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 broad band frequency content due to the high dv/dt of the circuit breaker current chopping and the chopping rate. Next, mitigation variations were examined such as snubber location, sizing and effectiveness on the transformer secondary.