05000219/LER-2015-002, Regarding Reactor Scram Due to Digital Protective Relay System Lockout
| ML15196A036 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 07/06/2015 |
| From: | Dostal J Exelon Generation Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RA-15-059 LER 15-002-00 | |
| Download: ML15196A036 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2192015002R00 - NRC Website | |
text
~ExeLon Generat ion RA-1 5-O59 10 CFR 50.73 July 8, 2015 u. s. Nuclea Regultory Comsso Attn: Document Control Desk wasigo, DC; 20556. -000 Oystr Creek Nuclea Geerwatng Stain Renewed Facilit Oprtn Liense No. DPR-18 NRC Docket No.60-219
,Subject:
Licensee Event Report (LER) 2015.002-00, Reactor SCRAM due Digital Protective Relay Systm Lockout Enl se LER 2015-002-00, Reacto Scram due to the Digital Protective Relay System lockout trip which occurred on May 7, 2015. This event did not affect the healt and safety of the public or plant personnel. This event did not result in a salfety system fucioa failure. Trhere are no regulatory comtet maude in this LEA sub~mittal.
Should you have any questions concrig thi letter please contac Mke McKenna
- Repetfly, Oystr Creak Nuclea Generatingo E~nclosure: NRC Form 368, LER 201 5-001-00 cc:
Adiisrtor, NRC Region I NRC Senior Resident nsetr-OytrrekNcerGnrdn to NRC Project Manager - Oyster Creek Nuclear Geeatn Station
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On May 7, 2015 at 1727 hours0.02 days <br />0.48 hours <br />0.00286 weeks <br />6.571235e-4 months <br />, a Main Turbine Trip and subsequent Reactor Scram occurred from a trip of the Main Transformer Differential Lockout Relay, 85T. The Main Transformer Differential Lockout Relay, 88T" is a protection function provided by Digital Protection Relay System U9, (DPR$ B). The 8OST trip signal was sensed arnd cleared, without operator action, In 17 milliseconds (ms) or I cycle prior to the 230KV output breakers opening. The results of extensive troubleshooting ruled out an actual fault in the Main Transformers or leo-phase Bus, and determined that the Main Transformer Differential Relay actuation was a spurious trip, and not due to an actual degraded equipment condition.
ENS 51055 was submitted on May 7, 2015 as required by 10 CFR 50.72 (b)(2)(iv)(B). This Issue Is reportable under 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted In manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(Iv)(B).
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Description of Event
On May 7, 2015 at 1727, a turbine trip and subsequent reactor scram occurred from a trip of the Main Transformer Differential Lockout Relay, 86T. The Main Transformer Differential Lockout Relay, 86T is a protection function
- provided by Digital Protection Relay System UB" (DPRS 1B). The results of extensive troubleshooting ruled out an actual fault in the Main Transformers or Iso-.phase Bus; therefore, the Main Transformer Differential Relay actuation
- was a false trip. The intermittent condition was sensed and cleared on its own In 17 milliseconds (ins) or 1 cycle prior to the 230 KV output breakers opening.
The results of the troubleshooting did not determine a definitive cause. The results of the troubleshooting ruled out an actual fault in the Main Transformers or Iso-phase Bus; therefore, the Main Transformer Differential Relay actuation was a false trip. The differential current on the 'A' Phase sensed by the Digital Protection Relay System B (DPRS B) was intermittent and of short duration. The 42% imbalance exceeded the set point of 30% on the A phase and triggered a A phase differential current fault resulting In 871T trip signal. The trip signal cleared automatically after 17 milliseconds. The possible causes are an intermittent signal in the Current Transformers (GTs) such as a short in the OTs' secondary wiring or an intermittent signal in the PPRS hardware such as a short in a circuit card componenL
Analysis of the Event
Following the actuation, all systems responded as expected: therefore, this event is of low safety significance.
Cause of Event
A complex troubleshooting plan was developed and implemented during the shutdown. As a result of the troubleshooting, no definitive cause of the trip could be ascertained. The root cause team reviewed the troubleshooting plan along with all the data obtained during the troubleshooting and could not positively identify the cause of the trip. Based on the information available and Operating Experience (OPEX) information reviewed, the team determined that the most probable cause was an intermittent short in the CT wiring associated with this trip circuit caused by aging cables adversely impacted by their environment. This circuit has been continuously monitored since the SCRAM. Since installation, the monitoring equipment identified only a single indication of a voltage differential however that Indication, which occurred on June 29, 2015, was caused byea grid disturbance and would not have processed a trip.
All applicable wiring was tested for insulation resistance and continuity. All test results were satisfactory. Age related cable and terminal block degradation (cracked insulation, exposed copper conductors, and corrosion on terminal blocks) was found on the Auxiliary Transformer CT wiring. Megger testing to ground of this wiring and the other CT wiring for the Main Transformers and Main Generator were acceptable. Additionally, since the DPRS data indicated that the condition cleared in l7milliseconds, it would be unlikely to find a hard ground during the testing.
While there is no conclusive evidence as to the, cause of failure It is considered to be the most likely cause.
A risk assessment was required per Procedure PI-AA-125-1001 since the primary or root cause was determined to be indeterminate. The evaluation determined that actions completed as a result of this event investigation were adequate to prevent recurrence and no further actions were required. Since all of the major equipment tested satisfactory and has subsequently been placed back in service after the event, the team concluded that the trip was caused by a spurious trip of the protection circuit. A modification has been installed that replaced the entire circuit NHL: FIVM
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US. NU'CLEAR REGULATORY COMMISSION (01.2014)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
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Oyster Creek, Unit 1 050002~19 t
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NARRATIVE
with a Sudden Pressure Relay trip circuit. Therefore, since all DPRS "E5l" trip functions including the Main Transformer Differential Relay, 87T, trip are disabled, a similar false trip from DPRS "B" would likely be prevented in the future.
Corrective Actions
To address the most probable cause of CT wiring or termhination experiencing an intermittent short, open (momentary high resistance), or induced transient due to aging, the following actions were taken:
A modification was installed replacing the Main Transformer differential trip with a two-out-of -three logic sudden pressure sensing and trip circuit system under Engineering Change Request (ECR) 15-00197.
All DPRS "B trip functions including the Main Transformer Differential Lockout Relay, 87T, trip were disabled. The modification effectively eliminated the potential for a spurious trip from any equipment associated with the DPRS "B" trip functions including the Main Transformer differential trip scheme.
Oyster Creek has established a project to define the critical cables that traverse these areas and to take actions to mitigate the risk of future events for other systems.
Previous Occurrencesi There were no previous occurrences of a spurious DPRS trip at Oyster Creek although several similar issues have been Identified throughout the induetry.
NRC FORM 388A (01-2014)