ML20198E739

Special Rept:On 970703,diesel Generator Failed Surveillance Test Procedure ST-6-092-311-2.Caused by Motor Operated Potentiometer (Mop) Set Below Its Normal 60 Hz Position. Stroked Mop Through Full Travel While Monitoring Resistance

ML20198E739
Person / Time
Site:	Limerick
Issue date:	08/04/1997
From:	Boyce R PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9708080084
Download: ML20198E739 (3)

Text

Rtbert W.Brycs 4 .

Plant Manager l "

4 Lrnenck Generatre Staton -

\

,,. 4 PECO NUCLEAR nco ree-Umenck Gereratrg Staten A Unit of PECO Energy Pocon2300 Sanatoga. PA 194644920 010 718 2000 T.S. 6.9.2 August 4, 1997 Docket No. 50-353 License No. NPF-85 U.S. Nuclear Regulatory Commission Attn:-Document Control Desk Washington, DC 20555

SUBJECT:

Limerick Generating Station, Unit 2 Special Report for a Valid Emergency hlfhflhlfl Diesel Generator Test Failure

REFERENCE:

Technical Specifications Sections 4.8.1.1.3 and 6.9.2 This Special Report is submitted pursuant to the requirements of Technical Specifications (TS) Section 6.9.2 as required by TS Surveillance Requirement 4.8.1.1.3. TS Surveillance Requirement

4. 8.1.1. 3 requires reporting of all diesel generator failures, valid or non-valid, within 30 days. -The report is required to include the

- information recommended in Regulatory Position C.3.b of Regulatory Guide (RG) 1.108, " Periodic Testing of Diesel Generator Units as -Onsite Electric Power Systems at Nuclear Power Plants," Revision 1, August 1977 On July 3, 1997 at 1400-hours, the D21 Emergency Diesel Generator (EDG) was prepared for a slow start in accordance with Surveillance Test (ST) procedure ST-6-092-311-2, "D21 Diesel Generator Slow Start Operability Test Run." After the proper prelube period, the EDG started and came up to a speed in the range of 300 to 400 rpm as expected. The Equipment

- Operator (EO), stationed in the D21 EDG room, gradually dialed the speed control knob on the mechanical governor clockwise until EDG speed rose to approximately 900 rpm. Once at 900 rpmi the EO placed the  !

3 Exciter shutdown / reset switch to' RESET. The field then flashed and EDG- V frequency could be. read directly from a gauge-on the EDG control panel.

Af ter waiting 10 seconds the EO turned the speed control knob fully clockwise to its full high speed stop. The expected result of this i action was for -EDG frequency to settle out at approximately -60 Hz; -

however, frequency actually rose to 62 Hz. At this point, the EO centacted the Main Control Room (MCR). The MCR operator attempted to lower EDG speed by momentarily moving the speed governor control switch in the MCR to LOWER. EDG frequency responded and the MCR operator stopped adjustment when frequency reached 60 Hz.

Operations personnel alerted the Instrumentation and Controls group (I&C) of the problem while the D21 EDG was still running. The System 9708000004 970804 PDR ADOCK 05000353 S PDR

August 4, 1997 Page p.of 3 Manager and an I&C technician performed a cursory inspection of the system. Initial observations did not identify anything out of the ordinary. The lead IGC technician then removed the cover to the Motor Operated Potentiometer (MOP) to observe its setting. He noted that the MOP was set below its normal 60 Hz position. Operations then secured the D21 EDG. As EDG speed decreased during shutdown, the MOP returned to its preposition setting as expected.

I&C initiated a Troubleshooting Control Form to investigate the event and determine the cause of the frequency excursion. Several inspections and tests were performed.

1. All terminations in the electronic governor control system were checked for tightness. There were no loose terminations found.

2. The MOP was stroked through its full travel while its resistance was monitored. No dead spots, points of high resistance or other inconsistencies were observed.

3. Preposition of the MOP was verified from the full increase

" .flockwise ) and full decrease (counterclockwise) positions. The MOP returned to center point smoothly from both directions.

4. The control cable between the electronic and mechanical governors was checked for continuity. No signs of discontinuity were found.

