Dielectric-Withstand Levels of Some 735-kV Air-Blast Circuit Breakers Under Both Direct and Synthetic Out-of-Phase Interrupting Tests

Simulations with the transient network analyzer at IREQ have revealed that following a load-shedding operation on the James Bay 735-kV power system, severe out-of-phase overvoltages could appear across the interrupting circuit breakers. The simulation results show that overvoltages could reach 2000 kV, while the maximum breaking current is as low as 5 kA rms. Since these overvoltage values far exceed the IEC specifications (1500 kV), direct and synthetic out-of-phase interrupting tests were performed on two types of breaker: a conventional air-blast circuit breaker of the type already in service and a modern improved design of the same type. The aim of the test program was to qualify high-voltage circuit breakers for the James Bay power system. The paper focuses on the analysis of the test results and deals especially with the decrease in dielectric strength of the breaker with the time that elapses after current interruption.