ML20042G421

LER 90-001-00:on 900410,non-essential Bus 5,4160/480-volt Transformer N-9215 Experienced Fault.Caused by Breakdown of Insulation on Secondary Coils of Transformer.Coils of Transformer rewound.W/900510 Ltr

ML20042G421
Person / Time
Site:	Fort Saint Vrain
Issue date:	05/10/1990
From:	Fuller C, Gramling J PUBLIC SERVICE CO. OF COLORADO
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-001, LER-90-1, P-90152, NUDOCS 9005140215
Download: ML20042G421 (7)

Text

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May 10, 1990 Fort St. Vrain Unit No. 1 P-90152 e

l{7 U. 5 Nuclear Regulacry Commitsion l 0 ATTN: Document Control Desk Wash-Ington, D.C, 20S55 i-4 Docket No. 50-257  %

WU2CT: Lice'isee Event Repcrt 00-001-00, Final Report i:

REFERENCE:

Facility Operating License No. DPR-34 _

Gentlemen:

Enclosed, please find a copy of Licensee Event Report No. 50-267/90-001-00, Final, submitted per the requirements of 10 CFR 50.73(a)(2)(x). -

If you have any questions, please contact Mr. M. H. Holmes at (303) 480-6960.

Sincerely, k d[N C. H. Fuller Manager, Nuclear Production

.and Station Manager CHF/imb Enclosure cc: Regional Administrator, Region IV ATTN: Mr. J. B. Baird Technical Assistant Division of Reactor Projects Mr. R. E. Farrell Senior Resident Inspector, FSV j 9005140215 900510 ADOCK 0500C g7 f ffDR

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LICENSEE EVENT REPORT (LER)

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.ut...cv m, ,. . . a . . , ..,, a % ,,, . .. . , n ei At 0541 hours0.00626 days <br />0.15 hours <br />8.945106e-4 weeks <br />2.058505e-4 months <br /> on April 10, 1990, with the plant permanently shutdown for defueling, the non-essential bus 5, 4160/480 volt transformer, N-9215, experienced a fault. Control room operators were immediately alerted to the fault by an audible noise and actuation of numerous control room trouble alarms.

Action was taken from the control room to disconnect 480 volt bus 5 from its power supply. Operations personnel dispatched from the control room discovered a small fire within the bus 5, 4160/480 volt transformer ce51 net. The fire was extinguished with two hand held CO2 extinguishers at 0550 hours0.00637 days <br />0.153 hours <br />9.093915e-4 weeks <br />2.09275e-4 months <br />. An auxiliary tender remained in the area to watch for reflash until s permanent watch was established.

• With the loss of non-essential bus 5, electrical power was lost to several non-essential loads including portions of the reactor and turbine building HVAC systems, some plant lighting, the reactor and turbine building overhead cranes,.

two reactor and turbine building motor control centers (MCCs), and the Loop II startup bypass drag valve, PV-21130-1. Loss of power to PV-21130-1 allowed the valve to drif t closed and isolate secondary coolant flow. Bus 5 was re-energized at 0930 hours0.0108 days <br />0.258 hours <br />0.00154 weeks <br />3.53865e-4 months <br />. Secondary coolant flow was re-established at 0954 hours0.011 days <br />0.265 hours <br />0.00158 weeks <br />3.62997e-4 months <br />.

Insulation breakdown on one of the bus 5 transformer's secondary coils has been identified as the cause of the fault. The coils of all three phases on this transformer were re wound.

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Fort St. Vrain has been permanently shutdown since August 18, 1989, and PSC is in the process of defueling the reactor. To date, the fuel elements from twelve ofthethirtysevenr3actorfuelregions(AC)*havebeenremovedfromtnecore and placed in the fuel stcrage wells. These twelve regions have been refilled with defueling elements composed of graphite and containing borenated graphite poison pint for re.ictivit/ control. There is no fuel in the defueling elements.

On April 10, 1990, the plant was at normal defueling conditions with the <

Pre:,trer, sed Concreto Reactor Vessel at atmospheric pressure. Ln avarage core helit.m outist tenper->ture of 120 degraes F. and core cooling being provided by the "C" helku circulator and the loop 11 cconomizer-esaporator-superheater (EES)rection[AB)*,bothrpera,tingoncondensateviatheemergwycondensata header, ,

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Ey[t O ESCR]pT10N; At 0541 hours0.00626 days <br />0.15 hours <br />8.945106e-4 weeks <br />2.058505e-4 months <br /> on April 10, 1990, the non-essential bus 5, 4160/a80 volt

• transformer, N-9215, [EC)* experienced a fault. Control room operators were immediately alerted to the fault by a noise within the transformer cabinet and actuation of numerous control room trouble alarms. Action was taken from the control room to disconnect 480 voit bus 5 from its power supply. Operations personnel were dispatched from the control room to evaluate the cause of the noise and alarms. Personnel observed white smoke in the area of 480 volt non-essential bus 5 and notified the shift supervisor. The shift supervisor proceeded to the area, directed that the rear panel to the bus 5 transformer cabinet be removed, and assisted in extinguishing a small fire within the cabinet. The fire was contained within the transformer cabinet and was extinguished with two hand held CO2 extinguishers at 0550 hours0.00637 days <br />0.153 hours <br />9.093915e-4 weeks <br />2.09275e-4 months <br />. An auxiliary tender remained in the area to watch for reflash until a permanent watch was established.

The loss of non-essential bus 5 resulted in a temporary less of secondary coolant flow to the Loop II EE5 section. Primary coolant flow was not interrupted and remained in service throughout the event. Primary coolant temperature remained relatively constant throughout the event.

