ML18058B246

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LER 92-039-00:on 921030,reactor Automatically Tripped Due to Loss of Load Resulting from Unstable Voltage to Turbine Control Sys Computers.Caused by Failure to Maintain Power Voltage to Ups.New Transformers installed.W/921125 Ltr
ML18058B246
Person / Time
Site: Palisades Entergy icon.png
Issue date: 11/25/1992
From: Roberts W, Slade G
CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-92-039, LER-92-39, NUDOCS 9212030486
Download: ML18058B246 (7)
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Text

consumers Power GB Slade GenerafMariager l'DWERINli .

MICHlliAN"S l'IUlliRESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 4904.3 .

November 25, 1992 Nuclear RegulatorY Commission*

Document Control Desk Washington, DC 20555

  • DOCKET 50~255 - LICENSE DPR PALISADES PLANT -

LICENSEE EVENT REPORT 92-039 - AUTOMATIC REACTOR TRIP ON LOSS OF LOAD RESULTING FROM UNSTABLE VOLTAGE TO THE TURBINE CONTROL SYSTEM COMPUTERS Licensee_ Event Report (LER)92-039 is attached.* This event is reportable in accordance with 10 £FR 50.73(a)(2)(iv) as an event that resulted in the automatic actuation of the reactor protective system; Gerald B Slade General Manager CC Administrator, Region III, USNRC NRC Re~ident Inspector - Palisades Attachment 9212030486 921125 PDR ADOCK 05000255 S . PDR

  • A cMS ENCRGY COMPANY

NAC Form 366

!9-BJI

  • 9*

U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB NO. 3160-0104 EXPIRES: 8/31 i66

. LICENSEE EVENT REPORT (LERI FACILITY NAME 11 I DOCKET NUMBER 121 PAGE 131 Palisades Plant 01s101010121s1s ' I OF 0 1s TITLE 141 AUTOMATIC REACTOR TRIP ON LOSS OF LOAD RESULTING FROM UNSTABLE VOLTAGE TO THE TITRRTNF. rnN'T'ROT * ~V~'T'F.M f'OMPTT'T't;>l2~ '

EVENT. DATE 161 LEA NUMBER 161 REPORT DATE 161 OTHER FACIL.ITIES INVOLVED !Bl SEQUENTIAL REVISION FACILITY NAMES MONTH DAY YEAR YEAR NUMBER NUMBER MONTH DAY YEAR N/A 0161010101 I

11 0 310 9 2 912 0 13 19 010 i 1. 1 .21s. 912 N/A 0161010101 I THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR I: ICh<<:lc one OI' mOl'9 of rM followifl(ll 11 1 I OPERATING N MOOE 191 I

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- 20.4061oll1 Hivl 20.40611111 IM - 60. 7 3 loll211iil 60.731oH211iiil LICENSEE CONTACT FOR THIS LER 1121

60. 7 3 loll211viiillBI 60.731oll211xl NRC Form 386AI .

NAME TELEPHONE NUMBER William L. Roherts, Staff Licensing Engineer sARtA,cj°~ J 1 I I 4 I - I aI 9 I , I 3 s

COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT 1131 MANUFAC- REPORTABLE MANUFAC-

  • REPORTABLE CAUSE SYSTEM COMPONENT TUR ER TO NPROS CAUSE. SYSTEM COMPONENT. TURER TO NPRDS I I I I I I I I I I *I I I I I I I I I I I I I I I I I I MONTH DAY YEAR n

SUPPLEMENTAL REPORJ' EXPECTED 1141 EXPECTED SUBMISSION Y.ES Vf yu~ cOl'np/*t* EXPECTEf SUBMISSION DATEI *

~NO DATE 1161 ABSTRACT ILimir ro 1400 - ... i.* .. appro*im.r.Jy fihfffl. llin~-* typewrin.n linul I 181 *,

I *1 I

On Octobet 30, 1992~ at 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br />, the plant was operating at 100% ~ower. As a result of a turbine gen~rator trip the reactor automatically tri~ped on a loss of load si~nal. The turbine generator trip was initiated by coincident s utdown of the primary an backu~

computers in the turbine generator digital electrohydraulic (DEHf control system. Pant response to the event was good, with no safety significant *devia ions or anomalies noted.

