TESTS SHOW ABILITY OF VACUUM CIRCUIT-BREAKER TO INTERRUPT FAST TRANSIENT RECOVERY VOLTAGE RATES OF RISE OF TRANSFORMER SECONDARY FAULTS

A Vacuum Circuit Breaker demonstrated its ability to interrupt short c ircuits with faster than normal rates of rise of Transient Recovery Vo ltage (TRV) at levels greater than those produced by most transformer secondary faults. Two recent exploratory test programs evaluated the i nterrupting ability of a 15kV Vacuum Circuit Breaker containing interr upters of the rotating are type with contacts made from a Chromium-Cop per powder metal mixture. The interrupting conditions covered a wide r ange of currents from 10% to 130% of the 28kA rated short circuit curr ent of the tested circuit breaker and a wide range of TRV rates of ris e, including: the relatively slow rate of rise, normally used in testi ng and found in most indoor circuit breaker applications, two faster r ates of rise equalling and exceeding those found in a known power plan t transformer secondary protection application, and the fastest rates of rise possible in the laboratory which exceed the requirements of mo st transformer secondary faults. These tests showed that the interrupt ing performance of the tested Vacuum Circuit Breaker was unaffected by the TRV rate of rise to the fastest rates available in the test lab. Such a Vacuum Circuit Breaker can therefore be used without TRV modify ing capacitors to slow down the rate of rise provided by the power sys tem. This ability is particularly important if analysis shows that the expected TRV from a transformer secondary fault has a fast rate of ri se beyond the recognized ability of an older circuit breaker to accept ably interrupt.