ML17289B179

LER 93-003-00:on 930121,determined That Under Certain Ac Electrical Distribution Sys Alignments,Fault Could Result in Unavailability of Both Offsite Power Sources.Caused by Inadequate Sys Analysis.Delay Times revised.W/930222 Ltr

ML17289B179
Person / Time
Site:	Columbia
Issue date:	02/22/1993
From:	John Baker, Poche R WASHINGTON PUBLIC POWER SUPPLY SYSTEM
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GO2-93-041, GO2-93-41, LER-93-003, LER-93-3, NUDOCS 9303020003
Download: ML17289B179 (9)

Text

~orna,za~moREGULATORY Doimczm INFORMATION DISTRI DiS BUTIO aVrrow SYSTEM ( RIDS Svsrzrvr

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ACCESSIO>i! HBR:9303020003 DOC.DATE: 93/02/22 NOTARIZED: NO DOCKET FACIL:50-397 WPPSS Nuclear Project, Unit 2, Washington Public Powe . 05000397 AUTH. NAME AUTHOR AFFILIATION POCHE,R.J. Washington Public Power Supply System BAKER,J.W. , Washington Public Power Supply System RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 93-003-00:on 930121,de'termined that under certain ac .

electrical distribution sys alignments, fault could result in unavailability of both offsite power sources. Caused by inadequate sys analysis. Delay.imes revised.W/930222 ltr.

i!ii DISTRIBUTION CODE: IE22T COPIES RECEIVED:LTR ENCL SIZE:

TITLE: 50'.73/50.9 Licensee Event Report (LER), ncident Rpt, etc.

NOTES:

RECIPIENT COPIES , RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL

,PD5 LA 1 1 PD5 PD 1 1 CLIFFORDiJ 1 1 INTERNAL: ACNW 2 2 ACRS 2 2 AEOD/DOA 1 1 AEOD/DSP/TPAB 1 1 AEOD/ROAB/DSP 2 2 NRR/DE/EELB 1 1 NRR/DE/EMEB 1 1 NRR/DORS/OEAB 1 1 NRR/DRCH/HHFBHE 1 1 NRR/DRCH/HICB 1 1 NRR/DRCH/HOLB 1 1 NRR/DRIL/RPEB 1 1 NRR/DRSS/PRPB 2 2 RRglB~ SPLB 1 1 NRR/DSSA/SRXB 1 1 REG FILE 02 1 1 RES/DSIR/EIB 1 1 R FILE 01 1 1 EXTERNAL EG&G BRYCE i J ~ H L ST LOBBY WARD 1 1 NRC PDR NSIC MURPHY,G.A 1 1 NSIC POORE,W. 1 NUDOCS FULL'XT 1 1 NOTE TO ALL"RIDS" RECIPIENTS:

PLEASE HELP US TO REDUCE WASTE! CONTACT THE DOCUMENT CONTROL DESK, ROOM Pl-37 (EXT. 504-2065) TO ELIMINATEYOUR NAME FROM DISTRIBUTION LISTS FOR DOCUMENTS YOU DON'T NEED!

FULL TEXT CONVERSION REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 32 ENCL 32

WASHINGTON PUBLIC POWER SUPPLY SYSTEM P.O. Box 968 ~ 3000 George Washington Way ~ Richland, Washington 99352 February 22, 1993 G02-93-041 Docket No. 50-397 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555

SUBJECT:

NUCLEAR PLANT WNP-2, OPERATING LICENSE NPF-21 LICENSEE EVENT REPORT NO.93-003 Transmitted he'rewith is Licensee Event Report No.93-003 for the WNP-2 Plant. This report is submitted in response to the report requirements of 10CFR50.73 and discusses the items of reportability, corrective action taken, and action taken to preclude recurrence.

