ML20116B849

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Revised Special Rept SR 95-03:on 950621,EDG 1-2 Failed to Load During Surveillance Testing.Caused by Loose Fuse Holder Contact Clip.Retensioned Loose Fuse Holder Contact Clip & Tightened Loose Wire Connection
ML20116B849
Person / Time
Site: Diablo Canyon Pacific Gas & Electric icon.png
Issue date: 07/22/1996
From:
PACIFIC GAS & ELECTRIC CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20116B844 List:
References
DCL-96-155, SR-95-03, SR-95-3, NUDOCS 9607300307
Download: ML20116B849 (8)
v • d • e


Text

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Enclosura 1 ,

h PG&E Letter DCL-96-155 l

l l-REVISION TO SPECIAL REPORT 95-03, EMERGENCY DIESEL GENERATOR 1-2 FAILURE TO SUCCESSFULLY LOAD DUE TO DEGRADED VOLTAGE REGULATOR PERFORMANCE  :

In accordance with Regulatory Guide (RG) 1.108, Section C.3.b, the following l information is included:

t (1) Diesel aenerator involved: Emergency Diesel Generator (EDG) l l 1-2 l (2) As of April 1.1996. the number of valid failures in the last 100 EDG 1 1-2 valid tests: 3 i (3) Cause of failure: The root cause could not be determined based upon available evidence. The most probable cause was identified to be a loose fuse holder contact clip in the 4 kV potential transformer sensing circuit to the automatic and manual rectifier bridges and the magnetic amplifier.

(4) Corrective measures taken:

a) Immediate corrective actions: PG&E retensioned a loose fuse holder contact clip and tightened a loose wire connection at the motor operated potentiometer (MOP). At this time, PG&E successfully tested EDG 1-2 in accordance with Surveillance Test Procedure (STP) M-9A, " Diesel .

Engine Generator Routine Surveillance Test." Temporary Volts-Ampere Reactance (VAR) monitoring equipment was installed on EDG 1-2 to monitor voltage regulator input and  ;

output parameters during the next monthly surveillance tests, b) To prevent recurrence: Although PG&E believes that the existing surveillance testing process, in conjunction with RG ,

1.108 requirements, provides appropriate identification of voltage regulator performance problems, the following actions will be taken to prevent recurrence:

! PG&E has revised the recurring task work orders for

preventative maintenance (PM) on the EDG electrical

} equipment to include a step that requires maintenance

, personnel to inspect for loose fuse holder contact clips.  !

9607300307 960722 -l-PDR ADOCK 05000275 S PDR -

Enclosura 1 ,

PG&E Letter DCL-96-155 I If- loose fuse holder contact clips are discovered, the fuse holder contoct clip will be retensioned and the fuse will be ,

. reinstalled.

(5) Time EDG was unavailable: EDG 1-2 was cleared to investigate i the cause of VAR fluctuations on June 22,1995, at 0140 PDT. ,

The control switch for EDG 1-2 was returned to automatic after j successful performance of STP M-9A on June 23,1995, at 0032 PDT. Thus, EDG 1-2 was unavailable for a total of 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> and 52 minutes.

(6) Current surveillance test interval: 31 days. l (7) Confirmation of proper test interval: As of April 1,1996, the total l [

number of valid failures in the last 100 valid tests for EDG 1-2 is 3, and the total number of valid failures in the last 20 valid tests for EDG 1-2 is 0; therefore, the 31 day test interval is in compliance t with the accelerated test schedule of Technical Specifications (TS)

Table 4.8-1.

1. Plant Conditions  !

Unit 1 was in Mode 1 (Power Operation) at 100 percent power.

II. Description of Event R

A. Summary In accordance with the requirements of TS 6.9.2 and 4.8.1.1.4 and  ;

Revision 1 to NRC RG 1.108, PG&E is -submitting this revision to

, Special Report 95-03 concerning failure -of EDG 1-2 to I successfully load during surveillance testing. 1 l

Using the guidance of RG 1.108, Sections B and C.2.e, this event i is considered to be a valid failure because EDG 1-2 did not successfully load due to malfunction of voltage regulation equipment that is required to be operable in the emergency operating mode. During this event, all other electrical power sources were available if called upon in the unlikely event of an actual emergency. EDG 1-2 was returned to operable status within the allowed outage time of TS 3.8.1.1 action statement b-therefore, the requirements of TS 3.8.1.1 were satisfied.