BD design the* DEH com~u~ers were being s~~pl ied from a single ~nin~erruptable ~ower sup~ly

( PS{. The cause of his ev~nt was a fa1 ure.of.the UPS to maintain ~ro~er vo tage at~ he

. inpu to the DEH ~ower supplies. Volta~e variations at the UPS out~u (i.e., the DEH 1nput

  • . power suppl1) cou d not be handled by t e DEH system and all four D H computers * . **
  • automatical y went to their fail safe condition tripping the turbine generator. . . .

Corrective action for this event included addin~ redundancy to the computer power circuits;

.adlusting volta9e and installing new cables on he existing power source; installing two new vo ta?e regulating transformers in the back-up power source; install in~ new power supplies in al four DEH data processing computers; and adding alarms to the DE control room console. .

' NRC Form 3118A li-831

  • .e LICENSEE EVENT REPORT (LERI TEXT CONTINUATION U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMB NO_. 3160-¢1 O*

EXPIRES:. 8/31186

-: FACILITY NAME 111 . DOCKET NUMBER 121 LER NUMBER 131 PAGE l'I .

SEQUENTIAL REVISION YEAR NUMBER

  • NUMBER

. Palisades Plant o I s Io Io Io I2 I s Is 9 12 - o I 3 I 9 - o I o . ol 2 oF ob

" EVENT DESCRIPTION On October 30, I992, at approximately 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br />, the plant was operating at IOO% power with all systems in normal full power alignment, when the reactor automatically tripped on a loss of load signal. The loss of load signal was a result of an unstable voltage (power supply) to the turbine digital electrohydraulic (DEH) control iystem [TG;CPU] computers.'

Subsequently all four of the DEH data processing units shut down, causing a turbine trip signal which enabled the associated Reactor Protective System loss of load trip. Low ste~m ~enerator levels in both generators injtiated an AFWAS and the start of one

Operations entered Emergency Operating Procedure (EOP) I, ~Post Trip Actions" and later EOP 2, "Trip Recovery" to stabi 1i ze the p1ant in hot sh.utdown. The trip recovery actions . *

. included emergency boration that was due to the failure of Bus IA to transfer to start~up power and. the resulting loss of 2 of the 4 operating primary coolant pumps.

  • A post trip review*was completed shortly after the event. The following are highlights from the post trip review report~ -
  • The 4I60 VAC non-safety related Bus IA supplying two of the.four primary coolant pumps~ failed to transfer to start-up power from station power, resulting in the loss of two primary coolant puinps.
  • The secondary rod pos.iti*on for control rod drive number I6 remained at its*.

previous indication of about 73 inches following the.trip~ the primary indication showed that the control rod had dropped to the bottom of the core.

Two of the four plant control room event recorders (3 and 4), did not activate on the trip. -

This event is rep6rtable in a~corda~ce with .IO CFR 50.73 (a)(2)(iv) as an event that

.resulted in the automatic actuation of the reactor protective system.

CAUSE OF THE EVENT The cause of this event was a failure of the uninterruptable power supply (UPS) to maintain proper voltage at the input to the DEH power supplies. Voltage variations at the UPS output (i.e. the DEH power supply input) could not be handled by the DEH system and.

the DEH computers automatically went to their fail safe condition .. All four DEH computers shut down, tripping the turbine generator. This turbine generator trip resulted in the 1oss of 1oad trip of the reactor. * * ..

ANALYSIS OF THE EVENT

  • The plant had previously tripped on July 1 and July 24, 1992, on Joss of load. sig~als

'generated from a loss of power to the DEH system. After the July 1, 1992 event, post trip testing had dup 1i cated the p1ant trip by what was determined to be loose computer .*

./.

NRC Form 388A (1-831 LICENSEE EVENT REPORT !LERI TEXT CONTiNUATION U.S. NUCLEAR REGUUITORY COMMISSION APPROVED OMB NO. 3160-<1.10'

  • EXPIRES: 8/3i186 FACILITY NAME 111 DOCKET NUMBER 121 LER NUMBER 131 PAGE 141 SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant Q 5 Q Q Q 2 5 5 9 2 - Q 3. 9 - Q Q Q 3 . OF Q connecting cables. Following the July 24, I992 plant trip, it had been determined that input voltage fluttuations to the DEH computer~ had been the probable cause for both the July I, I992 and the July 24~ I992 plant trips~ The July 24, I992 plant trip occurred coincident with the performance of a plant test which calls for the plant sequ~ncer tb simultaneously start a number of safety injection system loads. The starting of this equipment resulted in a mqmentary system voltage drop. This low voltage input was

_interpreted by the DEH as a loss of primary power. The DEH then attempted to switch to its secondary power source. The DEH's power supply voltage dropped to below approximately 4.5 volts while it was seeking a stable power source from its secondary power supply, and when the voltage went lower than 4.5 VAC, the DEH shut itself down to prevent damage.

