Description of Event:
On July 4, 2014, at 0314 a.m., Unit 3 was operating in generate mode, Circuit Breakers (CB) 392 and CB 396 tripped. Unit 3 was returned to service at 0622 a.m. on the same day.
The noted events were:
CB 392 TRIP
CB 396 TRIP
UNIT 3 GENERATOR PROTECTIVE LOCKOUT – TRIPPED UNIT 3 SWITCHYARD (SWYD) BREAKER (BKR) A REPLICA RELAY – OPEN UNIT 3 SWYD BKR B REPLICA RELAY- OPEN
UNIT 3 TRANSFER TRIP RECEIVER LOCKOUT -95-3D1
Alarms Received:
UNIT 3 GOV OIL SYSTEM – FAILURE
UNIT 3 PENSTOCK GATE OIL PUMP CTRL PWR OR RESTORE – FAILURE UNIT 3 SPIRAL CASE WATER PRESS IS AT 166.5
Unit 3 CIRCUIT RELAY OPERATION
DFR NO. 1 OPERATED
DFR NO. 2 OPERATED UNIT 3 EXCITER TRIPPED UNIT 3 EXCITER WARNING
UNIT 3 GEN PROTECTIVE LOCKOUT RELAY TRIPPED UNIT 3 CIRCUIT RELAY MALFUNCTION
Actions Taken:
Hydro Regional Dispatcher (HRD) contacted the Electrical System Dispatcher and notified of the Unit 3 trip. The HRD contacted the plant’s lead operator and requested an investigation of the Unit 3 trip. The lead operator reported to the HRD the unit tripped due to the lead operator blowing down of the governor oil tank.
The HRD contacted the substation operator to reset the relays in the switchyard and instructed the lead operator to restore Unit 3 to service. The restoration process included raising the Fixed Wheel Head Gate to its normal operating position.
Method & Findings:
The plant’s lead operator was blowing down the governor accumulator tank on Unit 3. While the Unit 3 governor was blowing down the operator left the area to perform other duties. The plant’s lead operator was then contacted by the HRD office informing the operator Unit 3 had tripped offline and the Fixed Wheel Head Gate had dropped.
The unabated addition of air to the Unit 3 Governor Oil Accumulator tank forced the oil from the tank and air into the governor oil system which caused the wicket gate servo motors to operate in an erratic manner and subsequent unit trip.
The protection systems (relays, current transformers, potential transformers, direct current systems, batteries) operated as expected.
Findings
The plant’s lead operator failed to follow the blow down procedure for the Governor Oil Tank when the air valve was left open, and the operator left the site to perform other duties. This event has been classified as an Operator Error.
Supporting Materials:
Corrective Actions:
The plant’s lead operator received training and counseling on following Standard Operating Procedures.
Sequence & Completion Dates for Actions Listed:
The corrective action was completed by July 31, 2014.
Lessons Learned:
Recommendations:
1. Operations personnel shall be reminded to follow approved Standard Operating Procedures on a periodic basis.