Sincerely, J. W. Baker WNP-2 Plant Manager (Mail Drop 927M)

JWB/RJP/cgeh Enclosure CC: Mr. J. B. Martin, NRC - Region V Mr. R. Barr, NRC Resident Inspector (Mail Drop 901A, 2 Copies)

INPO Records Center - Atlanta, GA Mr. D. L. Williams, BPA (Mail Drop 399)

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LICENSEE EVENT REPORT (LER)

AGILITY NAME (I) DOCKET NUMB R ( ) PAGE (3)

Washin ton Nuclear Plant - Unit 2 0 5 0 0 0 3 9 7 I OF ITLE (4)

INADEQUATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY POWER SOURCE AVAILABILITY EVENT DATE (5 LER NUMBER 6) REPORT DATE (7 OTHER FACILITIES INVOLVED (8)

MONTH OAY YEAR YEAR  ; SEQUENTIAL EVI 5 ION MONTH DAY YEAR FACILITY NAMES CKE HUMB R (S)

"' NUMBER UHBER 05 0 I 2 I 9 3 9 3 0 0 3 0 0 0 2 2 2 9 3 0 5 P ERAT ING HIS REPORT IS SUBHITTED PURSUANT TO THE REQUIREHENTS OF 10 CFR %1 (Check one or more of the following) (11)

DDE (9) 3 DHER LEVEL o.4o2(b) o.4os(c) 50.73(a)(2)(iv) 77.71(b) 0.405(a)(1)(i) 50.36(c)(1) 50.73(a)(2)(v) 73.73(c) 0.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vii) THER (Specify in Abstract 20.405(a)(1)(iii) 50.73(a)(2)(i) 50.73(a)(2)(viii)(A) elow and in Text. NRC 20.405(a)(1)(iv) 50.73(a)(2)(ii) 50.73(a)(2)(viii)(B) Form 366A) 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x)

LICENSEE CONTACT FOR THIS LER (12)

TELEPHONE HUMBER REA CODE R. J. PochT5, Licensing Engineer -

5 0 9 7 7 4 1 4 5 COMPLETE OHE LINE FOR EACH COMPONENT FAILURE DESCRIBED IH THIS REPORT (13)

HAHUFACTURER EPORTABLE CAUSE SYSTEH COMPOHEHT MANUFACTURER REPORTABLE CAUSE SYSTEH COMPONENT 0 HPRDS TO NPRDS (14) EXPECTED SUBMISSIOH MONTH DAY YEAR SUPPLEMENTAL REPORT EXPECTED ATE (15)

YES (If yes, complete EXPECTED SUBMISSION DATE) HO TRACt TTTO At approximately 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br /> on January 21, 1993, it was determined that a deficiency involving coordination of protective relays associated with the AC electrical distribution system could result in plant operation that was not within the plant design bases. Under certain AC electrical distribution system alignments a fault on either 4.16 KV vital bus could have resulted in unavailability of both offsite power sources and delayed powering of the nonfaulted 4.16 KV vital bus from its associated Emergency Diesel Generator (EDG), WNP-2 has not experienced a bus fault that resulted in a loss of offsite power and/or delayed synchronization of the EDGs due to inadequate breaker coordination. Consequently, the conditions described in this report did not have an adverse affect on safe operation of the plant, or the health and safety of plant personnel or the general public.

The root cause of this condition was a deficiency involving inadequate analysis of undervoltage and overcurrent protective features on the vital 4.16 KV AC electrical distribution system. As corrective action, delay times associated with this breaker coordination problem were revised. Additionally, the Bus Short Circuit Current Calculation and the Relay Coordination Calculation will be reviewed and updated.

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION ACILITY NAHE (1) DOCKET NUHBER (2) LER NUHBER (B) AGE (3) ear umber ev. No.

Washington Nuclear Plant - Unit 2 3 003 0 2 F 7 ITLE (4)

INADE(UATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN, POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY POWER SOURCE AVAILABILITY Plant n ii n Power Level - 0%

Plant Mode - 3 (Hot Shutdown)

Event De ri tion At approximately 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br /> on January 21, 1993, it was determined that a deficiency in the medium voltage (4.16 KV) vital AC electrical distribution system could result in plant operation that was not within the plant design bases. This deficiency involved coordination of overcurrent protection relays and undervoltage bus transfer logic. Under certain electrical distribution system alignments, a persistent fault on either 4.16 KV vital bus could have resulted in unavailability of both offsite power sources and delayed powering of the nonfaulted 4.16 KV vital bus from its associated Emergency Diesel Generator (EDG) .

The medium voltage AC electrical distribution system has two divisionally separate and redundant vital busses, SM-7 and SM-8 (see attached sketch), that supply 4.16 KV power, to safety-related plant loads.

