05000219/LER-2015-003
On November 9, 2015, while attempting to commence a #1 Emergency Diesel Generator (EDG) test procedure, the #1 EDG failed to start and the "EDG 1 Disabled' Alarm annunciated in the Main Control Room. Locally, the operator heard relays chattering on the generator end of Diesel Generator #1 and notified the control room that the SEQ alarm was illuminated. Subsequently, a fast start of the #1 Emergency Diesel Generator was attempted but was unsuccessful. The cause of the failure was determined to be a failed zero speed relay (ZSR). The ZSR failure was caused by an internal component fallure'associated with age degradation of the Time Delay Module of the relay assembly.
The identification of this failure resulted in EDG #1 being declared inoperable and entry into an unplanned 7-day LCO at 0348. The repairs concluded on November 10, 2015 at 0400-and the #1 EDG was declared operable at 0634. This issue is reportable under 10 CFR 50.73(a)(2)(i)(B) as an Operation or Condition which was Prohibited by the plant's Technical Specifications since the likely time of failure would have been during the previous EDG#1 run which was conducted on October 26, 2015. NRC FORM 388 (014014) ' NRC FORM 35$A U.S. NUCLEAR REGULATORY COM ION X01-2014) P Al; VE Y OM NO. 31 IRE 01/31/2011 Wasiinton, uu 20503. If a means used to impose an information does not dankly a care* valid OMB control minim, the NM may not conduct or sponsor, and a person's not Reportd itasons foamed are incorporated Into the 6c , process and fed back to industry. Sand comments regard ng burden estimate to the FOfA, Rimy and Intonation Collections Branch (T.5 F53), U.S. Nuclear Riguitary Coovniston, Washington, DC 20655-0001, or. by Internet ameil to Iricceiscis.RoourcsOnrc.gov, and to the Deek Mot, Office of informadon and?Watt/ Mao, NE08-10202, (31500104), Olgen of =Pent end Budget required to respond to, the information collection. 2: DOCKET 05000219, $. LER NUMBER 3. PAGE OF 4 2014 - 003 - 01 ' ContentsDescription of EventOn November 9, 2016 at 0348, when Operations attempted a normal start of the Emergency Diesel Generator
The Operators then notified the Main Control Room that the SEQ alarm Was illuminated. Due to this condition, EDG #1 was declared inoperable and an unplanned risk change to yellow.occurred. Notifications were performed for the unplanned risk change and a prompt investigation was initiated. The station entered a 7-day shutdown Limiting Condition for Operation (LCO) per Technical Specifications 3.7.C.2. As part of this LCO, EDG # 2 operability was verified. EDG #1 was quarantined for troubleshooting. Complex Troubleshooting identified that the Zero Speed Relay (ZSR) was not de-energized or dropped out as expected from the last EDG #1 load test on 10/26/2015. There is no alarm or panel indication for the failure. The energized state of the relay would have precluded any start of EDG #1 since that test. The repairs concluded on November 10, 2016 at 0400 and the #1 EDG was declared operable at 0634. Equipment Dericriptima The Emergency Diesel Generators are designed to start automatically and supply AC power to Class 1 E buses during degraded voltage or loss of voltage conditions on 1E, Loss of Offsite Power (LOOP), or a LOOP with a Loss of Coolant Accident (LOCA), for safe shutdown and coot down. The ZSR is manufactured by Vapor Corporation under model number 39430622. The relay is mounted in .a control panel enclosure located in the. MG compartment. This location experiences mild temperatures during EDG operations and standby conditions. The most recent replacement of the ZSR relay for both EDG #1 and EDG #2 occurred in 1996. The ZSR is a normally de-energized, time delay relay with a fixed time delay of 2 seconds. Upon an EDG start signal, the relay is energized by ESSB (Engine Speed Sensing relay) when the engine speed increases to 40rpm. The MR prevents the starter motor engagement for next start attempt if the engine has not stopped. The relay contact returns to normal state after 2 seconds timeout following 0 rpm. This interlock is not bypassed by a fast start signal. Analysis cif Event The failed relay was sent to Exeton's PowerLabs for functional testing and analysis. PowerLabs' Failure Analysis report (OYS-25105) concluded that the ZSR's time delay unit failed to de-energize, which left voltage on the ZSR coil that was sufficient to prevent ZSR dropout. The cause of the excessive output voltage from the Time Delay Unit is likely attributed to degraded internal components. Attempts were made to identify the specific component(s) that was faulty within the Time Delay Unit. However, when the outer cover of the Time Delay Module was removed the Internal components were found to be encapsulated in a hardened potting compound that prohibited evaluating its components. The