ML19354E565

LER 89-021-00:on 891225,electrical Protection Assembly Output Breakers Associated W/Bus a of Reactor Protection Sys Tripped Open.Cause of Trip Unknown.Voltage & Frequency Output of Motor Generator a monitored.W/900124 Ltr

ML19354E565
Person / Time
Site:	Brunswick
Issue date:	01/24/1990
From:	Harness J, Jones T CAROLINA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BSEP-90-0061, LER-89-021-01, NUDOCS 9001310375
Download: ML19354E565 (4)

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Carolina Power & Light Company Brunswick Nucicar Project {

P. O. Box 10429 Southport, NC 28461-0429 January 24, 1990  ;

FILE: B09-13510C 100FR50.73 SERIAL: BSEP/90 0061 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk i Washington, DC 20555 BRUNSWICK STEAM ELECTRIC PLANT UNIT 2 DOCKET NO. 50-324 LICENSE NO. DPR-62 LICENSEE EVENT REPORT 2-89-021 Gentlemen:

In accordance with Title 10 to the Code of Federal Regulations, the enclosed Licensco Event Report is submitted. This report fulfills the requirement for a written report within thirty (30) days of a reportable occurrence and is in accordance with the format set forth in NUREG-1022, September 1983.

Very truly yours, gt J. L. llarness, General Manager Brunswick Nuclear Project ,

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On December 25, 1989, the Unit 2 reactor was defueled with the fuel pool gates installed during a scheduled refueling outage. At 1103 the Electrical Protection Assembly (EPA) output. breakers associated with the A bus of the Reactor Protection System (RPS) tripped open. The expected isolations and actuations occurred. The reason for the trip is unknown because the trip indicating lights on the breakers do not seal in. The load at the time of the event was low; therefore, the cause for the trip is not thought to have been a

! magnetic or thermal trip. The voltage and frequency output of the A RPS l motor generator set was monitored and no problems were found. The over i voltage, under voltage and under frequency trip function setpoints were l calibrated and the results did not reveal a cause for the event. A l modification kit is available from General Electric which includes a provision for seal in trip indications but it has been determined not to be cost justified. The RPS bus and EPA breakers are functioning normally. No further actions are planned.

This event had no safety significance as the system functioned as designed.

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0l0 0l2 0F 0 l3 nxi w = == = = =ac s , ame., nn Event A trip of the A Reactor Protection Systeru (RPS) Electrical. Protection Assembly (EPA) (EIIS/JC/BKR) I and 2 output breakers resulting in automatic Engineered Safety Feature actuations.

Initial Conditions Unit 2 reactor was Jefueled and the fuel pool gates were installed during a scheduled refuel and maintenance outage. RPS A and B buses were being powered from their associated motor-generator (M-G) (EIIS/JC/MG) sets.

Event Description On December 25, 1989, at 1103, the Unit 2, numbers 1 and 2. EPA breakers associated with RPS bus A (Ells /JC/BU) tripped open. The control operator (CO) verified that expected automatic actions occurred. A Primary Containment Isolation System (PCIS) (Ells /BD) Group 6 [ Containment Atmosphere Control (CAC)

(Ells /DB) system] isolation occurred, the A and B Standby Gas Treatment (Ells /

VA/Bll) systems automatically started and a half SCRAM signal was received when the A RPS logic tripped. At the time of the event, Group 2 isolation valves .

were mechanically blocked open, the Reactor B:111 ding was under clearance isolated, drywell purge was secured and Group 3 and 8 valves were doenergized c for outage activities; therefore, the involved valves and dampers did not isolate. An obvious cause for the breaker trips was not apparent. Per a recomn.endation f rom the Instrumentation and Control (ISC) group EPA breaker 1 ,

and 2 were closed. Indications were normal. At 1210 the trips were reset and systems were returned to normal service. Work Request / Job Order (WR/J0) 89-BBYK1 was initiated to investigate this event. At 2247, RPS bus A was transferred to the alternate power source to allow for testing of the 1 and 2 ,

EPA breakers.

Event Investigation The EPA breakers are designed to trip whenever an under frequency, under  ;

voltage or over voltage condition exists at the output of their associated RPS M-G set. The breakers will also trip on magnetics or thermal overload.

It is not possible to determine which trip opened the involved breakers because the trip indications are for calibration purposes and are not provided with a scal-in function. This lack of a scal-in function prevents the determination of what tripped the breaker and which breaker tripped first. Since the breakers are in series, if EPA 1 tripped then EPA 2 would trip on under voltage'.

Theoretically, if EPA 2 tripped EPA 1 would not. However, it is possible that a resulting voltage regulator spike due to sudden unloading could trip EPA 1.

, At the time of the event, the bus load was low; therefore, it is assumed that .

the trip was not the result of a thermal or magnetic overload. To deterpine if W.CPum astAe6491

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012 11 0 10 013 0' o 13 1 m, ~ . - % =. mn a problem existed with the remaining EPA breaker trip setpoints a maintenance surveillance test (MST) was performed. 2MST-RPS21SA, RPS Electrical Protection Assembly Channel Calibration, was performed on EPA breakers 1 and

2. The test results did not indicate a problem which is thought to have tripped the breakers. The test did reveal that the under frequency time delay trip varied from 0.5 to 1.0 seconds. The trip is required by the test to be set at a time delay of 0.9 seconds plus or minus 0.1. In addition to the test, the output frequency and voltage of the RPS M G set was monitored. No problems were found. The most probabic cause of the variable time delay is defective components on the printed circuit (PC) board. The PC board was replaced and the new PC was satisfactorily calibrated in accordance with 2MST-RPS21SA on January 2, 1990. RPS A was transferred from the alternate power supply to its M G set on January 8, 1990. No further problems have been indicated.

Service Information Letter (SIL) 496, " Electrical I'retection Assembly Per-formance," dated August 23, 1989, addressed a number of performance problems identified by BWR owners. The SIL discussed and recommended methods for resolving the problems. CP&L reviewed the SIL and incorporated the recommen-dations which were applicable and beneficial to the Brunswick site. A modifi-

,, cation kit was recommended by the SIL which provides for seal in trip indica-tions and an associated reset switch. The modification kit was dc.termined not 3 to be cost justified.

Past similar occurrences involving EPA breaker trips have been reported in LER 1-86 018, 1-86-030, 1-87-009 and 2-88-013. LER 1-86-018 reported three EPA trips which occurred on July 11 and 12, 1986, and on August 2, 1986. The July trips were attributed to failure of the primary coil in the breaker transformer, T1, which was replaced. The cause of the August trip was not determined. LER 1-86-030 reported two EPA trips; one on Unit 2 on November -

20, 1986, and the other on Unit 1 on December 4, 1986. The cause of the two events was attributed to loose connections where sensor leads attac.h to the i breakers. The connections are now verified at six month intervals. LER 1 87-009 reported four EPA trips in March 1987. The cause was attributed to a defective underfrequency card and the card was replaced. LER 2-88-013 reported four EPA trips while on the alternate power supply on August 2, 1988. The cause was attributed to swings in the alternato power supply voltage. Voltage regulator transformers were installed to prevent this event in the future.

Root Cause and Corrective Actions ,

The cause of the event cannot be identified. No further corrective actions are planned.

Event Assessment This event had no safety significance as the system functioned as designed.

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NRC Perm 306A (649)

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