05000482/LER-2016-001, Regarding Power Potential Transformer Overloading Results in Emergency Diesel Generator Inoperability

Regarding Power Potential Transformer Overloading Results in Emergency Diesel Generator Inoperability

ML16095A207
Person / Time
Site:	Wolf Creek
Issue date:	03/28/2016
From:	Shawn Smith Wolf Creek
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
WO-16-0015 LER 16-001-00
Download: ML16095A207 (5)

LER-2016-001, Regarding Power Potential Transformer Overloading Results in Emergency Diesel Generator Inoperability

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
4822016001R00 - NRC Website

text

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'NUCLEAR OPERATING CORPORATION Stephen L. Smith Plant Manager U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 March 28, 2016 WO 16-0015

Subject:

Docket No. 50-482: Licensee Event Report 2016-001-00, "Power Potential Transformer Overloading Results in Emergency Diesel Generator lnoperability" Gentlemen:

The enclosed Licensee Event Report (LER) 2016-001-00 is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) and 10 CFR 50.73(a)(2)(v)(B), (C), and (D).

This letter contains no commitments. If you have any questions concerning this matter, please contact me at (620) 364-4093, or Cynthia R. Hafenstine (620) 364-4204.

SLS/rlt

Enclosure:

LER 2016-001-00 cc:

l\\tl. L. Dapas (NRC), w/e C. F. Lyon (NRC), w/e N. H. Taylor (NRC), w/e Senior Resident Inspector (NRC), w/e Stephen L. Smith P.O. Box 411 I Burlington, KS 66839 I Phone: (620) 364-8831 An Equal Opportunity Employer M/F/HCNET

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1013112018

~ (11-2015)

LICENSEE EVENT REPORT (LER)

(See Page 2 for required number of digits/characters for each block)

1.

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2. DOCKET NUMBER

3. PAGE 05000 482 1 OF 4

4. TITLE Power Potential Transformer Overloading Results in Emergency Diesel Generator lnoperability

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR I SEQUENTIAL I REV MONTH DAY NUMBER NO.

YEAR 05000 01 28 2016 2016 -

001 00 03 28 2016 FACILITY NAME DOCKET NUMBER 05000

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)

D 20.2201(b)

D 20.2203(a)(3)(i)

D 50.73(a)(2)(i)(C)

D 50.73(a)(2)(vii) 1 D 50.73(a)(2)(viii)(A)

D 20.2201cd>

D 20.2203<a>(3)(ii)

D 50.73(a)(2)(ii)(A)

D 20.2203<a>c1 >

D 20.2203<a><4>

D 50.73(a)(2)(ii)(B)

D 50.73(a)(2)(viii)(B)

D 20.2203<a><2>

D 50.36(c)(1)(i)(A)

D 5o.73<a><2><iii)

D 50.73(a)(2)(ix)(A)

10. POWER LEVEL D

20.2203<a><2><H>

D 50.36(c)(1)(ii)(A)

D 50.73(a)(2)(iv)(A)

D 50.73(a)(2)(x)

D 20.2203(a><2>cm>

100 D

20.2203(a)(2)(iv)

D 20.2203(a)(2)(v)

D 20.2203(a)(2)(vi)

D 5o.36(c><2>

D

50. 73(a)(2)(v)(A)

D 50.46(a)(3)(ii)

[1S.J 50. 73(a)(2)(v)(B)

D 50.73(a)(2)(i)(A)

[1S.J 50. 73(a)(2)(v)(C)

(Kl 50. 73(a)(2)(i)(B) l.19 50. 73(a)(2)(v)(D)

12. LICENSEE CONTACT FOR THIS LER D 73.11ca><4>

D 73.11ca><5>

DoTHER Specify in Abstract below or in PLANT CONDITIONS PRIOR TO THE EVENT 100 % reactor thermal power Mode 1

DESCRIPTION

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6. LER NUMBER YEAR I SEQUENTIAL I NUMBER 2016 001 REV NO.

00

3. PAGE 2 OF 4 On October 6, 2014, at approximately 1326 Central Daylight Savings Time (CDT) during a scheduled 24-hour Run, the '8' Emergency Diesel Generator (EOG) [EllS: EK] unexpectedly tripped and a fire was observed in the electrical excitation control cabinet (NE106). Just prior to the trip, Operations personnel observed smoke coming from the cabinet and identified the source as the Power Potential Transformer (PPT) [EllS: XPT]. The smoke was a deficiency first identified during a post maintenance test run on June 11, 2014. The PPT exhibited the same symptoms during the subsequent surveillances after June 11, 2014.