With the DEH not in operation a turbine trip was initiated. To correct this deficiency an uninterruptable power supply with an extended.battery capacity was added to the system to filter any erratic incoming voltage to the DEH data processing units power supplies; On October 30, I992, by design, the DEH computers were being supplied from a single uninterruptable power supply. Because of the low and unstable input voltage condition, the UPS sw.itched to. a bypass condition _where it was no longer trying to condition the .

  • incoming voltage. As a result of .this switching, the lOw and fluctuating voltage was now fed directly to the DEH computer DC po~er supplies. We estimate that the UPS had been:.

powering the DEH at approximately I32 VAC. When the UPS was bypassed the* unfiltered line voltage now powering the DEH DC power supplies was estimated to be approximately I08 VAC directly from the plant distribution panel. The DEH power supplies could ncit withstand

  • this -approximate -24 .VAC transient in its power feed and since the UPS was the prtm_a.ry power source to the two controlling and the two standby computers, all .four computers shut down and the turbine trip signal was initiated. * *
  • Subsequent investigation has shown that the UPS was designed to switch to its battery back-up at an input voltage of approximately 176 VAC. The voltage ~upplied to th~ UPS was determined to be, on average, approximately I82 VAC versus a normal input voltage of 208 VAC plus or minus IO%. It is estimated that the voltage t~ the UPS varied fro~ a low lev~l of 176 VAC to a highest level of I87 VAC preceding the tri~. It is believed that this low and varying voltage condition for extended periods of tiine led to the UPS inverter shutting down its rectifier and placing the UPS in the bypass mode.

The 4I60 VAC non-safety related Bus IA supplying two of the four primary coolant pumps failed to transfer to start-up power from station power, resulting in the loss of two

  • primary ~oolant pumps. This start~up ~ower breaker had been replaced with a spare following a similar failure to transfer during the July I, I992 plant trip: Investi~ation of the July-I failure to transfer found that the breaker.cubicle interlock was not properly aligned. Corrective actions were taken and the breaker operated successfully during three subsequent plant trips prior to the October 30, I992 plant trip.

NRC Form 388A e- U.S. NUCLEAR REGULATORY COMMISSION 11-831 APPROVED OM8 NO. 31S0--01*0,

EXPIRES
  • . 8/31 /86 LICENSEE EVENT REPORT (LERI TEXT CONTINUATION FACILITY NAME 111 DOCKET NUMBER 121 LER NUMBER 131 PAGE 141 SEQUENTIAL. REVISION YEAR *. NUMBER NUMBER Palisades Plant 0 I5 I0 I0 I0 I2 I5 I5 9 I2 - 0 I3I9 - 0. I0 0 I4 OF 0 I5 During diagnostic testing following tha October 30 plant trip, using both the spare installed breaker and the breaker that was removed in July, we were able to rec*reate the breaker failure to close. Review of a video tape of the breaker in operation showed that the cubicle interlock foot pedal bounces during breaker operation. The purpose of this interlock is to ensur~ that the breaker is tri~ped prior to moving the breaker within the cubicle or withdrawirig the breaker from the cubicle .. The bouncing of the foot pedal allows it to strike the trip lever and trip the breaker. Visual inspection also showed that the metal floor of the breaker cubicle has an approximate three-eights inch bow. This flex in the floor apparently amplifies the interlotk pedal bounce since installation of

. temporary shims allowed successful 'testing of the breaker. A temporary modification was installed to pin the interlock foot pedal to prevent inadvertent operation. Operation of the breaker is not affected, but the pin must be removed before tha breaker c~n be

  • withdrawn from the cubicle.

A blown fuse was found in the common DC supply to control room event recorders .3 and 4.

_The fuse was replaced. Work order history has shown that this fuse has a history of blowing without finding a root cause. Replacement fuses with visual indicatibn of blown condition were _insta1led and they will be checked frequently to assure that the event

  • recorders remain in service unt i 1 a re so 1ut ion_ to the fuse b1owing is fou_nd.

During initial troubleshooting to investigate why the secondary rod position for control rod 16 i~dicated 73._inches when the rod was at the bottom of the core, the relay for the secondary rod position was tapped and the indication returned to its proper reading. The secondary position indication problems are- being tracked by enginee-ring as* a lohg range operations concern item and therefore are being addressed on a programmatic basis.

