05000282/LER-2001-002

LER-2001-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2822001002R00 - NRC Website

EVENT DESCRIPTION

On June 18, 2001, while operating at 100% power, severe weather resulted in an electrical grid disturbance that caused 4KV Breakers CT11-1 to open. This resulted in the loss of the normal offsite source (CT11 transformer2) to safeguards Bus3 16. The load sequencer for Bus 16 operated as designed and transferred Bus 16 to its backup 4KV offsite source (1R transformer).

Upon investigating Operations personnel found no flags had dropped on any of the protective relaying4.

No lockout relay actuations had occurred. The initial conclusion was that the trip of CT 11 was due to a grid disturbance.

safeguards buses was then restored to operable status.

CAUSE OF THE EVENT

Investigation into the cause of the event included a work order to determine the state of the equipment.

Bus CT-11 and cooling tower motors5 121 and 122 were meggered to verify no bus or load fault existed.

Breaker CT11-1 was visually inspected and function tested out of the cubicle to verify proper breaker function. Data from the plant 345 KV substation fault recorder and data from the plant Emergency Response Computer System were used to determine grid and plant distribution system voltage perturbations and timing in relation to this event.

The investigation determined there were no equipment malfunctions. The undervoltage relay on bus CT-11 actuated on a valid undervoltage due to a grid event. The loss of voltage on bus CT-11 caused a valid actuation of the Bus 16 load sequencer.

ANALYSIS OF THE EVENT

As a result of the undervoltage on the grid, the plant safeguards AC systems behaved as-designed and automatically transferred a safeguards electrical bus from its normal source to its alternate source. This event resulted in the automatic, unplanned actuation of the 4KV Bus 16 load sequencer. Bus 16 is part of the Unit 1 emergency AC electrical power system and, thus, this event is reportable per 10CFR 50.73(a)(2)(iv)(A).

1 (EIIS Component Identifier: BKR) 2 (EIIS Component Identifier: XFMR) 3 (EIIS Component Identifier: BU) 4 (EllS.Component Identifier: RLY) 5 (EIIS Component Identifier: MO) 6 (EIIS System Identifier: EK) Effect on Safety System Functional Failure and Other Performance Indicators This event did not involve any plant equipment failures, thus, it is not a safety system functional failure.

Bus 16 was automatically switched to its alternate source, but was not de-energized. Thus, this event resulted in no unplanned unavailability. No other performance indicators are affected by this event.

Significance Determination This event demonstrates that the plant can successfully cope with a disturbance on the grid. The plant responded to the disturbance as it was designed to. There was no impact on safety. This event was assessed per the Maintenance Rule and determined to be non-risk significant.

CORRECTIVE ACTION

Potential actions to prevent recurrence are being evaluated under the Prairie Island Corrective Action Process.

FAILED COMPONENT IDENTIFICATION

None.

PREVIOUS SIMILAR EVENTS

While there have been a few instances where a safeguards bus load sequencer has actuated in the past four years, only one of these (LER 1-96-12) involved an actual grid disturbance.