05000266/LER-2013-001
| Point Beach Nuclear Plant Unit 1 | |
| Event date: | 02-06-2013 |
|---|---|
| Report date: | 04-05-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vi)(A) 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2662013001R00 - NRC Website | |
Description of the Event:
On February 6, 2013 at 1132 CST, all three phases of the Unit 1, 1X-03 High Voltage Station Auxiliary Transformer High Side Circuit Switcher, 1F89-112, opened for an unknown reason. An undervoltage condition occurred on 4.16 kV safeguard buses 1A-05 and 1A-06. This resulted in the starting of all four emergency diesel generators (EDGs) and the automatic loading of EDGs GO1 and G03 onto buses 1A-05 and 1A-06, respectively.
The 2X-03 high voltage station auxiliary transformer remained energized throughout the event. An Unusual Event (UE) was declared at 1143 CST based upon the interruption of offsite power to the essential buses. The UE was subsequently terminated at 1340 CST on February 6, 2013, when offsite power was restored to H-02 bus from H-01 bus and to the affected Unit 1 buses from the 2X-03 transformer. Unit 2 maintained offsite power throughout the event.
The undervoltage condition resulted in a separation of the vital buses from off-site power, as designed. The EDGs performed as expected, fast starting and loading onto safeguards buses 1A-05 and 1A-06, respectively.
The plant safety systems functioned as expected.
After assessing the cause of the loss of offsite power and confirming that the redundant circuit for offsite power remained available, offsite power was restored to the Unit 1 safeguards buses by synchronizing the running EDGs to the grid and closing the alternate feed from offsite power. Offsite power remained available to the affected unit (Unit 1) via this redundant path (2X-03).
The loss of offsite power did not constitute a safety system functional failure. Both Unit 1 and Unit 2 remained on-line throughout the event.
Cause of the Event:
The specific cause of this event was the opening of all three phases of the 1F89-112, high side circuit switcher, to the 1X-03 high voltage station auxiliary transformer. This resulted in the degraded voltage condition to safeguards buses 1A-05 and 1A-06. The one-out-of-one open logic for 1F89-112 circuit switcher was satisfied due to spurious contact actuation. The root cause was found to be the original design of the control logic circuitry lacked robustness to mitigate the spurious actuations.
Analysis of the Event
The 345 kV system supplies the high voltage station auxiliary transformers (1/2-X03), which provide the interface to the 13.8 kV system and are the normal offsite power supply for auxiliary loads associated with plant engineered safeguards. Under some conditions, if the normal offsite supply is not available, safeguards equipment can also be supplied from offsite power by back feeding through the main transformer.
The 13.8 kV system boundaries include the high voltage station auxiliary transformer (1/2X-03) to the high side connection of the low voltage station auxiliary transformers (1/2X-04), and various 480 V transformers.
The 13.8 kV system supplies offsite power to the station via the 4.16 kV and 480 V systems. The 13.8 kV and is normally supplied by high voltage station auxiliary transformer 1X-03. The H-02 bus supplies power to low voltage station auxiliary transformer 1X-04. Similarly, the H-03 bus supplies Unit 2 and is normally supplied by high voltage station auxiliary transformer 2X-03. The H-03 bus supplies power to the low voltage station auxiliary transformer 2X-04. The units can be interconnected to alternate supplies by arranging bus tie breakers to connect H-02 to H-01 and H-03 to H-01.
Safety Analysis
The spurious action of the circuit switcher resulted in a low voltage condition on the 1A-05 and 1A-06, safety-related buses, which started the four EDGs. However, the opening of the circuit switcher did not cause a lockout of 1X-03. This was in accordance with the design of the lockout sensor. An undervoltage alone does not cause a transformer lockout. As a result, the automatic transfer to close in the redundant offsite power supply in the switchyard was not initiated, and the GO1 and G03 EDGs proceeded to automatically load to the Unit 1 safety system buses, once they had reached operating voltage and frequency.
An assessment confirmed that the redundant circuit for offsite power remained available. Offsite power was restored to the Unit 1 safeguards buses by synchronizing the running EDGs to the grid and closing the alternate feed from offsite power. Offsite power remained available to the affected unit from this redundant path
Corrective Actions
The corrective action to prevent the recurrence is to modify circuit switchers 1F89-112 and 2F89-152 control logic circuitry to remove the spurious actuation. The actions have been entered into the corrective action program.
Similar Events
Component Failure Data
Description Model Number Circuit Switcher Type G, 3 Gap, Center-Break Manufacturer S & C