While a reactor trip on losi of lbad is a challenge to. the plant safety systems because of the plant trip, all plant safety systems performed as designed and no significant safety deviations or anomalies were noted~

CORRECTIVE ACTION The existing DEH DC power suppiies were changed to a.power supply which has improved switching and input voltage tolerance. -

The AC power supply to the back-up DEH tomputers was provi~ed with power from a source independent of the UPS. This provides a degree of redundancy for the priinary, and backup computers. -

Taps were changed on the transformer supplying power to the UPS to re~tor~ the voltage to a nominal 208 VAC from the previous 198 VAC. Further, additional electrical ca6le was run between the power panel and the UPS to reduce th~ voltage drop to less than 3% of nominal.

Voltage regulating transformers were installed to the DEH power supply inputs not supplied by the UPS, to help to condition the input voltage_ and to reduce the tisk of unregulated voltage reaching the computers.

NRC Form 398A U.S. NUCLEAR REGULATORY COMMISSION 19-831 ~OVED OMB NO. 3160-010' EXF'lRES: '8/31186 LICENSEE EVENT REPORT (LERI TEXT CONTINUATION FACILITY NAME 111 DOCKET NUMBER 121

  • LER NUMBER 131 SEQUENTIAL REV1!110N YEAR NUMBER NUMBER Palisades Plant
  • 0 5 0 0 0 .2 5 5 9 2 - 0 .3 9 - Q Q Q 5 OF Q 5 Alarms "UPS On Battery", and "UPS On Bypass", were added to the DEH system to.provide the controJ* room operators with an early warning of potential UPS problems.
  • Temporary voltage wave-form disturbance analyzers were installed on the DEH and UPS systems to monitor voltage conditi-0ns during system operation for the remainder of this operating cycle.

All of the above mentioned enhancements were successfully tested and prov~n to have significantly improved the tolerance of the DEH system to known or* expected voltage transients~

In addition to the temporary modific*tion that was installed to pin the trip lever of the start-up power breaker to prevent inadvertent operation, the following longer term actions are planned as part of our corrective action process: * *

  • Disas~emble and inspect the spare 4160 volt breaker to assure that no other problems exist with it. . *
  • Evaluate the replacement of existing springs, of all 2400/4160 volt breaker foot petals, with stronger springs to prevent or reduce pedal bounce during a clo_se operation. . *
  • Walk-down all 2400/4160 VAC bre~ker cubicles and inspect for warped or bowed floors and evaluate methods to reduce cubicle "floor boun~e" during breaker*

operation. * . ..

  • Review work order hi story files for evidence of any other medium voltage breaker malfunctions that may need to be investigated.

ADDITIONAL INFORMATION Related recent plant trips have been reported in Licensee Event Reports92-034 and 92-035 ..

LICENSING CORRESPONDENCE\COMMITMENT TRACKING RECORD-

SUMMARY

DATE: November 25, 1992 DOCKET 50-255 LICENSE DPR~20 - PALISADES PLANT LICENSEE EVENT REPORT 92-039 Automatic Reactor Trip On Loss Of Load Resulting From Unstable Voltage~o The Turbine Control System Computer~

SUMMARY

Transmits LER 92-039 which describes the October 30, 1992 plant trip. The.

reactor tripped when the turbine generator tripped. The turbine tri~ resulted from the DEH computers' response to a voltage transient which occurred when the UPS went to bypass and the DEH system was switched to a low voltage plant feed.

Previous Previous NRC Letters Dated: LC _ _ __ CPCo Letters Dated: LC LC - - -

LC -'--------'--- LC--

LC LC - -

UFI NO: 950-73*20*01*01 Individuals Originator: Concurrences: Concurrences: Providing Info:

WLRoberts TJPalmisano GBSlade TE Leva PMDonne1ly JLKuemin TCAnderson Special KEOsborne JLHanson MTNordin Di stri but ion: RSWesterhof MTNordin TE Leva *psE LOG PRC MTG*

NPAD LOG COMMITMENT TRACKING COMMITMENTS MADE:

Assigned Individual:

Related CA Document No: CTS Commitment No:. - - - - - - -

  • Commitment To.Be Made' Resident? Resident Document:

COMMITMENTS CLOSED:.

Provide report to NRC within 30 days of the event.

Related CA Document No:E-PAL-92-067 CTS Commitment.No:.

Additional -Information Needed for CTS Entry:

System Code:

Suggested Keywords:

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