environment during the testing was approximately 71-73 degrees F. Powertabs' report indicated that the Time Delay Module would fail to de-energize the relay at ambient temperature. NR - FO 34 (01.2014) In addition to the ZSR failure, potential leakage through surge suppressing rectifier CR-2A was considered. This device was replaced with the ZSR replacement. The CR-2A rectifier was also sent to PowerLabs for testing. PowerLabs analysis concluded that the rectifier was free of any deficiencies and did not contribute to the failure mechanism. Assessment of Safety ConsequencesTwo diesel generator units serve as the Standby Power Supply for the station by providing an emergency source of power to the 4.16 kV buses 1C and ID In the event of a loss of normal power. The diesel generator units are designed to start and load automatically, if required. Nonessential loads are automatically shed by undervoltage sensing devices on loss of offsite power to ensure that the units are not overloaded. The capacity of the units is sufficient to sequentially energize for starting all safety related pumps and auxiliaries required for a safe shutdown of the reactor in the event of a Design Basis Accident. The diesel generator units are independent of each other, with the exception of a common bulk fuel storage supply, and are provided with auxiliary systems to ensure reliable starting and continuous operation with no operator attention. Power to start the units is self-contained and is not dependent on the availability of any other source of normal plant power at the moment of initiation. There are two types of automatic start signals to the diesel generator units. The first signal will cause the Units to start and idle. The second signal is called the Fast Start Signal. The Emergency Diesel Generator allowable time response to a Loss of Offsite Power (LOOP) event is 20 seconds as a basis for Core Spray System response to accident conditions. The time response period Includes UV sensor pickup time, emergency bus logic to isolate and actuate the Emergency Diesel Generators and the period to bring the Emergency Buses to normal voltage level. The failure of the Zero Speed Relay would have precluded the #1 EDG from being able to start and load in the event of a degraded or toss of voltage condition. The #2 EDG would have continued to operate and would have supplied thelD 4160V bus as required to ensure power to the components required to achieve cold shutdown. The #2 EDG was evaluated to not be susceptible to a common cause failure and was maintained in an operable condition in accordance with station procedures. Cause of EventThe ZSR failed to drop-out and remained energized from the last EDG #1 bi-weekly load test and prevented the fast start of EDG #1 during the current load testing. There Is no alarm or panel indication for a ZSR failure. Complex troubleshooting identified the energized state of the ZSR; however, there were no visible degraded conditions kientified that would have caused the relay to remain energized. The relay was subsequently replaced and EDG # I was restored to an operable status. The failed relay was sent to Exelon's Powertabs for functional testing and analysis. PowerLabs' Failure Analysis report (0VS-25105) concluded that the ZSR's time delay unit failed to de-energize, which left voltage on the ZSR coil that was sufficient to prevent ZSR dropout. The cause of the excessive output voltage from the Time Delay Unit is likely attributed to degraded internal components. Attempts were made to identify the specific component(s) that was faulty within the Time Delay Unit. However, when the outer cover of the Time Delay Module was removed the internal components were found to be encapsulated in a hardened potting compound that prohibited evaluating its components. The environment during the testing was approximately 71-73 degrees F. PowerLabs' report indicated that the Time Delay Module would fail to de-energize the relay at ambient temperature. The apparent cause of the ZSR failure was determined to be en internal component failure associated with age degradation of the Time Delay Module of the relay assembly. Due to the internal components encapsulated in a dense potting material, it was not possible to determine what specific component of the Time Delay Module was failed. The following immediate actions were taken:
Corrective ActionsIn order to address the Apparent Cause the following actions were (or are being) taken:
Previous OccurrencesThere have been no previous events resulting from a failure of a normally de-energised Vapor Corporation relay. Component Data Component IEEE 605 System ID IEEE 8034 Function Emergency Diesel Generator EK DO NRC FORM 2A (01-2014) |
| Oyster Creek, Unit 1 | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2192015003R00 - NRC Website | |