Insulation Coordination for38kV Circuit Breakers

Based on a reviewoftheimpulse insulation requirements for38kVcircuit breakers andtheimpact of insulation coordination using modernarresters, a change is suggested inthewaytheimpulse requirements arestated for thecircuit breaker. Thefacets ofcircuit breaker design which may beaffected are indicated. Specifically itissuggested thatinthepresent stateoftheart, circuit breakers forgeneral application at34.5kVshould haveastandard requirement for 150kVBILphase toground, and200kVBILphase tophase andacrosstheopenbreak. Background Formanyyears200kVhasbeenstandard BILfor34.5 kVcircuit breakers. A 1950reference (1)gives theprotective characteristics ofarresters available atthattime.Manyofthe subtransmission circuits operating atapproximately 34.5kV were ungrounded, requiring theuse of100% or 37 kV arresters. Considering theprotective characteristics ofthe thenavailable arresters, 200kV BILwas required inthe circuit breaker inorderto assure coordination withthe arrester. Sincethattime, arresterprotective characteristics haveimproved substantially. Intheprocess ofdeveloping vacuum circuit breakers for 34.5kVservice, itbecameapparent that extrainsulation was being addedtothecircuit breaker simply tomeetthestandard requirement for200kVBIL.Itbecamequestionable whether this extrainsulation wouldhaveanybenefit fortheuserifthe breaker was to beapplied ina substation withmodern arresters. Thepurpose ofthis paperistoreview theinsulation coordination andprovide a basis forconsideration ofa more effective way ofspecifying theinsulation requirements for thecircuit breakers.