ML20248D194

AEOD/T915, Ground Fault Detection & Trip Circuit at Perry Unit 1

ML20248D194
Person / Time
Site:	Perry
Issue date:	09/25/1989
From:	Mazumdar S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20248D144	List: ML20248D139 ML20248D194 ML20248D204
References
TASK-AE, TASK-T915 AEOD-T915, NUDOCS 8910040152
Download: ML20248D194 (2)

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~AEOD TECHNICAL REVIEW REPORT

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I sVNIT: , Perry, Unit 1- TR REPORT NO.: AE0D/T915 1

1. ' DOCKET NO.:' 50-440'. DATE: 1989

n LICENSEE: Cleveland Electric Company ' September 25,S.

EVALUATOR / CONTACT:

Mazumdar

NSSS/AE: GE/ Gilbert:

SUBJECT:

GROUND FAULT DETECTION AND TRIP CIRCUIT AT PERRY UNIT 1 EVENT.'DATE: May.25, 1989 A

SUMMARY

0n May 25 1989, the licensee discovered that with the existing high resistance grounded scheme a single line-to-ground fault on an individual load could trip

'the EDG breaker before the individual load would trip. In this plant, certain

non-safety buses are fed from safety-related buses and a licensee investigation

, indicates that certain fire or seismic events can cause loss of offsite power j with a concurrent loss of Division 1 and 2 EDGs. As a corrective action, the y

' licensee.has disabled the EDG trip operation on single line-to-ground fault C .'and has added control room and local alarms to alert the operator of a ground

~ fault.

9

~ DESCRIPTION OF: EVENT In this; plant, the EDG neutrals are high resistant grcunded to limit single

.line-to-ground faults to 2 Amps using a neutral grounding transformer and a

grounding resistor. -The original design of the EDG protection circuit would initiate.the EDG trip ~whenever a voltage above a preset valve is sensed across.the grounding. resistor.

~On'May 25,'1989 during plant design engineering review, the licensee discovered v ithat with the original,EDG protection scheme, a single ground fault anywhere in the EDG distribution system could trip the EDG withou; tripping the faulty

< feeder breaker.

Furthermore,.in this plant certrin non-safety buses are fed from safety-related buses fed by Division'I and 2 ELGs. A license investigation has established that' because of.the phy.sical arrangement of non-safety electrical equipment in the plant ,a' single fire or seismic event could cause a loss of offsite power as well as, simultaneous ground faults on non-safety related loads supplied by each EDG.:'Thus,'a single event could cause loss of offsite power with a y . concurrent. loss of Division.1 and 2 EDGs.

SIMILAR EVENT SEARCH.

l Dak' Ridge National Laboratory (ORNL) conducted a search of the SCSS for events

, involving relays or control circuitry in the Emergency Power Generation

, . Systems, similar to the cvent at Perry described in LER 89-017. This search

• This document supports ongoing AE0D and NRC activities and does not represent the. position or requirements of the responsible NRC program office.

8910040152 890925 '

i. , PDR ADOCK 05000440

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-identified'441LERs. ORNL letter, July 28, 1989, specifying the search commands

, 'is attached. Our review of these LERs has. identified only one event (Surry 1,

,, LER87-028)inwhichanEDGtrippedbecausetheinstantaneousovercurrenttrip setting was not properly co-ordinated. -Thus, this issue appears to be limited in' extent.

IMPLICATION OF DISABLING GROUND FAULT TRIP

'In solid or. low resistance grounded systems every effort is made to isolate a faulty circuit at the earliest opportunity to minimize equipnent damage, fire

.and personnel hazard.

In a high resistance grounded system similar to the one in this plant in which

~

the single line-to-ground fault current is limited to two amperes, the system can continue to operate for extended period with a single line-to-ground fault.

The major draw back of. such extended operation with single line-to-ground fault is that in the event of a second fault between another phase and the ground, heavy fault current can flow in the system. Normally, the phase

'overcurrent devices protect the system against heavy faults. However, to avoid occurrence of such heavy faults, it is desirable to limit the operation with single line-to-ground fault to the shortest duration possible. This is achieved

-by initiating an alarm on the occurrence of the first line-to-ground fault which alerts the operator to' isolate the faulty circuit at the earliest opportunity. Thus, the licensee is now using the most desirable scheme for resistance grounded EDGs.

CONCLUSION As a high resistance grounded system can operate for extended period with a single line-to-ground fault,_the original protection scheme to trip the EDG imediately on-occurrence of the first line-to-ground fault is unnecessary.

On the other hand, the proposed change to disable the_EDG overcurrent trip and provide local and control room alarm in the event of a single line-to-ground c fault, permits continuation of EDG operation with orde.rly isolation of the faulty circuit. A_ search of SCSS indicates that this issue is plant specific and not a generic problem and no further AEOD action is required.

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