ENS 46201
ENS Event | |
|---|---|
13:14 Aug 24, 2010 | |
| Title | Loss of Offsite Power to Emergency Station Service Transformer |
| Event Description | During planned maintenance on the Startup Station Service Transformer (SSST), an undervoltage condition occurred on the 69 kV offsite power supply to the Emergency Station Service Transformer (ESST). At the time of this event, utility personnel were isolating 161 kV offsite power to the SSST from the grid. As a result, the offsite power voltage to the ESST lowered below the level where the essential 4160 VAC buses are automatically prevented from loading on the ESST.
From 0814 to 0816, BKR IFS and 1GS Auto Closure Not Permitted alarms were received when the system control operator was switching out the 161 kV line which supplies the SSST. ESST Secondary Winding voltage lowered to 4309 V and the 69 kV line voltage lowered to 69.4 kV. The nominal setpoint for the Closure not Permitted action is 4330 V. Actions were taken per the alarm response procedures. The ESST was declared inoperable as the automatic closure of the supply breakers to the 4160 V essential buses was precluded. With the SSST already inoperable, LCO 3.8.1 Condition C (two offsite circuits inoperable) was entered for the ESST inoperability. Required action C.1 directs declaring required features inoperable when the redundant required features are inoperable within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of discovery; and required action C.2 directs restoring one offsite circuit to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. At 0816, the system control operator placed the 69 kV capacitor bank in service, restoring the 69 kV line voltage to 71.5 kV and ESST Secondary Winding voltage to 4461 V. At 0846, the ESST was declared Operable following restoration of supply voltage, and meeting acceptance criteria of SR 3.8.1.1, and exited the above LCO required actions for the ESST. The plant remains in LCO 3.8.1, Condition A for the SSST inoperability for planned maintenance. This condition is being reported in accordance with 10CFR50.72(b)(3)(v) as a condition that could have prevented fulfillment of a safety function. Per the discussion in NUREG 1022, both offsite electrical power and onsite power are considered to be separate functions by GDC 17. Per the GDC, the offsite sources must be capable of automatically connecting to the essential buses within a few seconds. Due to the automatic transfer to the ESST being prevented, the auto transfer function described in the GDC was prevented. The lowest voltage recorded by the system control operator was 69.4 kV, corresponding to 4309 V on the essential 4160 V buses. The 69 kV line voltage was promptly restored, and the Closure not Permitted alarms cleared. The ESST could have been manually aligned to the essential buses during this event. The licensee notified the NRC Resident Inspector. |
| Where | |
|---|---|
Cooper  Nebraska (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| LER: | 05000298/LER-2010-003 Regarding Low Voltage on Emergency Transformer Causes Loss of Safety Function |
| Time - Person (Reporting Time:+3.72 h0.155 days <br />0.0221 weeks <br />0.0051 months <br />) | |
| Opened: | John Myers 16:57 Aug 24, 2010 |
| NRC Officer: | Joe O'Hara |
| Last Updated: | Aug 24, 2010 |
| 46201 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |