ML19224D667

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LER 79-017/01T-0 on 790626:fault Current Trip Occurred, Causing Reactor Scram & Interrupted Reactor Cooling.Caused by Water Spray in Switchgear,Resulting in Short Circuit Due to Voltage Perturbation
ML19224D667
Person / Time
Site: Fort Saint Vrain Xcel Energy icon.png
Issue date: 07/10/1979
From: Gahm J
PUBLIC SERVICE CO. OF COLORADO
To:
NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML19224D666 List:
References
LER-79-017-01T, LER-79-17-1T, NUDOCS 7907130391
Download: ML19224D667 (8)
v • d • e


Text

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EVENT DESCRIPTION AND PROB ABLE CONstuJENCES h fault current trip 1 2 lWhile operating at 1. 5'. the rmal p ower and ze rr ele c t ri c a l_p_ow e r.

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o ea l on non-essential 4 80 volt switchgear 5 occurred. This resulted in a reactor scram I Reportable per Technical Specification j o A l and 15 minute interruption of reactor cooling.

Did not ef fe.ct public health of safety. Similar events RO 76-01 and l g 3 lAC 7.5.2(a)5.

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44 47 3 41 42 3J 24 3$ 36 CAUSE DESCRIPTION AND CC A AECTIVE ACTIONS Rose coupling separated and sprayed water l 1 o l Resin barrels being flushed with feedwater.

linto switchgear cuasing a phase to phase short ciruult which resulted in bus voltare f

lt }ij Re-established forced circula- l

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REPORT CATE: J uly 10, 19 79 REPORTABLE OCCURRENCE 79-17 ISSUE O OCCURRENCE DATE: June 26, 1979 Page 1 of 7 FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P. O. B0X 361 PLATTTVILLE, COLORADO 80651 REPORT NO. 50-267/ 79-17/01-T-0 Final IDENTIFICATION OF OCCURRENCF:

On Tuesday, June 26, 1979, while operating the plant at approximately 1.5%

of rated power, a f ault current trip on non-essential 480 volt switchgear 5 occurred. This resulted in a reactor scram and 15 minute interruption of re-actor cooling.

This is reportable per Fort St. Vrain Technical Specification AC 7.5.2(a)S.

CONDITIONS PRIOR TO OCCURRENCE:

S teady State Power DESCRIPTION OF OCCURRENCE:

o Conditions Prior to Event Reactor critical at approximately 1.5% power.

Circulators: lA on s ec am turbine drive at 1,500 rpm.

1B self-turbining.

1C self-turbining.

1D on steam turbine drive at 1,500 rpm.

Total primary coolant flow 12%.

Average circulator inlet temperature 290*F.

Average core outlet temperature 462*F.

Reactor pressure 385 psia.

lA boiler feed pu=p supplying feedwater at approximately 1,000 psig and 150,000 pounds per hour per loop.

444 276

  • REPORTABLE OCCURRENCE 79-17 ISSUE O Page 2 of 7 DESCRIPTION OF OCCURRENCE- (con tinued) :

Conditions Prior to Event (continued) :

Electrical plant configuration as shoun on Figure 1 and described below:

Main generator disconne:t links open.

House power being supplied by backfeecing from the unit auxili-ary transformer (UAT) to 4160 volt switchgears 1 and 3.

4160 volt switchgear 1 to 2 bus ti circuit breaker closed.

Two normal bearing water pumps operating in each loop.

Backup bearing water not in service, nor required by Technical Specifica-tions.

Conditions Which Leaa Up to Event Time: From 1600 to 1700 Hours. Operations and Chemistry personnel were adding new resin to the cation tank from level 6 in the turbine building (Elevation 4811') at the funnel used for resin addition.

The resin bar-els were being flushed with feedwater and a hose was con-nected to the barrel by a Chicago coupling.

Time: Approximate 1v 1720 Hours. The personnel adding the resin lef t the area to get another barrel of resin. When they returned, they no-ticed that the coupling had come loose and water was being sprayed onto the 480 volt switchgear 5 cabinet. The water was secured and a visual inspection of the exterior of the switchgear indicated that no proble=s e xis te d. No unusual odors were detected nor were any temperature changes on the switchgear noticed, so flushing continued.

Event Time: 1724 Hours. 480 volt switchgear 5 tripped due to a short circuit caused by water spray f rom the hose which was being used for resin barrel flushing. Two phases on the transformer were blown.

An explosion accocpanied by yellow and gray smoke was noted by the per-sonnel near the switchgear.

Upon observing the explosion and smoke f rom the switchgear, the personnel working near the switchgear notified the control room, kkk  !.

REPORTABLE OCCURRENCE 79-17 ISSUE O Page 3 of 7 DESCRIPTI,0N OF OCCURRENCE (continued) :

Event (con tinue d) :

Tice: 1724 Hours (continued). The high and low side breakers on the 4160/480 volt transformer tripped. This was verified locali; by the per-sonnel working near the switchgear.

The emergency was announced and the fire alarm was sounded. When the shif t supervisor arrived on the scene, he determined that there was no fire in 480 volt switchgear 5.

The reactor operator placed the handswitches for the circuit b.eakers to the af fected transformer in the pull-to-lock position, assuring that the breaker, remained tripped. .

