ML20045G861

LER 93-003-00:on 930610,dual Unit Trip Occurred Due to Partial Loss of Offsite Power.Flashover Protection Relay for Breaker 552-61 Replaced & Training for Personnel W/ Access to Relays Will Be reinforced.W/930709 Ltr

ML20045G861
Person / Time
Site:	Calvert Cliffs
Issue date:	07/09/1993
From:	Cruse C, Muth D BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-93-003, LER-93-3, NUDOCS 9307160080
Download: ML20045G861 (8)

Text

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g-BALTIMORE GAS AND ELECTRIC

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CALVERT CLIFFS NUCLEAR POWER PLANT 1650 CALVERT CLIFFS PARKWAY

• LUSBY. MARYLAND 20657-4702 l

CHARLES H. CRUSE PLANT GENERAL MANAGER CALVERT CurrS J uly 9, 1993 U.f. Nuclear Regulatory Commission Uc-hington, D.C. 20555 ATTENTION: Document Control Desk

SUBJECT:

Calvert Cliffs Nuclear Power Plant Unit Nos. 1 and 2; Docket Nos. 50-317 and 50-318; License Nos. DPR 53 and DPR 69 Licensee Event Report 93-003 Dual Unit Trip Due to Partial Loss of Offsite Power The attached report is being sent to you as required under 10 CFR 50.73 guidelines. Should you have any questions regarding this report, we will be pleased to discuss them with you.

Very truly yours,

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/ jf ) ' x' CHC/DWM/bj d Attachment c ::: D. A. Brune, Esquire J. E. Silberg, Esquire R. A. Capra, NRC D. G. Mcdonald, Jr. , NRC T. T. Martin, NRC P. R. Wilson, NRC R. I. McLean, DNR J . H. Wal ter, PSC Director, Office of Management Information and Program Control 140043 9307160080 930709 E /

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.NRC FORM E36 (5-92)

• U. S. NUCLEAR REGULATORY COMMISSION h ,3 ESTMTID BlflDEM PER FESPONSE TO COMRYWiTHTK3 INFC% TION

• cot 1ECIlON FEQUEST: 50.0 HFiS F0FWAFD COMMENTS FEGNUNO LICENSEE EVENT REPORT (LER) se ESTWTE TOTHE INF OfmTM MECORDS WNEMENT

• BRr#CH (MNBB 7714). U S. NUCLEAR FEGULATORY tmmeent WASHINGTOPI DC206550001, AND TOTFE PAPEFWORKFEDUCTION PIDFiCT (See teverse for required number of dioits/ characters for each block) AM MW NM AND N,MMW DCM FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3)

Calvert Cliff s, Unit 1 05000 317 1 OF 07 TITLE (4)

Dual Unit Trip Due to Partial Loss of Offsite Power EVENT DATE (5) LER NUMBL3 (6) REPORT DAT E (7) OTHER FACILITIES INVOLVED (a)

MONTH S EN RNSION DAY YEAR YEAR MONTH DAY YEAR NUMBER N M ER Calvert Cliffs, U2 05000 318 06 10 93 93 - 003 -

00 07 09 93 05000 OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENT *, OF 10 CFR fCheck one or more) f11)

MODE (9) 20.402(b) 20 405(c) ,. )( 50 73(a)(2)(iv) 73 71(b) 20.405(a)(1)0) 50.36(c)(1) 50.73(a)(2)(v) 73,73 ge)

LEVEL 100 20.405(a)0)0i) 50 36(c)(2) Sn73(a)mQ OTHEn

00) 20.405(a)(1)(iii) 50.73(a)(2)C) 50.73(a)(2)(viii)(A)

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(Specity in Abstract below and in 20.405(a)(1)(iv) Text, NRC form 366A) 50.73(a)(2)0i) 50.73(a)(2)(viQ(B) 20.405(a)(1)(v) 50.73(a)(2)0ii) 50,73(a)(2)(x)

