05000353/LER-2008-001
| Limerick Generating Station, Unit 2 | |
| Event date: | 01-12-2008 |
|---|---|
| Report date: | 02-02-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3532008001R01 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. Post maintenance testing was being performed on D23 Emergency Diesel Generator (EDG) (EIIS:DG) (EIIS:EK), which was inoperable since January 6, 2008 to conduct a planned inspection.
There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Saturday, January 12, 2008, D23 Emergency Diesel Generator (EDG) governor tuning per RT-6-092-503-2 "D23 Diesel Generator Governor Tuning Response Test" was in progress during post maintenance testing of the inoperable D23 EDG. At 0904 hours0.0105 days <br />0.251 hours <br />0.00149 weeks <br />3.43972e-4 months <br /> the EDG was supplying the bus in isochronous mode when the test directed starting the 2C Residual Heat Removal (RHR) pump. Following the pump start the bus voltage increased above the test limit of 4900 V for greater than 5 seconds. The Plant Reactor Operator opened the EDG output breaker and then secured the EDG engine as directed by a contingency step in the test.
The bus undervoltage condition actuated the D23 Division 3 4kV Safeguard Bus undervoltage relay, which initiated the automatic start logic (0.5 second time-delay) for D23 EDG followed by an automatic close signal (1.0 second time- delay) to the D23-201 offsite source feeder breaker, which re-energized the bus. The EDG continued to run per design after the EDG output breaker was manually opened until the operator secured the EDG engine. It was during this time period that the automatic start logic was initiated and therefore, with the diesel already running, no actual EDG start occurred.
�■ � The investigation determined that the failure of the emergency diesel generator voltage regulator (EIIS:RG) was due to an intermittent failure of the #1 rectifier bank. The #2 rectifier bank was placed in service and the #1 rectifier bank was removed from service. D23 EDG post maintenance testing was successfully completed and D23 EDG was restored to operable status on January 14, 2008 at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br />.
The loads on the affected bus were de-energized for approximately one second. The most significant effects on plant equipment were a trip of the 2A Reactor Water Cleanup Pump, a trip of the 2C RHR pump, and a trip of the 2A Instrument Air Compressor. The Containment Leak Detector Radiation Monitor primary containment isolation valves (PCIVs) closed due to loss of power.
This event resulted in a valid actuation of emergency A.C.
electrical power. The 8-hour ENS notification required by 10CFR50.72(b)(3)(iv)(A) was performed on Saturday, January 12, 2008 at 1909 hours0.0221 days <br />0.53 hours <br />0.00316 weeks <br />7.263745e-4 months <br /> (#43898). Since the EDG automatic start minimum actuation logic for the bus undervoltage condition actuated, this event is being reported as an automatic actuation of emergency A.C. electrical power.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The 4kV safeguard bus was de-energized for approximately 1.0 second, per design, between the time the EDG output breaker was manually opened and the time the offsite source breaker automatically closed.
The D23 EDG governors (EG-A and EG-B) were replaced during the EDG 24-month inspection; therefore, governor tuning was required during the post maintenance testing. The governor tuning test placed the EDG in service supplying the bus loads in isochronous mode with the offsite source breakers open. A load was applied to the EDG by starting the 2C RHR pump.
The engineering evaluation of the overvoltage condition determined that the EDG, motors, and transformer were not adversely affected by the short duration overvoltage condition. Instruments powered from the bus were walked down to ensure no undetected failures occurred.
The vendor failure analysis consistently duplicated the failure during as-found testing of the rectifier chassis. A voltage drop was identified at a bolted connection where torque was less than expected on the flyback diode anode heatsink. Corrosion induced voltage drops were also identified at several locations. The voltage drops combined to cause a silicon controlled rectifier (SCR) to fail to return to the "off" state which is also known as a "latch on" condition.
Cause of the Event
The actuation was caused by a failure of the D23 EDG voltage regulator that was due to an intermittent failure of the #1 rectifier bank. The failure was primarily caused by looseness at a bolted connection and corrosion at the rectifier flyback diode.
Corrective Action Completed The D23 EDG voltage regulator was swapped to the #2 rectifier bank and the D23 EDG governor tuning was successfully completed.
The #1 rectifier was replaced.
Failure analysis was completed on the degraded rectifier.
Corrective Action Planned Preventive maintenance (PM) on EDG voltage regulators will be performed during each upcoming EDG overhaul.
The voltage regulator PMs and maintenance procedure will be revised to include visual inspection and tightness checks of the silicon controlled rectifier (SCR) bolted connections.
Previous Similar Occurrences There were no previous similar occurrences of emergency diesel generator voltage regulator failures that resulted in bus overvoltage conditions in the last five years.
Component data:
Equipment:
Component Number:
Manufacturer:
Model Number:
D23 Emergency Diesel Generator 2C-G501 384C Fairbanks Morse Engine Division 00884777