Assessment of TRV parameters and overvoltages in three-phase ungrounded faults

The transient recovery voltage requirements laid down in IEC/IEEE 62271-37-013 for testing generator circuit-breakers are determined for the first-pole-to-clear in case of a three-phase grounded fault despite it is recognized that the rate-of-rise in such case is somewhat smaller and therefore less severe than the rate-of-rise in case of ungrounded faults. The occurrence of a three-phase ungrounded fault in power stations is rare but cannot be completely ruled out. Such fault may arise, for example, inside the panel of MV-switchgear with non-segregated busducts or when active arc mitigation devices which transform an arc fault into a three-phase ungrounded bolted short-circuit are employed. The transient recovery voltage requirements for three-phase ungrounded faults are assessed in the present work. Also the voltage transients occurring at the fault location and generator neutral are investigated.