LICENSEE CONTACT FOR THIS LER (12)

NAME TE LEPHONE NUMBER (include Area Code)

D. W. Muth, Compliance Engineer 410 260-3592 COMPLETE ONE LINE FOR EACH COMPONENT F AILURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT A R R o CAUSE SYSTEM COMPONENT

" 0 TURER NPRDS TUPER NPRDS I

SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YEAR YES SusuiSSiON (if yes, enmplete EXPECTED SUBMISSloN DATE)

X e DAT E (15)

ABST RACT (Lim #t to 1400 spaces, i.e., apprommately15 single space typewntten imes) (16)

On Thursday, June 10, 1993 at 8:28:13 a.m., a flashover relay in the Calvert Cliffs switchyaro ectuated, causing the loss of the 500 kv " Red" bus feeding Unit 2. Unit 2 inw:diately tripped on low coolant fl ow. Unit I was manually tripped 25 seconds later by operators anticipating a total loss of offsite power.

The most probable cause of the Unit 2 trip is induced vibration of the ,

flashover relay. The decision to trip Unit 1 resulted from the Control Room Supervisor's having concluded that a loss of offsite power was underway.

We will train appropriate personnel on avoiding inadvertent relay actuation.

We will minimize the protective features Icft operative wher. breakers are opened for extended periods. We will evaluate improvements to operator training.

IdC Fesm356}50

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l NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 )

Fea EXPIRES 5/31/95 1

, ESTIMATED BURDEN PER RESPONSE To COMPLY WITH THIS INFoRMATON CoLLECTON REcuEST: 50.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) COMMENTS REGAFONG DUHDEN ESTIMATE To THE MoRMATON AND RECoHDS MANAGEMENT BRANCH (MNBS 7714). U S. NUCLEAR TEXT CONTINUATION REGutAToRy CoMMissON wamGToN, Do rosss.onoi. AND To THE PAPERWORK REDUCTON PROJECT (3t504104), OFFICE oF MANAGEMENT AND BUDGET. WASHINGTON, DC 20503.

F AcluTV NAME (1) DOCKET NUMBER (2) (IR NUMBER (3) PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 -

003 -

00 02 0F 07 TEXT (if moes space is foquired, use addetsonal cope of NRC Form 306A) (17)

I. DESCRIPTION OF EVENT on Thursday, June 10, 1993 at 8:28:13 a.m., a flashover relay in the Calvert-Cliffs switchyard actuated, causing the loss of the 500 kv " Red" bus feeding Unit 2. Unit 2 tripped on a low reactor coolant flow signal. Unit 1 was manually tripped 25 seconds later. Unit 2 wrs in MODE 3 at 2250 psia and 532 degrees Fahrenheit and Unit 1 was in MODE 1 at 100 percent power at the time of the event.

At the time of the trip, modifications were being made in the switchyard to accommodate a new 500 kV transmission line. All 500 kv breakers were closed at t the time of the trip except 552-61, 62, and 63, which were tagged out for the i modification work (see Figure 1). The work was being done to connect the new line between breakers 552-62 and 552-63. Workers had finished constructing new )

panels and adding new wiring and relays and were performing wiring checks and l cleaning up when the flashover protection relay for breaker 552-61 actuated.

Flashover protection is designed to isolate a generator breaker if the breaker  ;

is open and current is still present either due to an are across the breaker gap (a "flashover") or failure of the breaker to open. For breaker 552-61 flashover protection is provided by a Westinghouse Type KC-4 relay which contains a fast acting overcurrent device for each phase (A, B, and C). Current transformers located at the breaker and main generator sense current through the breaker, indicating flashover. Breaker auxiliary switches permit flashover protection when the breaker is open.

