, The substation gantry structures shall be designedelectrical equipment and to terminate transmission line to terminate the overhead line download span. Which mayconductors. The structures may be of steel, wood, RCC or enter + 30 degrees horizontally and +15 degrees vertically.PSC. They need foundations according to the soil conditions The yard structures may be hot dip galvanized orof the side. Generally, fabricated steel structures are used in painted. Galvanized structures require less maintenance. Butthe substations due to various advantages. The design of the in some highly polluted locations, painted structures providedstructures is affected by the phase clearance, ground more corrosion resistance.clearance, types of insulators, length and weight of buses Normally adopted phase spacingsand other equipment. 11 KV 1.3 m Steel beams and girders shall be designed to preventfailure by bending, flange buckling vertical and horizontal shear 33 KV 1.5 mand web crippling. The depth of the lattice box girders shall be 66 KV 2.0 to 2.2 mabout 1/10 to 1/15 of the span and square in section. Maximum 110 KV 2.4 to 3 mbeam defluxion shall not normally exceed 1/250 of the span 220 KV 4.5 mlength. All bolts and nuts for structures shall be not less than 400 KV 7.0 m16 mm diameter, except in light loaded section, where theymay be 12 mm dia. 8. Bus Design The design load on columns and girders shall include The present day trend is to use rigid bus rather than(i) Conductor tension (ii) Earth wire tension (iii) Wt. of insulators strain bus due to various reasons. Rigid bus can be constructedand hardwares (iv) Fraction load (about 350 kg) (v) Weight of at a lower profile and are aesthetically pleasing. Increased capacityman & tools to works on them (about 200 kg) (vi) Wind load for the bus can be provided and corona level is lower. 72
  • 9. Please purchase PDFcamp Printer on to remove this watermark. 8.1 Rigid Bus Where f = Natural frequency of span in Hertz L = Span length in feet Aluminium bus materials used for rigid bus may be of different shapes. They may be round tubings, square tubings, E = Modulus of elasticity PSI channels, angles or integral web designs. i = Moment of inertia of cross sectional area (in 4) Round tubing is used in all voltages, whereas M = Mass per unit length square tubing is used only at lower voltages. Channel (Mass = Wt./32.2) K = Constant (1.0 for pinned ends and bus is the same as square tubings except that they 1.5056 for fixed ends) provide more capacity. Angle bus is used only at Assuming there will be spans with pinned ends distribution voltages. Integral web bus is structurally where K = I strong and is used for high current and long spans- generally at lower voltage. 3 r f = 2.153 x 10 8.2 Capacity L2 The rigid bus must be able to carry the excepted 2 2 OD + ID maximum load current without exceeding the temperature Where r = 4 limit. The capacity of the bus shall also be checked for maximum temperature under short circuit conditions using r = radius of gyration (inches) the equation. OD =Outside dia. of tubing (inches) ID = Inside dia. of tubing (inches) I = K. A. 4 1 / t x10 L =Span length (feet) Where I = Symmetrical rms current in amps. As vibration may be induced in the bus by the action A = Cross sectional area in inches of 50 Hz. current, a natural frequency of 50 or 100 may be t = Time in seconds avoided. K = Coefficient for alloy bus at maximum temp. Another force which creates vibration in the bus is specified. the wind flowing across the tubing. The maximum aeolian frequency f in Hertz will be Maximum Value of K for various Temperature aluminimum Alloys 3.26V 0 f = 200 C 5.50 to 5.71 OD 2500C 6.28 to 6.52 Where V = maximum wind speed (mph) 0 300 C 6.94 to 7.18 OD =Conductor out side dia. in inches 8.4 Short Circuit Force The general practice is to limit the temperature of rigid Short circuit force produced between two parallel aluminium bus to 1000C for emergency ratings and 2500C for conductors, in the event of a line to line fault can be expressed short circuit duty. as follows. 8.3 Vibration 2 43.2.I sc A span of rigid bus has a natural frequency expressed f = 7 as follows : 10 ( D) 2 1 Where f =short circuit force (Ib/ft.) K Ei f= 2 ( )2 Isc = Symmetrical rms short circuit current (amps) 24L M D = Conductor spacing centre to centre (in) 73
  • 10. Please purchase PDFcamp Printer on to remove this watermark. For a 3 phase fault, the maximum instantaneous 8.8 Substation Bus Accessories force will be (i) Tubular bus conductor System Nominal Diameter 2 Voltage KV External (mm) Internal (mm) 37.4.I sc 72.5 42 35 F = 7 145 60 52 10 ( D) 60 49.25 8.5 Bus support system 89 78 The bus support system must be capable of taking the 89 74 following weights. 101.6 90.1 i. Weight of the tubing 245 101.6 85.4 ii. Weight of damping materials 114.3 102.3 iii. Wind on the tubing 114.3 97.2 iv. Short circuit force calculated 114.3 102.3 The resultant load establishes minimum strength of tubing material, span length and expected deflection, the bus 114.3 97.2 deflection shall not generally exceed 1/200 of the span length 420 127 114.5 without ice loading. 127 109.0 8.6 Corona For rigid bus arrangement 7000mm spacing between For HV and EHV substations, the diameter of the bus should be checked for corona discharge. Bus tubing can be phases are given for 400 KV and 4500mm for 220KV. considered satisfactory, if the voltage gradient at the surface (ii) ACSR Conductor for strain bus. does not exceed 2 KV/cm. The voltage gradient can be Suitable ACSR conductors having the desired capacity determined by shall be used for bus stringing. According to requirement, quadruple Moose, Twin Kundah and Single Kundah ACSR E g = Conductors are used for strain bus. r. In D / r Some Commonly used conductor for Where g = voltage gradient (KV/cm) Bus Stringing E =Line to neutral voltage (KV) System r =Bus outside radius (inches) Voltage (KV) Bus Conductor D = Bus spacing (feet) Lynx ACSR, Kundah ACSR 8.7 Strain Bus 72.5 19/3.53 AAC 145 Panther ACSR, Kundah ACSR Strain Bus is widely in use in most of the stations due to 19/4.22 AAC the ease of construction. Even in stations where rigid buses 245 Kundah ACSR, Moose ACSR are predominant, some spans will be invariably of strain bus 19/5.36 AAC, 37/5.23 AAC construction . The design is followed based on simple sag- 400 Moose ACSR tension calculations. a. Conductor tension The following conductor tensions are generally taken The down drops from the strain bus appear as a for designing the switchyard structures for bus arrangement. concentrated load and depending on the length and weight of Details 400KV yard 220 KV yard 132/110 KV yard the dropper, tension on the bus will vary considerable. (Kgs/conductor) (Kgs/conductor) (Kgs/conductor Where bundled conductors are used in strain bus, the Line termination 2000 1000 1000 Main Bus/Sub Bus 1000 900 800 types of spacers used may have an influence on the resulting Interconnections tension. If rigid spacers are used, then during short circuit, 1000 900 800 between yards the two conductors will attempt to draw together and can Earth wire 800 600 600 cause for increase tension in the strain bus. 74