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Section –4Selection of Circuit Breaker1.0 IntroductionCircuit breaker is a mechanical switching device capable of making, carrying andbreaking current under normal circuit condition as well as under specified abnormalcircuit condition such as short circuit etc. Circuit breakers are generally classifiedaccording to interrupting medium used to cool and elongate electrical arc permittinginterruption. Selection of outdoor circuit breakers for switchyards 12 kV and above upto245 kV (highest system voltage) as regards types, rating, performance requirements andtests for AC high voltage circuit breakers that are installed in SHP outdoor switchyardafter the step up transformer on outgoing transmission line feeders. Special requirementfor rating of AC high voltage generator circuit breakers between the generator andtransformer terminals are also discussed.1.1 ReferencesRelevant National and important international standard in this connection are as follows:-1. IS: 13118 - Specification for high-voltage alternating current circuitbreakers2. IEC: 56 - High voltage alternating current circuit breakers3. IEEE: 37010 - IEEE Application Guide for AC high voltage circuit breakers4. IEEE 37013 - AC high voltage generator circuit breaker rated onsymmetrical current basis2.0 Classification2.1 Following types of circuit breakers formerly used in high voltage outdoor substationsare no longer in use and are being phased out.i) Bulk oil circuit breakers (Dead Tank Design) – In these circuit breakers oil contentsis used for arc extinction and also for insulating live parts from the tank which is deadand generally earthed (ground).ii)iii)Minimum oil breakers (Live Tank Design) – In these circuit breakers oil isprimarily used for arc extinction and not necessarily for insulating live parts fromearth (ground). The tank of these circuit breakers are insulated from earth ground.The circuit breakers are phase separated. These circuit breaker were widely used upto72 kV level and are being phased out from existing installation.Air blast circuit breaker – circuit breaking in these circuit breakers occurs in a blastof air under pressure. These circuit breakers were widely used upto 765 kV system.These circuits breakers are being phased out.4-1
3.0 Type of Circuit BreakerFollowing types of circuit breakers are in use now-a-days for max. voltage class used for25 MW hydro station.i) SF-6 – Sulphur Hexa Flouride Breakers 36 kV to 220 kV classii) Vacuum circuit breakers upto 36 kV classiii) Air circuit breaker upto 12 kV (Generator circuit breaker)Sulphar Hexafluoride as an Arc Quenching Agent:- Pure sulphar hexafluoride gas isinert and thermally stable. It possesses very good arc quenching as well as insulatingproperties which make it ideally suitable for use in a circuit breaker. Sulphar hexafluorideremains in a gaseous state upto a temperature of 9 0 C at 15 kg/cm 2 pressure its density isabout five times of air and the free heat convection is 1.6 times as much as that of air.Apart from being a gas, it is non-inflammable, non-poisonous and odourless. Whenarcing takes place through the gas, some by-products are produced due to breakdown ofthe gas. These by-products are a hazard to the health of the maintenance personneltherefore should be properly taken care of.At a pressure of three atmospheres the dielectric strength of sulphar hexafluoride is about2.4 times that of air and compares very well with that of oil. Even when gas is exposed toelectric arcs for fairly long periods, it has been found that decomposition effects are smalland the dielectric strength is not materially affected. On the other hand the metallicfluorides at the temperatures of the arc are good insulators and the arc is therefore, not atall harmful to the breaker.Gas circuit breaker generally employ SF-6 (sulphar hexafluoride) as an interruptingmedium and sometimes as an insulating medium. In “single puffer” mechanisms, theinterrupter is designed to compress the gas during the opening stroke and use thecompressed gas as a transfer mechanism to cool the arc and to elongate the arc through agrid (arc chutes), allowing extinguisher of the arc when the current passes through zero.In other designs, the arc heats the SF6 gas and the resulting pressure is used forelongating and interrupting the arc. Some older low-pressure SF6 breakers employed apump to provide the high pressure SF6 gas for arc interruption.Gas circuit breakers typically operate at pressures between six and seven atmospheres.The dielectric strength of SF6 gas reduce significantly at lower pressures, normally as aresult of lower ambient temperatures. Monitoring of the density of the SF6 gas is criticaland some designs will block operation of the circuit breaker in the event of low gasdensity.Circuit breakers are available as live-tank or dead-tank designs. Dead –tank designs putthe interrupter in a grounded metal enclosure. Interrupter maintenance is at ground leveland seismic withstand is improved versus the live-tank designs. Bushings are used forline and load connections which permit installation of bushing current transformers for4-2
Page 2 and 3:
Section -1Guide for Design of Outdo
Page 4 and 5:
The terms atmospheric overvoltages
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Table 2.31. Basic Data Example 1.Hi
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Spark gaps specified for fitting to
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2.4.2 Working Safety Clearances125
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Highest System Voltage rating kV Sp
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2.6 Insulator TypeTypes of insulato
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Bending .. .. 1265 kg/sq.cm.Shear .
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v) Installations in difficult site
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During maintenance, it is essential
Page 22 and 23: Areva Compact Air Insulated Substat
Page 24 and 25: The bus bar sizes should meet the e
Page 26 and 27: disconnect switch contacts in such
Page 28 and 29: (iv)Electro magnetic type interlock
Page 30 and 31: ) Temperature rise of 11 kV Class C
Page 32 and 33: 4.6 TestsThe CTs shall be provided
Page 34 and 35: 10. 1.2/50 micro seconds impulse 32
Page 36: 7.0 Lightning ArrestorsLightning ar
Page 39 and 40: and this aspect has to be kept in m
Page 41 and 42: Fig. 12.1: Layout of 12 kV Agnoor S
Page 43 and 44: Fig. 12.4 Layout of 72.5 kV Switchy
Page 45 and 46: 464006800 7600 3000LA30002000 2000X
Page 47 and 48: Fig. 12.6:145 kV Outdoor Switchyard
Page 49 and 50: Fig. 12.7: 245 kV Outdoor Switchyar
Page 51 and 52: 2-20
Page 53 and 54: 3.0 Transformer RatingThe full load
Page 55 and 56: OFAFOil Immersed with forced oil ci
Page 57 and 58: The transformer may be operated wit
Page 59 and 60: 8.10 Parallel OperationThe transfor
Page 61 and 62: (a)(b)33 kV Transformers (MVA)1.001
Page 63 and 64: (e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p
Page 65 and 66: 18.0 Oil Containment and Fire Prote
Page 67 and 68: ) The limits of temperature rise me
Page 69 and 70: Example Calculation -1Generator Tra
Page 71: TRANSFORMER RATING AND CHARACTERSTI
Page 75 and 76: iii) during arcing of SF6 breaker,
Page 77 and 78: 3.4 Protection Classes for Switchge
Page 79 and 80: The rated voltage is expressed in k
Page 81 and 82: 5.7 Rated Short Circuit Making Curr
Page 83 and 84: Effect of circuit conditions on rec
Page 85 and 86: 5.17 Rated time quantities and oper
Page 87 and 88: 25405017.5 812.516254024 812.516254
Page 89 and 90: component of fault current in addit
Page 91 and 92: Steps involved in applying method a
Page 93 and 94: to earthxvii. Difference in the ins
Page 95 and 96: Datai) Sub transient reactance of g
Page 97 and 98: Annexure-1Current waveAA = envelop
Page 99: Annexure-3Fig. 3.3 Arc extinction4-
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