When the relay actuated, it tripped breakers 552-21, 41, and 63 (which was already open), isolating the 500 kv Red Bus (see Figure 1). Loss of the Red Bus deenergized 13 kv Reactor Coolant Pump (RCP) Buses 21, 22, 23, and 24 which power the four Unit 2 RCPs; and 13 kv Service Buses 21 and 22 which power 4 kv Buses 14, 22, 23, and 24. Bus 14 supplies power to Unit 1 safety-related equipment, including Control Room panel 1Y10 which provides primary indication of Unit 1 control rod position. 4 kv Bus 24 supplies power to Unit 2 safety-  !

related equipment and 4 kv Buses 22 and 23 supply non-safety-related Unit 2 i equipment. The loss of power to the Unit 2 RCPs caused Unit 2 to trip on a low )

flow signal at 8:28:13 a.m.

As a result of the loss of 4 kv Buses 14 and 24, all normal Control Room lighting went out. Emergency lighting came on as designed. Plant operators are trained to recognize that one of the symptoms of a loss of offsite power is the loss of Control Room lighting. In addition to noting the loss of Control Room light ag, the Unit 1 Control Room Supervisor (CRS) noted that the Red Bus I

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.NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 gHra EXPIRES 5/31/95 ESTIMATED BLADEN PER RESPONSE To COMPLY WITH THIS INFoRMATioN COLLECTION REQUEST. 50 0 HRS. FORWARD LICENSEE EVENT REPORT (LER) COMMENTS REGARoWG BUHDEN ESTNATE TO THE WFORMATioN AND RECORDS MANAGEMENT BRANCH (MNBB 7714). U.S NUCLEAR TEXT CONTINUATION REautAToRy CouuissioN. WASHINGTON. oC 20$ssami. ANo To THE PAPERWORK REDUCTION PROJECT (3150 0104). OFFICE oF MANAGE MENT AND BUDGET, WASHINGTON. oC 20503 FACIUTY NAME (1) DOCKET NUMBEM (a LER NUMBER (a PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 -

003 - 00 03 0F 07 TEXT (it more space is required, use additional copH,e of NRC Fon 366A) (17) potential light was out and that he had lost all but the backup control rod position indication. He noted that the Trip Circuit Breakers had not tripped but was aware that work was going on in the switchyard and believed that the plant was in the process of losing power to both units. He therefore ordered j the Unit 1 Reactor Operator to trip the unit. Unit 1 tripped at 8:28:38 a.m.,

25 seconds after the Unit 2 trip.

Emergency Diesel Generators (EDGs) 12 and 21 started up on the loss of power to 4 kv Buses 14 and 24, respectively. Emergency Diesel Generator 21 breaker automatically closed in on 4 kv Bus 24 and the Shutdown Sequencer loaded the bus. As designed, EDG 12 had to be manually aligned to 4 kv Bus 14 since this was an Underyc1tage Trip vice a Safety Injection Actuation Signal condition.

EDG 12 was manually aligned to 4 kv Bus 14 at 8:34:23, roughly five minutes after event initiation. Since 4 kv Bus 11 was not deenergized, EDG 11 was not required to start.

1 The Shift Supervisor declared an Unusual Event at 8:50 a.m. After verifying that a common fault did not exist, the switchyard configuration was returned to its pre-event alignment by 11:50 a.m. After completing additional recovery actions, the plant exited the Unusual Event at 1:05 p.m.

II. CAUSE OF EVENT The most probable cause of the Unit 2 trip is ir.duced vibration of the KC-4 relay. However, the specific source of the induced vibration could not be absolutely established. While work was in progress in the general area, none needed to be done in the immediate vicinity of the relay. None of the workers in the Switchyard Control House reported being near the relay at the time of the trip. All other means of relay failure, including valid breaker flashover, accidental electrical operation, internal ground fault, and defect in the relay, have been examined and eliminated as possible causes. The breaker was electrically disconnected, preventing possible flashover. No work was ongoing in circuitry connected to the relay that could have accidentally energized it.

Inspection and testing found the relay within specifications and acceptable for use.

