05000331/LER-2010-001
| Document Numbersequentia Revmonth Day Year Year Month Day Year 05000L Number No. | |
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| ENS 45603 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| LER closed by | |
| IR 05000331/2010002 (14 May 2010) | |
| 3312010001R00 - NRC Website | |
I. Description of Event:
On January 1, 2010, while operating at 100% power, the 'A' Emergency Diesel Generator (EDG) automatically started from a valid undervoltage condition, but did not load onto its respective 4160 VAC bus. The momentary undervoltage condition was caused by a momentary fault on the 161 kilovolt (kV) Vinton Line. The momentary fault was caused by a failed lightning arrester on the 161 kV line in the Duane Arnold Energy Center (DAEC) switchyard. The 'A' EDG supply breaker was not required to close onto its respective bus as it remained powered from its normal power supply during and after the undervoltage event. Offsite power remained fully operable during and following the event.
As designed, the 'A' Emergency Service Water (ESW) system automatically started. Additionally, in response to the momentary undervoltage condition, the '13' Well Water Pump, the RWCU System, and the Spent Fuel Pooling Cooling System tripped.
II. Assessment of Safety Consequences:
There were no structures, systems, or components (SSCs) that failed during this event. The automatic starting of the 'A' EDG in response to a short-duration under-voltage condition had no personnel or radiological safety significance. Since the 'A' EDG was capable of fulfilling its safety function throughout the event and offsite power remained fully operable during and following the event, there was no nuclear safety significance.
This event did not result in a Safety System Functional Failure.
Therefore, this event did not result in any radiological or nuclear concern which would impact the health and safety of the public.
III. Cause of Event:
An Apparent Cause Evaluation (ACE 2020) was completed for this event. The ACE detelinined that the `A' EDG started automatically during a condition where there was no Loss of Offsite Power or Degraded Voltage condition. This occurred because the sequential loading relay for the 'A' EDG provided, as designed, a non-safety start signal when its respective essential bus, 1A3, voltage momentarily dropped below 65% of nominal. Bus voltage dropped when a lightning arrester in the switchyard failed, causing a fault which pulled 'A' phase-ground voltage on the 161kV bus down to approximately 4kV. The protective relaying cleared the fault within 4.5 cycles (0.075 seconds), well within the design perfoii�lance of the switchyard protection equipment.
The sequential loading relay gave a start signal during what is considered a normal grid transient before the relaying and circuit breakers in the switchyard substation were able to remove the fault. Therefore, any fault affecting the local grid has the potential to start the EDGs.
The apparent cause of this event is a lack of coordination of the EDG automatic start logic and the DAEC switchyard protective relaying. Specifically, the EDG automatic start logic does not have a built in tolerance for grid disturbances.
A contributing factor was the failure of the lightning arrester on the 161kV Vinton line, which was most likely caused by a combination of moisture ingress while the line was previously de-energized for planned maintenance and time in service beyond its expected life.
IV. Corrective Actions:
Corrective Actions to Address Condition All three lightning arresters on the Vinton line were replaced on 1/29/10.
Corrective Actions to Address Apparent Cause Modifications will be made to the 'A' and 'IV standby diesel generator start logic to introduce a time delay into the non-safety bus under voltage start so that the start signal coordinates with the grid relaying.
This modification is currently due 12/15/2010.
Corrective Action to Address Extent of Condition The lightning arresters on the 161kV Fairfax and Hiawatha lines have been replaced.
V. Additional Information:
Previous Similar Occurrences:
A review of LERs over the previous 5 years revealed the following similar occurrences:
EIIS System and Component Codes:
EK — Emergency Onsite Power System EB — Low Voltage Power System — Class lE FK — Switchyard System Reporting Requirements:
This event was reported under 10CFR50.72(b)(3)(iv)(A), 'Any event or condition that results in valid actuation of any of the systems listed in paragraph (b)(3)(iv)(B) of this section' due to the Auto Start of the 'A' EDG. Reference EN # 45603.