B. Background -

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Enclosuro 1 l PG&E Letter DCL-96-155 The exciter-voltage regulator provides and controls current to the y field winding of the EDG to maintain the voltage within the regulator band from no load to full load. The generator exciter-voltage regulator is designed with controls in the main control room and in the local diesel generator room. The exciter-voltage regulator is designed to operate in two modes: (1) isochronous, as l an independent source or (2) droop, in parallel with the offsite 230 kV or 500 kV systems. During performance 'of STP M-9A, the L. exciter-voltage regulator is operating in the droop mode because the EDG is paralleled through the 4160 V system to the PG&E grid.

STP M-9A implements TS 4.8.1.1.2 and is normally performed on  ;

i a 31-day testing frequency. STP M-9A starts and runs each EDG for a minimum of one hour. STP M-9A is performed to demonstrate proper startup to verify that the required voltage and frequency are ,

automatically attained within acceptable limits and time. l l C. Event l-  !

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l On June 21,1995, at 2118 PDT, testing of EDG 1-2 was initiated in  ;

accordance with STP M-9A. EDG 1-2 started, accelerated, and loaded. At 2134 PDT, VAR oscillations occurred on EDG 1-2, approximately ten minutes after paralleling to Bus G.

Subsequently, Operations reduced load and separated EDG 1-2  ;

from Bus G, shut down the EDG, and declared EDG 1-2 inoperable. Maintenance personnel began troubleshooting the l VAR fluctuations by opening the EDG voltage regulation electrical  ;

panel. A loose fuse holder contact clip and a loose connection at l  !

the MOP were discovered. Both connections were tightened. No l other abnormalities were identified. j 1

l On June 23,1995, at 0103 PDT, EDG 1-2 was declared operable after the successful performance of STP M-9A.

Between June 23 and August 19,1995,17 EDG 1-2 STP M-9As
j. were performed and no VAR swings were observed.

D. Inoperable Structures, Components, or. Systems that Contributed to the Event None.

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Enclosuro 1 PG&E Letter DCL-96-155 E Dates and Approximate Times for Major Occurrences

1. June 21,1995, at 2134 PDT: Event / Discovery date. EDG 1-2 started per STP M-9A but did not successfully complete the surveillance test due to VAR oscillations.
2. June 23,1995, at 0103 PDT: EDG 1-2 was declared l operable after successful performance of STP M-9A.

F. Other Systems or Secondary Functions Affected None.

G. Method of Discovery Plant personnel identified the problem during the performance of STP M-9A.

i H. Operator Actions None.

i 1. Safety System Responses l None.

l 111. Cause of the Event l A. Immediate Cause

! PG&E has determine that the most probable immediate cause of this event was a loose fuse holder contact clip in the fuse holder in the 4 kV potential sensing circuit to the automatic and manual j rectifier bridges and the magnetic amplifier. A loose fuse holder contact clip can change the resistance of the circuit depending on the contact of the fuse to the fuse holder and affect voltage regulation.

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Enclosuro 1 J PG&E Letter DCL-96-155 B. Root Cause f PG&E conducted a root cause analysis including a thorough investigation into maintenance practices, a review of vendor and 1 industry operating experience, and a review of historical '

information with respect to loose MOPS and fuse holder contact  !

clips on all six EDGs. However, conclusive evidence of a root  ;

cause has not been found; therefore, the root cause is indeterminate.

The following categories of possible root causes for the EDG 1-2 '

valid failure were reviewed:  !

1. Component Defects '

Both loose connections were tightened and each component .

tested satisfactorily. Thus, there is no evidence of a defect in the fuse, fuse holder contact clip, or MOP that would have' l caused or contributed to the EDG 1-2 valid failure.

2. Installation j Fuse holder contact clips: The original fuse holder contact  :

clip tension may have been minimal. This condition may  ;

have deteriorated over time during installation and removal of fuses and caused the loose fuse holder contact clip.

MOP wire: Based on interviews with Technical Maintenance- [

technicians investigating the event, the wire on the MOP ,

was verified tight during the initial troubleshooting (swing i shift) on June 21,1995. On June 22,1995, the MOP wire t was discovered loose, therefore; the loose wire connection .

was most likely caused by troubleshooting activities.  ;

1 The MOP was previously inspected during the Unit 1 sixth l refueling outage (1R6), on April 10,1994. All connections were -inspected and tightened as necessary. In addition, l STP M-9A has been performed on a 31-day frequency and EDG 1-2 has never had a valid failure due to a loose electrical connection. Therefore, evidence does not exist to indicate that original installation of the wire connection caused or contributed to the EDG 1-2 valid failure.

l Enclosuro 1 PG&E Letter DCL-96-155 i4 j 3. Operating Environment i Mechanical vibration levels: The vibrational forces from an

[ operating EDG could not have caused either the fuse holder

l' l contact clip, MOP wire connection, or components to vibrate l loose. The electrical cabinet containing both degraded j c

. omponents are mounted on the EDG room floor. Vibration

levels at the cabinet are not large enough to loosen fuse I holder contact clip or wire terminations.