Contributing to the Unit 2 trip was that the flashover protection for breaker 552-61 was enabled when it was not needed to protect an electrically isolated breaker. Only breakers 552-62 and 63 actually needed to be tagged out for the work being performed. The decision to open breaker 552-61 was made after the crews reported to the switchyard and was an unnecessary change in system configuration. The protective circuitry for breaker 552-61 should have been

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 ,

pean EXPIRES 5/31/95

, ESTIMATED BURDEN PER RESPONSE To COMPLY WITH THtS INFoRMATON COLLECTION REoUEST: 50.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) CouuENTs REGARDiNo sURDEN ESriuATE To THE iNroRuAfioN AND RECORDS MANAGEMENT ORANCH lMNBB 7714), U S. NUCLEAR +

TEXT CONTINUATION Recut 4ToRv comssoN. WASHINGTON. oC roess-000i, AND To THE PAPERWORK REDUCTION PROJECT (31S0104), OFFICE oF ,

MANAGEMENT AND DVDGET, WASHINGTON, DC 20503.

F AC8UTY NAME (1) DOCKET huMBER (2) . LER NUMBER (3) PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 -

003 - 00 04 0F 07 TEXT lit more spac.e a recuved, une additional copes of NRC Form 366A) (17) considered as part of the change in the system configuration and not been left enabled when the breaker was opened.

The decision to trip Unit 1 resulted from the CRSs having quickly analyzed various inputs and concluded that a loss of offsite power was underway. Ilis decision was conservative. However, manual trip of the unit was not optimal

• since Unit 1 would not have tripped on loss of the Red Bus. The CRS believed, based on the loss of Control Room lighting and trip of Unit 2, that the plant was in the process of losing power to both units. He did not fully recognize  ;

and synthesize all available information. This was due in part to distractions caused by the automatic shutdown of Unit 2. The CRS had been recently trained l on the loss of 4 kv Bus 14 and the resultant loss of panel 1Y10. He had also received training on loss of offsite power. The situation in which he found  ;

himself in this event, however, was sufficiently unfamiliar that he appropriately took conservative action.

III. ANALYSIS OF EVENT l i

The worst-case Loss of Coolant Flow transient described in the Updated Final l Safety Analysis Report assures a simultaneous loss of all four RCPs on a unit .

l operating at 100 percent power and concludes that no significant safety i consequences will result from this event. This analysis is bounding for the Unit 2 trip. Unit 1 was tripped manually and shut down normally. There are no  ;

significant safety consequences resulting from this event. l l

This item is reportable under the provisions of 10 CFR 50.73 (A)(2)(iv) as a Reactor Protection System actuation.

IV. CORRECTIVE ACTIONS Immediate Corrective Actions A. The flashover protection relay for breaker 552-61 was replaced. The old relay was examined for possible defects. None were found.

Preventive Actions A. Access to relay protection equipment is restricted. However, in light of the number of people working in the Switchyard Control House at the time of this incident, and the number of inadvertent relay actuations that have occurred in the Baltimore Gas and Electric Company system, we will review possible additional relay

NHC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 (5 s2) , EXPlRES 5/31/95 ESTIMATED BURDEN PER RESPONSE To COMPLY WITH THts INFoRMATION COLLECTION FIEQUEST: 50.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) CouMENTs RtGARDiNG BURoEN EstiuArE To THE iNFORuATiON AND RECoHDS MANAGEMENT BRANCH (MNBB 7714), U.S NUCLEAR TEXT CONTINUATION REGouTORY CoMMSSM WASHWGToN. DC 205554XH, AND To '

THE PAPERWORK FIEDUCTION PROJECT 0150 0104). oFFICF oF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

F ActuTV NAME (1) DOCKET NUMBER (M LER NUMBER (M PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 -

003 -

00 05 0F 07 TEXT (It more space is reqwred, use additenal copees of NRC Form 366A; (17) protection. This review will consider different methods to desensitize the relay as much as possible, including increased relay gap, as well as additional panel bracing. We will also evaluate the use of distinctive colors, labels, or signs to identify the relays that are more sensitive and susceptible to trips.