These busses are typically aligned to the main generator via the normal station transformer (TR-Nl) when the main generator is on-line; however they can also be, and occasionally are, aligned to either the startup transformer (TR-S) or backup transformer (TR-B) during power operations. Busses SM-7 and SM-8 are typically aligned to receive power'rom TR-S during plant startups and periods when the plant is shut down, with backup power available from TR-B.

Design basis concerns involving loss of offsite power supplies and delayed powering of the nonfaulted vital bus existed under the previous 'design when vital busses SM-7 and SM-8 were aligned to either TR-S or TR-B. Due to the cumulative effect of protective feature time delays associated with the transfer of loads from TR-S to TR-B, the most limiting condition with respect to delayed EDG output breaker closure resulted when the plant was initially aligned to TR-S. However, the design basis effect was essentially unchanged regardless of whether the vital busses were aligned to TR-S or TR-B at the start of the event.

Vital busses SM-7 and SM-8 are both supplied from the same (Y) winding on TR-S. As a result, a fault on either vital bus when they are aligned to this transformer would cause an undervoltage condition on the 4.16 KV (Y) transformer winding that would be evidenced as an undervoltage condition on both vital busses. A bus fault cannot affect both vital busses when they are aligned to TR-Nl because each vital bus is powered from a separate winding when it is aligned to this transformer.

After sensing an undervoltage condition on SM-7 and SM-8, primary undervoltage protective devices (27) associated with these busses would have immediately initiated an automatic start of their respective EDG.

With the previous undervoltage logic, vial bus source breakers aligned to TR-S would have tripped after approximately 2.0 seconds, initiating automatic transfer of the vital bus source from TR-S to TR-B. Due to

TEXT FACILITY NAME (I)

CONTINUATION'GE LICENSEE EVENT REPORT (LER)

DOCKET NUMBER (2) LER NUMBER (B) (3) ear umber ev. No.

Washington Nuclear Plant - Unit 2 0 5 0 0 0 3 9 7 3 003 00 3 OF 7 ITLE (4)

INADEQUATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY POWER SOURCE AVAILABILITY inadequate coordination of the overcurrent and undervoltage protective relay time delays, this transfer would have occurred before the overcurrent device on the incoming line to the faulted bus, which had an approximate 2.2 second delay time, could energize its lock-out feature. This lock-out feature is designed to open the breaker on the incoming line to the faulted bus arid prevent other source breakers from closing onto the faulted bus. As a result, the faulted vital bus would not have been isolated, and would have transferred to the backup source, TR-B, instead. At this point, the safety bus source transfer logic (timing) would proceed as if the bus fault condition was a loss of power source event.

There are two undervoltage relays on each of the incoming lines to backup source breakers B-7 and B-8.

One relay senses bus undervoltage and trips its breaker at 69% or less of rated voltage. The other relay

'provides a permissive to allow closure of it's source breaker if the secondary of TR-B is 94% or greater of rated voltage. The undervoltage transfer logic would have permitted backup source breakers B-7 and B-8 to close onto the vital busses approximately 4.0 seconds after fault initiation. Vital busses SM-7 and SM-8 share a common winding on TR-B. Consequently, if the bus fault had not cleared by this time, breakers B-7 and B-8 would have tripped open almost immediately due to the fault related undervoltage condition that would, exist on both SM-7 and SM-8.

This breaker trip occurs immediately because undervoltage protective relays on the incoming lines from TR-B that are set to operate at 69% of rated voltage did not have a time delay. Again, because of a coordination deficiency involving time delays for undervoltage and overcurrent relays on the incoming line from TR-B, the overcurrent lock-out feature would not have prevented the TR-B supply breaker from closing onto the faulted bus. Instead, the undervoltage logic would have allowed the supply breaker from TR-B to repeatedly and unsuccessfully cycle onto the faulted bus until either the fault cleared or the supply breaker to the faulted bus failed, and could'have resulted= in the inability to successfully sequence the vital busses between TR-S to TR-B.

