05000302/LER-1990-010

LER 90-010-00:on 900626,determined That Voltage Dips Caused by Sluggish Voltage Regulator Response Exceed Reg Guide 1.9 Limits.Caused by Improper Load Sequence Interval.Test Plan Being Developed for Testing Voltage regulator.W/900727 Ltr

ML20055J049
Person / Time
Site:	Crystal River
Issue date:	07/27/1990
From:	Boldt G, Stephenson W FLORIDA POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
RTR-REGGD-01.099, RTR-REGGD-1.099 3F0790-16, 3F790-16, LER-90-010, LER-90-10, NUDOCS 9008010005
Download: ML20055J049 (4)

LER-2090-010,

Event date:
Report date:
3022090010R00 - NRC Website

text

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i Florida l Power CORP 0H ATION i July 27, 1990 <

3F0790-16 U. S. Nuclear Regulatory Commission Attention: Document Control. Desk  ;

Washington, D. C. 20555

Subject:

Crystal River Unit 3 Docket No. 50-302 Operating License No' DPR-72 Licensee Event Report No.<90-010

Dear Sir:

Enclosed is-Licensee Event Report'(LER)90-010 which is submitted in  ;

accordance with 10 CFR 50.73.-  ;

Sincerely, b G. L'. oldt i

Vice resident, Nuclear Production-WLR: mag Enclosure 21.3 ,

xc:- Regional . Administrator, Region II  !

Senior Resident-Inspector 1

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I 9003010005 900727 l F DR ADOCK 0300 2

[6 M-GENERAL OFFICE: 3201 Thirty-fourth Street South

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Voltage Dips caused by Sluggish Voltage Regulator Response Exceed Reg. Guide 1.9 Limits  ;

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f On June 26, 1990, Crystal River Unit 3 (CR-3) was operating in MODE 1 (POWER.

OPERATION) at 70.2% reactor power, conducting core physics testing following a l refueling outage. While reviewing data gathered during Emergency Diesel  !

l Generator (EGDG) testing in Refuel 7, it was determined that some voltage dips for the diesel generators were not in accordance with REG. GUIDE 1.9 and a load sequence timer interval was not as expected. Voltage dips to slightly j less than 75% of nominal voltage were identified for two of the six load 1 blocks on the "A" EGDG and one of the block loads on the ."B" EGDG. A load "

sequence time interval of 4 seconds verses 5 seconds nominal was also identified between two of the blocks on the "A" EGDG. Full. evaluation is not ,

complete, however, the cause of the voltage dip appears to .be a sluggish -

response of the EGDG voltage regulator. The abbreviated time. interval was

! most probably caused by an electrical contact not making contact. Corrective I

action to test and fine tunc the voltage regulator has been planned to improve >

its response. Procedure revisions will be made to incorporate test steps to assure the contacts are making proper contact and to increase the frequency of the surveillance. These actions will assure proper load sequence time intervals.

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EVENT _DESCRIPTIONt

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On June 26, 1990, Crystal River Unit 3 (CR-3) was operating in MODE 1 (POWER OPERATION) at 70.2% reactor power, conducting core physics testing following a refueling outage. During the recently completed refueling outage, Florida Power Corporation modified the block loads and loading sequence of the Emergency Diesel ,

Generators (EK,DG) (EGDG). As part of the post-modification test program, FPC monitored voltages on the 4160 VAC Buses, 480 VAC Engineered Safeguards Buses (EB,BU) (ES Buses) and selected 480/120 VAC loads during the integrated ES response test. This Engineered Safeguards actuation simulates a full ES ,

actuation followed by a loss Of Offsite Power (LOOP). This test was followed by satisfactory performance of the normal surveillance procedures which provided information verifying the operability of the EGDGs. During the review of the test data, it was determined that some voltage dips for the Emergency Diesel Generators were not in accordance with REG. GUIDE 1.9, " SELECTION, DESIGN, AND QUALIFICATION OF DIESEL-GENERATOR UNITS USED AS STANDBY-(0NSITE) ELECTRIC POWER SYSTEMS AT NUCLEAR POWER PLANTS" and a load sequence time interval was not as

! expected.

REG. GUIDE 1.9 specifies that "at no time during the loading sequence should the ... voltage decrease to less than .. 75 percent of nominal". It also specifies that "... voltage should be restored to within 10 percent of nominal within 60 percent of each load-sequence time interval". The. test data shows that for the "A" Diesel Generator voltage dipped lower than the allowed 75 .

percent level by dipping to 72.8 and 71.2 percent when load blocks two and four '

respectively were applied. The "B" EGDG allowed a voltage dip to 74.2 percent for block one loading, The load sequence time interval between block three and block four-loading onto the "A" EGDG was expected to be 5 seconds long and was actually 4 seconds in 3 duration. The abbreviated interval caused block four loads to be applied to the EGDG before the generator voltage had fully recovered from block-three loads.

This may have contributed to the second voltage dip on the "A" EGDG.

CAUSE:

l The excessive dips of the "A" and "B" EGDG voltages were most probably the result of sluggish voltage regulator [EK,RG) responses as well as, in the case of the "A" generator voltage going into block four, an improper load sequence interval.

The improper load sequence interval may have been a result of one contact 1

[EK,CNTR) failing to reclose during the testing sequence; however, testing failed

^

to reproduce this condition.

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While the test data pointed out that the responses of the Emergency Diesel Generators did not meet all the guidance of REG. GUIDE 1.9, it also showed that - '

the system was functional. The voltage dip which occurred when block one loads l were applied to the EGDG were anticipated and accounted for in the Final Safety l Analysis Report (FSAR). The FSAR analysis recognized the occurrence of an initial voltage dip of 28 percent. To compensate for this dip, motor starters were used in the system that would hold in during the transient. Therefore, the i system would work even if the strict guidance of R.G.1.9 was not met. Earlier ,

calculations by FPC have identified those contacts which experience the worst '

voltage dips and which would therefore be most likely to fail to remain closed.

The contactors are specified to be able to remain closed if their voltage were ,

to decrease to a value of 65 percent for two seconds. Since the voltage during "

the dip remained greater than 65 percent and the dip lasted for less than 0.75 seconds, none of the contactors would drop out or " fail". FPC has also tested similar relays and shown the contactors will hold in at voltages even less than 65 percent nominal. There was no impact on public safety from this event.

CORRECTIVE ACTION:

1) A test plan is being developed for testing the EGDG voltage regulator and governor response. Testing will occur in the October 1991 mid-cycle outage.

2) The surveillance frequency for the testing of the_ time delay relays has been changed from every refueling to quarterly.

3) Surveillance procedures for testing the Engineered Safeguards Actuation ,

channels are being revised to provide for testing the single contact which i stuck and its counterparts to assure they are properly aligned prior to l further testing which depends on the proper sequence interval controlled by. j those contacts.  !

4) The sequence interval timers will be replaced with components having greater -

setpoint accuracy. <

i PREVIOUS SINILAR EVENTS There have been no previous similar events reported in the operating history of CR-3.

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