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

At approximately 5.48 am on August 13, 1991, phase B of the main setup transformer of Niagara Mohawk Nine Mile Point Unit 2 nuclear power plant experienced a failure resulting in degraded voltage in phase B of the electrical AC distribution system. The author describes the events at the plant and an engineering analysis of the event. 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. Safety-related instrumentation remained unaffected by these transient conditions. There was no radioactive release, and the plant was brought to a cold shutdown in an orderly manner