B. We will reinforce training for personnel with access to relays on the methods employed to indicate sensitive relays and the means to avoid inadvertent actuation.

C. We will evaluate a policy that requires review of protective features when transmission system breakers are opened for an -

extended period of time. The goal of such a policy would be to ensure that protective features are left enabled only if they are needed.

D. To assist operators in maintaining a questioning attitude when contacted about switchyard operation and maintenance, we plan to develop a matrix of protective features associated with 500 kv breakers and incorporate it into the Operations procedure governing operation of the 500 kv switchyard.

E. We will discuss this event in operator training, with particular

• emphasis on the organization's support for taking conservative actions even when, in hindsight, the result is less than optimal from the power production standpoint.

F. We will expand operator training to provide add.itional criteria for operators to evaluate events involving loss power.

G. We will evaluate improvements to simulator training to better model distractions associated with dual-unit transients.

HRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO,3150-0104 m sa - EXPIRES 5/31/95 ESTIMATED BUFOEN PER RESPONSE TO COMPT.Y WITH THis INFOP'4A10N (XX.LECTON FEQUEST: 60.0 HRS. FORWARD LICENSEE EVENT REPORT (LER) COMMENTS REGARDING BURDEN ESDMATE TO THE INFORMATON AND RECORDS MANAGEMENT BRANCH (MNBB m4), U B. NUCLEAR TEXT CONTINUATION REGUMTORY CoMMGSON, WASHINGTON, DC N0001, AND TO THE PAPERWORK REDUCTON PROJECT (3150-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.

FACluTV NAME (1) DOCKET NUMBER (a EER NUMBER (3) PAGE l4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 -

003 - 00 06 OF 07 TEXT (if more space is required, use addthonal copies of NRC Form 300N {17)

V. ADDITIONAL INFORMATION A. Affected Component Identification:

IEEE 803 IEEE 805 Component or System EIIS Funct System ID Flashover Protection Relay BDMP FK 500 kv Breaker BKR FK Reactor Coolant Pump P AB Reactor Trip Breakers BKR JC Emergency Diesel Generator DG EK B. Previous Similar Events:

LER 50-318/87-012 described a similar condition involving a dual unit trip on loss of both 500 kv buses due to a tree touching one line and a defective logic circuit card tripping breakers on the other line. This event did not involve induced vibration on a relay.

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ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS WORMATON COLLECTON REQUEST: so.0 HRS. FORWARD LlCENSEE EVENT REPORT (LER) OOMMENTS REGMONG BURDEN ESTNATE TO THE MORMATON AND RFCORDS MANAGEMENT BRANCH (MNBB 7714), U.S. NUCLEAR TEXT CONTINUATION REGutatoRY OOMussoN.wASHmTON, DC aosssaci. ANo TO j THE PAPERWORK REDUCTON PROJECT (31500104) OFFCE OF MANAGEMENT AND BUDGET, WASHINGTON DC 20503

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FACIUTY NAME (1) OOCKET NUMBER (@ LER NUMBER (4 PAGE (4)

Calvert Cliffs, Unit 1 05000 3 1 7 93 - 003 - 00 07 0F 07 TEXT or .p.c. N n auw = mons cop.es W NFC Form 366A) i 7) r To 13 kv System  ;

500 kv Red Dus3 Switchyard._-, 552-21 552-41 552-61 Breaker  :

From Unit 2 Generator 552-22 552-43 552-62 '

From Unit 1 Generator 552-23 552-63 ,

j; 500 kv Black Bus I J } [

Existing 500 kv Existing 500 kv To 13 kv New 500 kv Transmission Line TransmFssion Line System Line i

FIGURE 1 500 kv SWITCHYARD RING BUS F

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