05000277/LER-2011-003
| Peach Bottom Atomic Power Station (Pbaps) Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2772011003R00 - NRC Website | |
Unit Conditions Prior to the Discovery of the Event Units 2 was in Mode 1 operating at approximately 100% rated thermal power and Unit 3 was in Mode 5 for its 18th refueling outage when this event occurred on 9/21/11. At the time of the event, undervoltage relay functional testing and Loss-of-Coolant Accident (LOCA) / Loss of Offsite Power (LOOP) functional testing was in progress for the E-33 4 kV bus. The E-33 4 kV bus was considered inoperable during the performance of this testing. The E-33 4 kV bus functional testing was a planned evolution being performed during the P3R18 refueling outage.
The four site Emergency Diesel Generators (EDGs) are common to both Unit 2 and Unit 3.
Description of the Event
On 9/21/11 at approximately 1020 hours0.0118 days <br />0.283 hours <br />0.00169 weeks <br />3.8811e-4 months <br />, an unplanned, valid actuation of the E-3 EDG (EllS:
EK) occurred during surveillance testing. This actuation occurred as a result of an undervoltage condition of the E-33 4 kV bus for a time period longer than planned during the performance of surveillance test ST-O-054-753-3, "E33 4kV Bus Undervoltage Relays and LOCA LOOP Functional Test." At the time of the event, ST-O-054-753-3 was being performed, in part, to test the capability of the E-33 bus to transfer from the 00A019 offsite power source to the 00A020 offsite power source by tripping the 00A019 offsite power source feeder breaker (3-54-1708).
When the 3-54-1708 feeder breaker (EllS: BKR) was tripped, the 4 kV bus voltage lowered and the 00A020 offsite power source feeder breaker (3-54-1701) should have closed within a quarter-second to re-energize the 4 kV bus. However, as a result of a malfunction of logic relay 3-54-183-1708 (EllS: 2), the bus transfer was delayed for approximately 3 seconds. This resulted in the actuation of an undervoltage relay resulting in the valid acutation of the E-3 EDG.
The undervoltage relay is required by Technical Specifications (TS) and is designed to start the EDG in the event that the offsite source auto-transfer fails to occur.
Subsequent to the initiation of the E-3 EDG, the 00A020 offsite source feeder breaker closed.
Because the transfer to the 00A020 offsite source occurred prior to the E-3 EDG reaching full speed and voltage, the E-3 EDG output breaker was not required to close. As a result of the E- 3 EDG actuation, the Emergency Service Water (ESW) / Emergency Cooling Water (ECW) pumps also started as designed.
The E-3 EDG and the ESW/ECW pumps (EllS: BI) were promptly secured by licensed operations personnel and the surveillance test was halted to evaluate the unexpected equipment response.
This event was reported to the NRC as a valid system actuation at 2247 hours0.026 days <br />0.624 hours <br />0.00372 weeks <br />8.549835e-4 months <br /> on 9/21/11 (EN 47286).
The 3-54-183-1708 time delay relay was replaced on 9/22/11.
Analysis of the Event
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(iv)(A) — Valid System Actuation — During the E-33 4 kV bus undervoltage relay and LOCA/LOOP functional test, an unplanned, valid actuation of the E-3 EDG occurred.
There were no actual safety consequences associated with this event.
The standby AC power supply consists of four EDGs (common to both Unit 2 and Unit 3) whose safety objective is to provide a reliable source of electrical power to eight safety related 4kV buses for safe shutdown of the Unit 2 and Unit 3 reactors. Since two off-site power sources are connected to each of the eight 4 kV buses, the failure of one off-site power source supplying power to the bus results in transfer to the second off-site source. The EDGs are automatically connected to their associated 4 kV emergency buses after the generator voltage and frequency are established, bus voltage is zero, and all bus loads are tripped. The necessary safeguard loads are then automatically sequenced onto the EDG powered buses.
The 4 kV buses are continuously energized by an offsite source during planned power operation. The voltage for each 4 kV bus is monitored by two different undervoltage functions:
loss of voltage and four levels of degraded voltage conditions. Upon failure of the preferred off- site source, an undervoltage relay trips the off-site source breaker and initiates auto-transfer to the alternate source in approximately a quarter-second. If the auto-transfer fails, the relay starts the associated EDG as required by TS 3.3.8.1 Table 3.3.8.1-1, Loss of Power Instrumentation.
As a result of this event, the auto-transfer did not occur promptly. The auto-transfer occurred in approximately 3 seconds instead of a quarter-second as a result of a degradation of the 3-54- 183-1708 logic relay. As designed, the TS undervotage relay operated and the E-3 EDG started. Subsequent to the initiation of the E-3 EDG, the Unit 3 off-site source feeder breaker closed. Because the transfer to the Unit 3 off-site source occurred prior to the E-3 EDG reaching full speed and voltage, the E-3 EDG output breaker was not required to close. As a result of the E-3 EDG actuation, the Emergency Service Water (ESW) / Emergency Cooling Water (ECW) pumps also started as designed.
During the period of time that the 3-54-183-1708 was degraded, had a design basis LOCA / LOOP event occurred, this condition would not have affected the plant response to this event.
If a LOCA and a loss of the 00A019 offsite power source occurred, the impact on the ability to power the E-33 bus would not have been consequential. The TS 3.3.8.1 undervoltage functions were unaffected by this condition.
11 - 003 - 00 Cause of Event The cause of the event was due to the failure of the 3-54-183-1708 time delay relay to properly function (ie., the relay actuated in approximately 3 seconds instead of the required quarter- second time.) The relay is calibrated on a 4-year frequency and was most recently calibrated on 9/15/11 with as-found acceptable results. This relay has historically demonstrated good performance during calibration checks.
The failed relay was sent offsite for laboratory analysis. At the laboratory, the relay was tested multiple times resulting in a measured time delay of approximately a quarter-second (i.e., normal times). Although the relay operated normally at the laboratory, recommendations provided by the laboratory personnel are being evaluated in the corrctive action program.
The auto-transfer relay 3-54-183-1708 is an 'Agastat' relay provided by Amerace Corporation (Model No. ETR14D3AC2002003).
Corrective Actions
The 3-54-183-1708 time delay relay was replaced on 9/22/11.
Additional actions and extent of condition considerations are being evaluated within the corrective action program.
Previous Similar Occurrences There were no previous LERs identified involving similar relays affecting the 4 kV power source protection relays.