I Electrical vibratie ' levels: The vibrational forces generated j by the energizac voltage regulation circuit could not have j caused.either component to vibrate loose. The voltage and  !

power levels are not high enough to induce electrical  ;

!- vibration. Therefore, evidence does not exist to indicate that i either mechanical or electrical vibrational forces caused the j- . components to loosen. ,

l 4. Routine Maintenance

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Fuse holder contact clips: Fuses inserted and removed l l numerous time during maintenance may have changed the holder contact clip tension. Also, the fuse holder may have l

. been damaged during fuse replacement. During 1R6, on i

April 26,1994, all EDG 1-2 fuses were visually inspected .

satisfactorily. A review of historical information with respectL '

to loose fuse holder contact clips on -the EDGs has l determined that no previous events have occurred.  !

Therefore, contact clips were noloosenedevidence exists during routineto support that the fusel

-maintenance.

MOP: A PM check is performed after maintenance is performed on any component in the EDG system to ensure  ;

proper tightness of all wire terminations. In addition, during l every refueling outage a PM check is performed to verify i l

that all wire connections in this circuit are tight. During 1R6 on April 10,1994, all MOP -connections were inspected for tightness. Therefore, no evidence exists to indicate that the wire terminations were not tightened during the PM.

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. Enclosuro 1 PG&E Letter DCL-96-155

5. Vendor and Industry Operating Experience Discussions with the vendor and other industry experts indicated that the VAR oscillations could have been caused by the loose fuse holder contact clip or loose wire on the MOP.

A review of vendor and industry operating experience indicates that most EDG failures involving malfunctioning voltage regulation circuitry resulted from malfunctioning MOPS and not loose MOP connections or fuse holder .  ;

contact clips. No problem with the operation of the MOP {

was found during this event.

IV. Analysis of the Event Safety-related (Class 1E) electrical loads are supplied from three 4160 V vital buses (F, G, and H) for each unit. Each vital bus can be supplied from the 500 kV switchyard, the 230 kV switchyard, the main generator, or l the EDGs. The EDGs can provide power for engineered safeguards (Class 1E) motors and loads used for emergency core cooling, reactor shutdown, and other vital safety functions when the main generator and offsite power sources are not available in the event of a loss-of-offsite power or other design-basis event.

During a design-basis event, with EDG 1-2 in a degraded condition, one bus of vital 4160 V power would not have been available for Unit 1 support. However, EDGs 1-1 and 1-3 were available to provide power to the other two Unit i vital 4160 V buses during the time that EDG 1-2 was inoperable. Subsequent to this event, EDGs 1-1 and 1-3 were successfully tested in accordance with STP M-9X, " Operability Verification."

Since only two vital buses are necessary to support Diablo Canyon Power Plant accident analysis, the health and safety of the public were not adversely affected by this event.

V. Corrective Actions i A. Immediate Corrective Actions

1. The voltage regulator MOP loose wire connection was tightened. The MOP was functionally tested on its full range.

4 Enclosuro 1 -

PG&E Letter DCL-96-155

2. The loose fuse holder contact clip holding the fuse was l retensioned to the proper tightness and the fuse was reinserted. ,
3. EDG 1-2 was successfully restarted and fully loaded. The VARs were verified to be fully adjustable from the control-room.

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4. EDG 1-2 was successfully tested in accordance with STP M-9A.
5. Temporary VAR monitoring equipment was installed on EDG 1-2 to monitor voltage regulator input and output parameters during the next monthly surveillance test.

B. Corrective Actions to Prevent Recurrence The recurring task work orders for PM on the EDG electrical equipment was revised to include a step that requires maintenance ,

personnel to inspect for loose fuse holder contact clips. If a loose ,

fuse holder contact clip is discovered, the fuse holder contact clip will be retensioned and the fuse will be reinstalled t

PG&E has determined that no corrective actions are necessary to preclude a loose wire connection at the voltage regulator MOP.

The wire connection at the MOP is checked for tightness on an 18-month frequency. There is reasonable evidence to conclude the wire was loosened during troubleshooting activities and there have been no other cases of loose wire connections at the MOP.

Therefore, this event alone does not warrant an increase in the PM frequency.

VI. Additional Information A. Failed Components i

l None.

B. Previous Similar Events None. I 1

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