A failure investigation was performed using available site information as well as offsite hardware failure analysis (HFA), modeling, testing, and third party reviews. It was concluded that the smoking and eventual failure of the PPT on October 6, 2014 was most likely due to overloading. The overloading of the PPT likely resulted from failure of a diode in the power rectifier [EllS: RECT] of the EOG excitation system. Failure of the diode most likely occurred during load transients on June 11, 2014, resulting from a governor actuator [EllS: EK, 65] malfunction. The failure of the diode was the only structure, component or system (SSC) that was inoperable at the start of the event and contributed to the event.

Upon failure of an exciter power rectifier diode, current boost to the generator field is reduced. The voltage regulator compensates by increasing the output of the power amplifier, supplied by the PPT, beyond the load capability of the PPT. The overloaded condition of the PPT leads to increased temperatures which accelerate insulation breakdown and reduce service life. Though a diode failure results in high temperatures of the PPT, the exciter system can sustain this condition for a short period of time. Sustained high temperatu'res of the PPT eventually lead to voltage break over of the winding insulation, creating a rapidly progressing turn to turn short. The short initiates on the primary side due to the higher voltage, resulting in a phase to phase fault on the generator output. As the PPT is down stream of the boost current transformers (CT) [EllS: XCT], the CT's are exposed to the fault current, generating a high current/voltage transient on the power rectifier, subsequently damaging an additional diode. Generator protective relays, detecting the fault, trip the EOG.

The A-EOG was started within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of the inoperability of the 8-EDG for Common Cause Failure, as required by Technical Specification (TS) 3.8.1 Condition 8.3.1. The 'A' EOG was loaded and thermography was performed on the PPT to identify any overheating conditions. The thermography data determined that the 'A'-EDG had tested satisfactorily and remained operable on October 6, 2014.

The '8' EOG was declared operable on October 9, 2014 at 1717 CDT.

REPORT ABILITY Technical Specification (TS) 3.8.1, 'AC Sources - Operating,' requires two diesel generators capable of supplying the onsite Class 1 E power distribution subsystem(s) be operable in Modes 1, 2, 3 and 4. As a result of the HFA, the 'B' EDG was most likely inoperable from June 11, 2014 until October 9, 2014. This exceeded the allowed outage time for one EDG. This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a Condition prohibited by TS from June 11, 2014 until October 9, 2014.

The 'B' EDG was most likely inoperable from June 11, 2014 until October 9, 2014. During the time period, the 'A' EDG was taken out of service for maintenance on July 21, 2014 creating a condition where both trains may have been inoperable. This event is reportable in accordance with 10 CFR 50.73(a)(2)(v)(B), (C) and (D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (B) Remove residual heat; (C) Control the release of radioactive material; or (D) mitigate the consequences of an accident.

ROOT CAUSE The station did not have the ability to assess the degradation of the PPT within the EDG's excitation system that led to the continual operation of a degraded component, resulting in significant equipment failure. Additionally, there were limited preventative maintenance, obsolescence issues that had not been addressed, limited knowledge of the exciter, and the design of the system lacked overcurrent protection/detection of the PPT.

The station continued using the PPT after it was identified as degraded on June 11, 2014. When the smoking was first identified, the PPT was determined to be degraded, but could still perform its safety function due to the EDG satisfactorily performing its surveillances.

The most probable cause of the event is that a single diode failure led to the thermal failure of the PPT.

Due to the reduced contribution of field current and voltage from the Power Current Transformer (PCT) circuitry from a single diode failure, the voltage regulator would task the PPT to supply the remainder of the required current to the field. This increased current would increase the internal temperatures of the PPT, leading to degraded windings within the PPT. A single diode failure would not be noticed immediately as the PPT is appropriately sized to maintain field current and voltage, for short durations.

This condition could only be noticed by the observed smoke from the PPT. The most likely scenario is that the PPT was compromised and failed prior to the companion diode. The turn to turn short on the primary side would cause a high current demand that would be observed by the PCT. That current would then exceed the capability of the companion diode, causing it to fail, shorted. Once the first diode failed the diesel could not perform its function for more than 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The first indication of EDG excitation deficiency was on June 11, 2014.