The D21 EDG was then slow started on July 3, 1997 at approximately 1640 hours0.019 days <br />0.456 hours <br />0.00271 weeks <br />6.2402e-4 months <br />, for a troubleshooting run to determine if the event would repeat. Upon flashing of the field and increasing the speed control knob to its full clockwise position, the generator settled out at the expected frequency of 60 Hz. The following live voltages were checked on the electronic governor to verify its functionality.

1. All three phases of the AC input voltage.

2. The summing junction (zener) voltages.

3. The speed setting voltage.

4. The speed sensor voltage.

All voltages were found to be within the manufacturer's specifications.

The EDG was then paralleled to the grid without incident. Load was then increased to 500 Kw and then 1,000 Kw with no observable problems. The safeguard busses feeding the D21 safeguard bus were then swapped. No problems were encountered during the transfer. Load was then reduced and the D21 EDG was secured at approximately 1715 hours0.0198 days <br />0.476 hours <br />0.00284 weeks <br />6.525575e-4 months <br /> on July 3, 1997.

On July 4, 1997 at 0544 hours0.0063 days <br />0.151 hours <br />8.994709e-4 weeks <br />2.06992e-4 months <br />, the D21 EDG was tested in accordance with procedure ST-6-092-311-2. The run was successfully completed and at 0810 hours0.00938 days <br />0.225 hours <br />0.00134 weeks <br />3.08205e-4 months <br /> the D21 EDG was declared operable.

Troubleshooting did not identify any obvious cause of this event.

However, during this event, the I&C technician observed that the MOP was below its 60 Hz reference voltage setpoint. This indicated that the MOP may have been responsible for the frequency fluctuation. A work activity was initiated to replace the MOP during the next system maintenance outage.

i

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. '$ugust4, 1997 P. ages 3,of 3 This event was reviewed using the guidance of RG 1.108, Revision 1, August 1977, Section C.2.e (5) with respect to EDG failures. The frequency fluctuation resulted in a valid test failure of the D21 EDG.

Since this is the second test failure for the D21 EDG in the last 20 valid demands, the ST procedure monthly frequency is required to be performed weekly in accordance with TS Section 4.8.1.1.2.a.

On July 10 and 17, 1997, the D21 EDG was successfully tested in accordance with procedure ST-6-092-311-2. On July 24, 1997, the MOP was replaced. During performance of the test run to setup the MOP, the EDG frequency again fluctuated. Due to the difficulty in troubleshooting an intermittent problem, a conservative decision was made to replace both the electronic and mechanical governors.

On July 26, 1997, during testing of the replaced components, the D21 EDG output voltage increased above its setpoint and remained at that level despite the opening of the load breaker. This transient resulted in a number of equipment problems including the automatic closure of the Drywell Chilled Water System primary containment isolation valves, an Engineered Safety Feature (ES F) actuation . This ESF actuation will be reported separately in Limerick Generating Station LER 2-97-008.

Troubleshooting of this transient revealed that one of the two rectifier banks was malfunctioning. On August 1, 1997, the operable rectifier was placed in service and was successfully tested.

On August 1, 1997, during performance of the 24-hour endurance run for the D21 EDG, an unrelated problem was identified with the scavenging air blower. The EDG was secured and an inspection revealed that the upper inboard seal was indicating signs of leakage. A conservative decision was made to replace the scavenging air blower. On August 4, 1997, the scavenging air blower was replaced and successfully tested.

At 0600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br /> on August 4, 1997, performance of the 24-hour endurance run commenced. The D21 EDG is expected to be returned to an operable condition on August 5, 1997.

All components (i.e., MOP, electronic and mechanical governors, rectifier, and scavenging air blower) will be sent to their respective manufacturers for failure analyses. A supplement to this *;eport is expected to be issued by November 28, 1997, to address the cause(s)and corrective measures.

For additional information, please contact Mr. Timothy A. Moore at (610) 718-3400.

Very truly yours, t

DMS/JWB cc: H. J. Miller, Administrator Region I, USNRC N. S. Perry, USNRC Senior Resident Inspector, LGS