05000373/LER-2005-004
| Event date: | 08-18-2005 |
|---|---|
| Report date: | 10-14-2005 |
| Initial Reporting | |
| ENS 41931 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3732005004R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
A. CONDITION PRIOR TO EVENT
� Unit(s): 1 Event Date: 08/18/05� Event Time: 1440 CDT � Reactor Mode(s): 1 Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On 08/18/05, the 1B diesel generator (DG)(Ell was being run in accordance with surveillance procedure LOS-DG-R1B, "lB Diesel Generator Twenty-Four Hour Run.
During this test, the DG output breaker is closed onto 4160 VAC bus 143 and the DG is loaded in parallel with the grid. At approximately 1440 hours0.0167 days <br />0.4 hours <br />0.00238 weeks <br />5.4792e-4 months <br /> CDT, the SAT feed breaker (ACB 1432) to bus 143 tripped. The 1B DG continued to run and supply power to bus 143; however, the control room operators observed that the 1B DG cooling water pump was not running and promptly shutdown the 1B DG, leaving bus 143 de-energized.
Bus 143 is the Division 3 power supply to the High Pressure Core Spray (HP)(BG] system. With both the normal and alternate power supply unavailable, HP was declared inoperable. This condition was reportable in accordance with 10CFR50.72(b)(3)(v) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems needed to mitigate the consequences of an accident. An 8-hour ENS notification (#41931) was made at 1804 hours0.0209 days <br />0.501 hours <br />0.00298 weeks <br />6.86422e-4 months <br />.
The 1B DG and the HP system were restored to operable status at 1410 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.36505e-4 months <br /> on 8/22/05.
C. CAUSE OF EVENT
A root cause investigation was conducted which determined that the cause of the ACB 1432 trip was a phase-to-ground fault in the primary winding of potential transformer (PT) T1 in the 1B DG voltage regulator.
The ground fault caused fuse F30 on the Hi side of the primary winding of the PT to open. The Ti PT supplies a reference voltage to the input of the voltage regulator for the DG. When the bus voltage decreases, the reference voltage also decreases and the voltage regulator then increases the generator field current to return the bus voltage to the proper 4160 V level. When fuse F30 opened, the voltage regulator increased the generator field current to the output limit in an attempt to correct a perceived low voltage on bus 143.
With the generator still paralleled to the grid, the voltage could not be increased above 4160 volts. As a result, the VAR output from the generator increased from approximately 1.3 MVAR to approximately 7.8 MVAR. The increased VAR output caused DG loading to increase approximately 200% to approximately 940 amps. At this high current level, the DG overcurrent relay tripped approximately 40 seconds later, which was consistent with the time overcurrent trip setting on the overcurrent relay at 940 amps.
The SAT feed breaker tripped due to the DG overcurrent trip condition. The 1B DG continued to run and supply bus 143 following the trip of the SAT feed breaker.
But because the DG was no longer paralleled to the grid, the generator VARs decreased significantly while the generator output voltage increased to approximately 7000 volts. 480-volt bus 143-1 voltage increased to approximately 800 volts, which resulted in the trip of the 1B DG cooling water pump. The thermal overload for the pump motor tripped from high current due to over excitation of the motor windings from the high bus voltage.
The following components also failed due to the high voltage on bus 143-1:
- Potential Transformer supplying relay and metering from Bus 143, including two 1 amp fuses
- 480V / 120V control power transformer for the 1B DG Lubricating Oil Circulating Pump
- K70 Agastat relay (120V) for the Oil Circulating Pump
- 480V / 120V control power transformer for the 1B DG Lubricating Oil Immersion Heater
- K71 Agastat relay (120V) for the Immersion Heater
- 480V / 120V control relay for Div. 3 125 VDC Battery Room Exhaust Fan 1VDO6C (discovered after Bus 143 returned to service)
- 1VDO1YA/B hydro motor for 1B DG ventilation system The actual cause for the phase-to-ground fault in the T1 PT could not be determined. The most probable cause for the shorted winding is a manufacturing defect. Some voiding in the rubberized insulating material surrounding the failed wire was discovered during the failure analysis, but no conclusive evidence was found to link this to the failure.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. Reactor Core Isolation Cooling (RCIC)[BN] and the Division 1 and 2 ECCS systems were fully operable during the event. A review of the probabilistic risk assessment (PRA) values determined that there was minimal impact on overall plant risk from the event.
This event is considered a safety system functional failure.
E. CORRECTIVE ACTIONS
- The Division 3 buses were meggered acceptably prior to restoring power (complete).
- The 1B DG 24-hour surveillance run was performed satisfactory (complete).
- A modification to add a load limiting capability to the DG voltage regulator to prevent a future bus overvoltage condition will be evaluated (AT# 364534 16)
- A comprehensive evaluation of the Division 3 bus components exposed to the overvoltage condition will be completed and formally documented. Actions will be initiated as appropriate. (AT# 364534-17).
F. PREVIOUS OCCURRENCES
A search for potential transformer failures in DG voltage regulators was conducted and found no previous reportable occurrences at LaSalle.
G. COMPONENT FAILURE DATA
Basler 4200/240 V Potential Transformer, Model Number BE11173001