Subsequently, if the SM-7 and SM-8 undervoltage time delay permissives to close the EDG output breakers were satisfied, and the EDGs were at rated speed and voltage (which normally occurs 8 seconds after start signal), transfer of the safety busses to their respective EDG would have been enabled approximately 5.0 seconds after automatic transfer of both busses from TR-S to TR-B (7.0 seconds after fault initiation) if the undervoltage logic had not reset. However, transfer of the EDGs to SM-7 and SM-8 may not occur simultaneously. If the source breaker for the nonfaulted bus closes onto TR-B before the source breaker for the faulted bus, it is possible that the undervoltage logic for the nonfaulted bus will reset. Resetting the undervoltage logic would extend the time delay permissive for transferring the EDG on the nonfaulted bus beyond design basis limits.

For the faulted bus, if the primary undervoltage logic was not reset, the EDG output breaker would close after the original time delay permissive was satisfied and the EDG had obtained rated speed and voltage.

EDG overcurrent relays (51V) would then pick up, provided there was no LOCA signal present, and

LICENSEE EVENT REPORT LER)

TEXT CONTINUATION h

AGILITY NAHE (1) OOCKET NUHBER (2) LER NUHBER (8) AGE (3) ear umber ev. No.

Washington Nuclear Plant - Unit 2 '0 0 3 9 7 0 5 0 3 003 00 4 F- 7 ITLE (4)

INADEQUATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC,AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY POWER SOURCE AVAILABILITY energize the EDG lock-out relay. If a LOCA signal was present, the 51V relays are bypassed and the emergency generator would have eventually developed an internal fault due the heavy currents being supplied to the bus fault. The generator differential relays would then pick-up, thereby energizing the lock-out feature, shutting down the affected EDG, and resulting in loss of power to the faulted safety division.

mmedia e orrec ive Ac i n As immediate corrective action, breaker B-7 was opened at 1330 hours0.0154 days <br />0.369 hours <br />0.0022 weeks <br />5.06065e-4 months <br /> on January 21, 1993, in order to provide separation between busses SM-7 and SM-8, and a design change was initiated to resolve breaker coordination deficiencies on SM-7 and SM-8. Bus separation was maintained until the design change could be implemented.

Further Evalua i n and rrec ive Ac ion A. F rther Evalu i n The breaker coordination issue described in this report was determined to be reportable pursuant to the requiiements of 10CFR50.72(b)(1)(ii)(B) as a condition that is outside the design basis of the plant because it could have resulted in loss of both offsite power sources and adversely affected the ability of the EDGs to start and synchronize within the time required. Necessary notification were made at 1302 hours0.0151 days <br />0.362 hours <br />0.00215 weeks <br />4.95411e-4 months <br /> on January 21, 1993. This condition is also reportable under 10CFR50,73(a)(2)(ii)(B).

2. Breaker coordination issues associated with current/voltage relaying -for SM-7 and SM-8 were identified as an area of potential concern during the WNP-2 Electrical Distribution Safety System Functional Inspection (EDSSFI) that was performed in January of 1992. When this inspection was performed, a review of the relay coordination calculation was already in progress, but had not been completed. Also, update of the fault current calculation, which is

'. necessary for evaluation of breaker coordination, was planned, but had not yet been performed. The breaker coordination deficiency issue was not fully identified during the EDSSFI due to the complexity and scope of reviews necessary to evaluate the existing breaker coordination scheme. The breaker coordination was identified by a plant engineer during a review of electrical distribution system calculations that was being performed as part of an ongoing Electrical Calculation Improvement Program.

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION AGILITY NAHE (1) DOCKET NUHBER (2) LER NUHBER (8) AGE (3)

Year Number ev. No.

Washington Nuclear. Plant - Unit 2 0 5 0 0 0 3 9 7 3 003 00 5 F 7 ITLE (4)

INADEQUATE COORDINATION OF PRIHARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EHERGENCY POWER SOURCE AVAILABILITY li

3. Coordination of overcurrent protective reiaying with the undervoltage transfer logic is required for vital electrical busses, but was, not adequately considered in the original AC electrical distribution design. In accordance with 10CFR50, Appendix A, Criterion 17; the design of the offsite power system is required to minimize the likelihood of simultaneous failure under operating and postulated accident and environmental conditions.

B. ~R~Clge The root cause identified for this condition was a Plant/Equipment - Design Configuration/Analysis category deficiency involving inadequate analysis. Previous analyses for the AC distribution system did not adequately consider coordination of the undervoltage and overcurrent protective relay schemes. This analysis inadequacy has existed since AC electrical distribution calculations were initially prepared by the architect/engineer.