CORRECTIVE ACTIONS

The Power Potential Transformer and associated cabling have been replaced.

The Current Transformers that feed the loss of fuse relay have been replaced.

All power diodes in each Power Rectifier on both diesels have been replaced.

Further corrective actions are being tracked by Condition Report (CR) 88665.

SAFETY SIGNIFICANCE

3.PAGE 4 OF 4 The 'B' EOG was most likely inoperable from June 11, 2014 until October 9, 2014. During that time period, the 'A' EOG was taken out of service for maintenance on July 21, 2014, creating a condition where both trains may have been inoperable. When the 'A' and 'B' EDGs are inoperable, there are no remaining safety related on-site stand-by AC sources. However, in the event of a complete loss of AC electric power Wolf Creek Generating Station (WCGS) has three (3) non-safety related "Station Blackout" diesel generators that are capable of supplying power to the required engineered safety feature (ESF) functions following a station blackout event. There was no demand for on-site power during the time that both DGs were likely inoperable as power was available from the off-site power sources.

OPERATING EXPERIENCE/PREVIOUS EVENTS A root cause was completed in 2010 on Safety System Function Failures (SSFFs) that identified safety system failures were occurring due to inadequate preventive maintenance (PM). In addition, Root Cause Evaluation (RCE) 24445, 'NRC Performance Indicator (Pl) Unplanned Scrams per 7000 Critical Hours Exceeding Threshold,' identified that the root cause was "Content and timeliness of PM activities were insufficient to support reliable plant operation." The corrective action for RCE 24445 was to 'Develop and Implement a PM Optimization Plan.' A RCE (CR 23119) for 10 SSFFs in a two-year period and the RCE (CR 24445) for four unplanned scrams in a year period both identified insufficient as a root cause. The action plan to address the preventive maintenance issues was coordinated between the two root cause evaluations and was implemented through CR 24445.

• PLANT CONDITIONS PRIOR TO THE EVENT 100 % reactor thermal power Mode 1

DESCRIPTION

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6. LER NUMBER YEAR I SEQUENTIAL I NUMBER 2016 001 REV NO.

00

3. PAGE 2 OF 4 On October 6, 2014, at approximately 1326 Central Daylight Savings Time (CDT) during a scheduled 24-hour Run, the '8' Emergency Diesel Generator (EOG) [EllS: EK] unexpectedly tripped and a fire was observed in the electrical excitation control cabinet (NE106). Just prior to the trip, Operations personnel observed smoke coming from the cabinet and identified the source as the Power Potential Transformer (PPT) [EllS: XPT]. The smoke was a deficiency first identified during a post maintenance test run on June 11, 2014. The PPT exhibited the same symptoms during the subsequent surveillances after June 11, 2014.

A failure investigation was performed using available site information as well as offsite hardware failure analysis (HFA), modeling, testing, and third party reviews. It was concluded that the smoking and eventual failure of the PPT on October 6, 2014 was most likely due to overloading. The overloading of the PPT likely resulted from failure of a diode in the power rectifier [EllS: RECT] of the EOG excitation system. Failure of the diode most likely occurred during load transients on June 11, 2014, resulting from a governor actuator [EllS: EK, 65] malfunction. The failure of the diode was the only structure, component or system (SSC) that was inoperable at the start of the event and contributed to the event.

Upon failure of an exciter power rectifier diode, current boost to the generator field is reduced. The voltage regulator compensates by increasing the output of the power amplifier, supplied by the PPT, beyond the load capability of the PPT. The overloaded condition of the PPT leads to increased temperatures which accelerate insulation breakdown and reduce service life. Though a diode failure results in high temperatures of the PPT, the exciter system can sustain this condition for a short period of time. Sustained high temperatu'res of the PPT eventually lead to voltage break over of the winding insulation, creating a rapidly progressing turn to turn short. The short initiates on the primary side due to the higher voltage, resulting in a phase to phase fault on the generator output. As the PPT is down stream of the boost current transformers (CT) [EllS: XCT], the CT's are exposed to the fault current, generating a high current/voltage transient on the power rectifier, subsequently damaging an additional diode. Generator protective relays, detecting the fault, trip the EOG.

The A-EOG was started within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of the inoperability of the 8-EDG for Common Cause Failure, as required by Technical Specification (TS) 3.8.1 Condition 8.3.1. The 'A' EOG was loaded and thermography was performed on the PPT to identify any overheating conditions. The thermography data determined that the 'A'-EDG had tested satisfactorily and remained operable on October 6, 2014.

