Overvoltages due to switching off an unloaded transformer with a vacuum circuit breaker

When a vacuum breaker is used to disconnect an unloaded transformer, high o vervoltages may result on the secondary side. The phenomena or current chop ping and multiple reignitions after contact opening, shortly before current zero, play a significant role in generating these overvoltages. Furthermor e, the transient recovery voltage (TRV) is also influenced by virtual curre nt chopping which may lead to fa;st interruptions and reignitions. The pape r reports on a vacuum breaker model implemented in the ATP-EMTP program. Si mulations have been performed to determine overvoltages generated by the va cuum breaker, before and after suppression. Network elements were simulated by simplified models of limited complexity.