Although FSV emergency procedures do not require 10 CFR 50.72 NRC notification on fires of less than 10 minutes duration, plant management decided to make NRC notification as a conservative measure. At 0655 hours0.00758 days <br />0.182 hours <br />0.00108 weeks <br />2.492275e-4 months <br /> NRC notification was made per 10 CFR 50.72(b)(1)(vi),

At 0930 hours0.0108 days <br />0.258 hours <br />0.00154 weeks <br />3.53865e-4 months <br />, non-essential bus 5 was re-energized via a cross-tie from 480 voit non-essential bus 4 [EC)*. At 0954 hours0.011 days <br />0.265 hours <br />0.00158 weeks <br />3.62997e-4 months <br />, secondary cooling was re-established in the Loop II EES section.

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An examination of the faulty bus 5 transformer identified that insulation breakdown'had occurred on the secondary coils of the damaged transformer phase. .

It is believed that this insuletion breakdown occurred prior to the fire and resulted in a short within the secondary coil windings of the transformer.

There are no indications that the insulation breakdown occurred for reasons other than normal component wear. The transformer cooling ports that allow air '

flow through and around the coils were inspected and verified to be open and .

functional.

ANALYSIS OF CVENT:

Although this event met netther 10 CFR 50.72(b)(1)(vi) criteria of an avent that "significantly hampers site persor.nel in the perfurmance of duties necessary for

>afe plant operation", or FSV wergency procedur* "$!P Class" criteris for "

declaring an unusual event (i.e.. fire lo, ting mow inar. ;C minutes), a conse:vative dGeision was made to make NRC tiotification pte 10 CFR .

50.72(b)(1)(vi). Netificazion wat made at 0655 hours0.00758 days <br />0.182 hours <br />0.00108 weeks <br />2.492275e-4 months <br /> on April 10, 1990. This event is therefore ocing reported herein consistent with the 10 CFR 50.72 notification and the requirements of 10 CFR 50.73(a)(2)(x).

The immediate action taken from the control room to open the bus 5 transformer feed breaker upon initiation of the transformer fault reduced both the damage to the transformer and the magnitude of the fire. The resulting fire was confined to one of the three phase coils of the non-essential bus 5, 4160/480 volt transformer and was contained within the bus 5 transformer cabinet. The fire did not threaten any safety related equipment that is relied upon to maintain core cooling during accident conditions and was readily extinguished with two hand held CO2 fire extinguishers. The fire produced little smoke and PSC verified that there was no smoke damage to surrounding equipment or instrumentation.

Non-essential bus 5 supplies equipment loads that are classified as "non-

essential" for maintaining reactor core cooling. These loads include the l turbine and reactor building overhead cranes, the plant warehouse, turbine and l reactor building MCCs, and some reactor and turbine building HVAC equipment, i

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0l0 0l4 or 016 gene . .an= anch.manwnn One of the MCC loads lost during the event was the hydraulic unit that powers drag valve PV-21130-1. When operating the secondary coolant system in the startup bypass mode (as it was during the event), PV-21130-1 is in the flow path from the Loop II EES discharge to the decay heat exchanger. Upon loss of hydraulic power, PV-21130-1 fails "as is" but drifts closed due to system pressure. Therefore, upon loss of bus 5, the hydraulic power system of PV-21130-1 shut down resulting in a loss of hydraulic fluid flow and eventual valve closure. This isolated the flow path from the EES to the decay heat exchanger and therefore, temporarily interrupted secondary coolant flow. This temporary lots of secondary coolant flow was of no immediate cen:ern for the following renons:

• Th( core decay heat load at the time of the event was such that all ccre l cooling (both primary and secondary) could oe off for up to 181 hours0.00209 days <br />0.0503 hours <br />2.992725e-4 weeks <br />6.88705e-5 months <br /> before

, a calculated average bulk core temperature of 760 degrees F was reached. 1 i

• Alternate secondary cociant flaw patns were available and could have been~

placed in service if needed.

+ One PCRV lir.er cooling loop is capable of removing existing core decay heat production. Both PCRV liner cooling loops were operable and in service during this event.

• Primary coolant flow remained in normal service during the avent.

Based on this analysis, it is concluded that this event did not pose a threat to plant safety or to the health and safety of the public.

CORRECTIVE ACTION:

The 4160 volt feed breaker to the non-essential bus 5, 4160/480 volt transformer was opened from the control room.

Operations personnel extinguished the fire with two hand held CO2 fire extinguishers. An auxiliary tender remained in the area to watch for reflash until a permanent watch was established.

480 volt ncn-essential bus 5 was re-energized from the 480 volt non-essential bus 4 cross-tie at 0930 hours0.0108 days <br />0.258 hours <br />0.00154 weeks <br />3.53865e-4 months <br />.

Secondary coolant flow was re-established at 0954 hours0.011 days <br />0.265 hours <br />0.00158 weeks <br />3.62997e-4 months <br />.

Equipment and instrumentation that were exposed to smoke were inspected for smoke damage. No damage was identified.

All three phase coils of the faulty transformer were re-wound and the transformer was returned to service, g eo. .u .u.opo aea+ere sso ais

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The non-essential bus 4, 4160/480 volt transformer has been cleaned, inspected, and tested. No deficiencies or potential problems were identified. The reactor plant HVAC auxiliary transformer and the turbine plant HVAC auxiliary transformer will be cleaned, inspected, and tested by May 30, 1990. These three transformers are of the same type and vintage as the bus 5 transformer that experienced the fault. It should be noted that essential bus 4160/480 volt transformers 1, 2, and 3 are of a different vintage and type (i e., oil cooled) and are monitored for degradation through oil sampling and analysis.

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