C. ~ Fu her orrective Action

'he following corrective actions have already been taken, or are in progress:

f

1. A design change was implemented uncler BDC 93-0021-OA to increase the time delay for primary undervoltage relays for SM-7 and SM-8 and resolve breaker coordination issues associated with the supply from TR-S. This change will ensure that overcurrent relays are able to provide breaker trip and lock-out functions necessary to properly isolate a fault on either vital bus. Also, a time delay was provided for the 69 percent undervoltage protective feature associated with TR-B. There was previously no time delay associated with this protective feature. This time delay will provide coordination between undervoltage trips on the backup source breakers, B-7 (B-8); protective features on the associated line; and branch circuit overcurrent protection. These design changes were completed for SM-7 and SM-8 on January 25, 1993, at 0948 hours0.011 days <br />0.263 hours <br />0.00157 weeks <br />3.60714e-4 months <br /> and 2355 hours0.0273 days <br />0.654 hours <br />0.00389 weeks <br />8.960775e-4 months <br />, respectively.

2. 'lectrical distribution system calculations are currently being reviewed and updated on an ongoing basis under the Electrical Calculation Improvement Program. The condition described in this report involved the Bus Short Circuit Current Calculation and the Relay Coordination Calculation. Review and update of these calculations is expected by April 30, 1993.

LICENSEE EVENT REPOR ER)

TEXT CONTINUATION ACILITY NAHE (i) DOCKET NUHBER (2) LER NUHBER (6) AGE (3) ear umber ev. No.

Washington Nuclear Plant - Unit 2 0 0 0- 3 9 7 0 5 3 03 0 6 F 7 1TLE (4)

INADEQUATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY-POWER SOURCE AVAILABILITY

~fi if'NP-2 has not experienced a bus fault that resulted in a loss of offsite power and/or delayed closure of EDG,output breakers due to inadequate breaker coordination. Consequently, the conditions described in this report did not have an adverse affect on safe operation of the plant, or the health and safety of plant personnel or the general public. Additionally, the condition described in this report can only affect both vital power divisions when plant loads are aligned to offsite power sources via TR-S or TR-B. This alignment is typically used only when the main generator is off-line and the plant is not at a substantial power level. Therefore, if a persistent fault condition had existed on SM-7 or SM-8 when the plant was aligned to TR-S or TR-B its significance would have been minimized.

imil r Even Two previous events involving inadequate coordination of undervoltage protection have been reported in LERs89-034 and 92-027. These previous events resulted from'design deficiencies involving inadequate coordination of protective features for 480 volt Motor Control Centers, and did not involve coordination of protective features for the 4.16 KV vital busses.

EIIS Informati n EIIS Reference

$ y~tem ~Com non AC Distribution System LK Transformers (TR-N1, TR-B, TR-S, TR-Ml/2/3/4) LK TD Breakers LK BKR Safety-Related Busses (SM-7 8c SM-8) EB BU Emergency Diesel Generators (EDG) ED DG Main Generator TB GEN1

LICENSEE EVENT REPOR LER)

TEXT CONTINUATION AGILITY NAHE (I) DOCKET NUHBER (2) LER NUHBER (B) AGE (3) ear umber ev. No.

Washington Nuclear Plant - Unit 2 9

0 5 0 0 0 3 7 3 03 00 7 F 7 ITLE (4)

INADEQUATE COORDINATION OF PRIMARY UNDERVOLTAGE BUS TRANSFER LOGIC AND OVERCURRENT PROTECTION RESULTS IN POTENTIAL LOSS OF OFFSITE POWER AND DELAYED EMERGENCY. POWER SOURCE AVAILABILITY 500 KV E- TR-M1/ 2/ 3/4 25 KV MA I N GENERATOR

. E-TR-N1 E- TR- S Y X 4.16 KV 4. 16 KV 4. 16 KV 6.9 KV N1- 1 S- 1 N1-2 S-2 N1-3 5-3 SM- 1 SM- 2 SM- 3 1-7 E-TR-8 115 KV 94%V 69%V 27 27 7

4. 16 KV I

'4 27'-B 69%V 99%V 51 7 27 8-7 8-3 SM- 8 7- DG1 8- DG2 51 DG1- 7 DG2- 8 EDG- 1 EDG-2 AC Distribution System Diagram (For Information Only)