The f ault on 480 volt switchgear 5 was reflected into the 4160 volt buses and 480 volt buses resulting in a decrease in 480 volt bus voltage before the high and low side breakers on switchgear 5 tripped to isolate the trans fo rme r.

As a result of the voltage drop on the 480 volt buses, the voltage relays installed to trip / inhibit the normal bearing water pump breakers on a loss of 480 volt essential buses were de-energized. This tripped both operating bearing water pumps in each loop. Since backup bearing water was not in operation, nor required by LCO 4.2.1 or LCO 4.2.2, all four circulators tripped on loss of bearing water.

A Icop 1 shutdown and two loop trouble scram automatically occurred.

The tecporary auxiliary boiler tripped causing an interruption of steam flow to the 1A boiler feed pump and subsequent decrease in feedwater flow to Loop 2 steam generators. Condenser vacuum was lost as well as operation of startup bypass valves, PV-22129-1 and PV-2?l30-1.

The yellow lights on Channels A and S of the Plant Power Loss modules were on indicating that low voltage had occurred on 480 volt buses 1 and 2. No automatic emergency diesel generator operation occurred nor was any required since the condition was not a loss of voltage, but a voltage fluctuation.

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The primary coolant flow was interrupted for a period which lasted 15 minutes.

APPARENT CAUSE OF OCCURRENCE:

Water spray into switchgear as a result of hose coupling separation caused short circuit and bus voltage perturbation.

444 278

REPORTABLE OCCURRENCE 79-17 ISSUE 0 Page 4 of 7 ANALYSIS OF OCCURRENCE:

The 480 volt switchgear 5 is located in the turbine building on level 6 (Ele-vation 4811') and feeds non-essential motor c.ontrol centers in the reactor building and turbine building. Located in the switchgear is a 4160 volt to 480 volt transformer. Af ter water sprayed into the switchgear (and the trans-former), a phase t. phase short circuit occurred which blew two phases of the high (4160 volt) side of the transformer and cables between the switchgear and the b us . Since the fault which occurred was phase to phasa and not phcse to ground, the high side circuit breaker overcurrent trip was not instanta-neous , but delayed per the design intent. Therefore, a voltage decrease was reflected onto 4160 volt switchgears 1 and 2 and all 480 volt buses before the high and low (480 s elt) side circuit breakers on switchgear 5 tripped to isolate the transformer.

As a result of the voltage drop on the 480 volt buses, the voltage relay's in-stalled to trip / inhibit the normal bearing water pump circuit breakers and initiate backup bearinz water (BU3W) on a loss of 480 volt essential buses were de-energized or dropped out. This tripped both of the operating bearing water pumps in each loop. Since backup bearing vater was not in operation, nor required by LCO 4.2.1 or LCO 4.2.2, all four circulators tripped on loss of bearing water.

A Loop 1 shutdown and two loop trouble scram automatically occurred.

The temporary auxiliary boiler tripped causing a loss of steam flow to the 1A boiler feed pump and subsequent decrease in feedwater flow to the Loop 2 s team generators. The LA boiler feed pump was the only feedwater pump in cer-vice at the time.

No automatic emergency diesel generator operation occurred nor was any required since the condition was not a loss of voltage but a voltage fluctuation. There-fore, no conditions existed on the reserve auxiliary transformer which would have necessitated emergency diesel generator operation.

A request for a revision to Specification LCO 4.2.18, Primary Coolant Depres-surization, was sent to the Nuclear Regulatory Cocaission January 3,1979, in our letter P-79001. The proposed revision to the basis of the LCO and the curves attached to the proposal illustrate that a significant increase in time is allowed for starting of depressurization with an average core outlet tem-perature less than 1,000* F. Refer to Figure 4.2.18-2 of proposed change.

With the core outlet temperature at 462*F, the time available prior to start of depressurization would be approximately six hours and 50 minutes. Under the conditions which existed, the time allowable for restoring core cooling is approximately twelve hours. Thus the fifteen minute interruption of coolant flow would not have any adverse effects.

444 279

REPORTABLE OCCUREFNCE 79-17 ISSUE O Page 5 of 7 CORRECTIVE ACTION: .

Forced circulation was re-established using condensate on the steam generators and as motive force to the IB and 1D circulator Pelton drive. Power was re-stored to 480 volt switchgear 5. Bus 5 had been left isolated until it was confirmed that there was not a possibility of closing in on a f ault on the b us . Bus 5 was energizei via the Bus 4 to Bus 5 tie breaker. The transformer was replaced with a spare, the 480 volt cables f rom the switchgear to the bus were replaced, the 4160 volt circuit breaker was tested and found to operate properly, and the transformer was returned to service.

FAILURE DATA /SI>ELAR REPORTED OCCURRENCES :

Sisilar occurrences were reported in Reportable Occurrence Report No. 50-267/76-01 and Reportable Occurrence Report No. 50-267/77-14.

P R0GRA>f>L1 TIC I)'E ACT:

None CO DE IMP ACT :

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REPORTABLE OCCURRENCE 79-17 ISSUE O Page 7 of 7 Prepared by: o Miqhael J. Fertjs Technical Servfces Engineer Reviewed by: / e

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/ Technical Services Supervisor Reviewed by: -

Frank M. Mathie Operations Manager Approved by: N 3C ' b*' W- #

v Don Warembourg Bbnager, Nuclear Production 444 282

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