Experimental Evaluation of Switching Induced Transformer Resonance Mitigation

Vacuum circuit breaker switching induced transients can cause frequency induced medium voltage transformer dielectric damage, where the two are coupled with short cable. The most common mitigation method is to add a resistor/capacitor snubber at the transformer terminals. While this approach has proved highly effective, there is little discussion of variations on the snubber application. This paper describes the results of switching tests of a vacuum circuit breaker connected to a 1-MVA transformer through medium voltage cable. First, the mechanism was investigated. The tests show the resonance is induced by the broadband frequency content due to the high dv/dt of the circuit breaker current chopping and the chopping rate. Next, snubber performance at various circuit locations was tested. The snubber was most effective at the transformer medium-voltage terminals and at the load side of the circuit breaker.