Degraded voltage resulting in non-safety UPS failure at Nine Mile Point Unit 2 August 13, 1991

At approximately 5:48 a.m. on August 13, 1991, phase B of the main stepup transformers of Niagara Mohawk´s Nine Mile Point Unit 2 (NMP2) Nuclear Power Plant experienced a failure resulting in degraded voltage in phase B of the electrical AC distribution system. The duration of the degraded voltage lasted 12 cycles, the time required to clear the fault and to fast transfer the house loads to alternate offsite sources. The protective relaying schemes accomplished this without any abnormalities. Due to the nature of the fault and its protection, the turbine tripped, resulting in an automatic reactor scram. However, during the first transfer under degraded voltage conditions, five nonsafety-related uninterruptible power supplies (UPSs) tripped. The tripping of these UPSs resulted in the loss of plant process computers, control room annunciation, and a significant portion of nonsafety-related instrumentation and control circuits. The events of that day and the effects of the tripping are described and analyzed here.<>