The '8' EOG was declared operable on October 9, 2014 at 1717 CDT.

REPORT ABILITY Technical Specification (TS) 3.8.1, 'AC Sources - Operating,' requires two diesel generators capable of supplying the onsite Class 1 E power distribution subsystem(s) be operable in Modes 1, 2, 3 and 4. As a result of the HFA, the 'B' EDG was most likely inoperable from June 11, 2014 until October 9, 2014. This exceeded the allowed outage time for one EDG. This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a Condition prohibited by TS from June 11, 2014 until October 9, 2014.

The 'B' EDG was most likely inoperable from June 11, 2014 until October 9, 2014. During the time period, the 'A' EDG was taken out of service for maintenance on July 21, 2014 creating a condition where both trains may have been inoperable. This event is reportable in accordance with 10 CFR 50.73(a)(2)(v)(B), (C) and (D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (B) Remove residual heat; (C) Control the release of radioactive material; or (D) mitigate the consequences of an accident.

ROOT CAUSE The station did not have the ability to assess the degradation of the PPT within the EDG's excitation system that led to the continual operation of a degraded component, resulting in significant equipment failure. Additionally, there were limited preventative maintenance, obsolescence issues that had not been addressed, limited knowledge of the exciter, and the design of the system lacked overcurrent protection/detection of the PPT.

The station continued using the PPT after it was identified as degraded on June 11, 2014. When the smoking was first identified, the PPT was determined to be degraded, but could still perform its safety function due to the EDG satisfactorily performing its surveillances.

The most probable cause of the event is that a single diode failure led to the thermal failure of the PPT.

Due to the reduced contribution of field current and voltage from the Power Current Transformer (PCT) circuitry from a single diode failure, the voltage regulator would task the PPT to supply the remainder of the required current to the field. This increased current would increase the internal temperatures of the PPT, leading to degraded windings within the PPT. A single diode failure would not be noticed immediately as the PPT is appropriately sized to maintain field current and voltage, for short durations.

This condition could only be noticed by the observed smoke from the PPT. The most likely scenario is that the PPT was compromised and failed prior to the companion diode. The turn to turn short on the primary side would cause a high current demand that would be observed by the PCT. That current would then exceed the capability of the companion diode, causing it to fail, shorted. Once the first diode failed the diesel could not perform its function for more than 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The first indication of EDG excitation deficiency was on June 11, 2014.

CORRECTIVE ACTIONS

The Power Potential Transformer and associated cabling have been replaced.

The Current Transformers that feed the loss of fuse relay have been replaced.

All power diodes in each Power Rectifier on both diesels have been replaced.

Further corrective actions are being tracked by Condition Report (CR) 88665.

SAFETY SIGNIFICANCE

3.PAGE 4 OF 4 The 'B' EOG was most likely inoperable from June 11, 2014 until October 9, 2014. During that time period, the 'A' EOG was taken out of service for maintenance on July 21, 2014, creating a condition where both trains may have been inoperable. When the 'A' and 'B' EDGs are inoperable, there are no remaining safety related on-site stand-by AC sources. However, in the event of a complete loss of AC electric power Wolf Creek Generating Station (WCGS) has three (3) non-safety related "Station Blackout" diesel generators that are capable of supplying power to the required engineered safety feature (ESF) functions following a station blackout event. There was no demand for on-site power during the time that both DGs were likely inoperable as power was available from the off-site power sources.

OPERATING EXPERIENCE/PREVIOUS EVENTS A root cause was completed in 2010 on Safety System Function Failures (SSFFs) that identified safety system failures were occurring due to inadequate preventive maintenance (PM). In addition, Root Cause Evaluation (RCE) 24445, 'NRC Performance Indicator (Pl) Unplanned Scrams per 7000 Critical Hours Exceeding Threshold,' identified that the root cause was "Content and timeliness of PM activities were insufficient to support reliable plant operation." The corrective action for RCE 24445 was to 'Develop and Implement a PM Optimization Plan.' A RCE (CR 23119) for 10 SSFFs in a two-year period and the RCE (CR 24445) for four unplanned scrams in a year period both identified insufficient as a root cause. The action plan to address the preventive maintenance issues was coordinated between the two root cause evaluations and was implemented through CR 24445. *