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~- 414 Nicollet Mall . .

.!Minneapolis, Minnesota 55401 1927 --

Telephone (612) 330 5500 -,

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April 9, 1990 '10 CFR Part 50Section 50.73 ,

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- ('| Director of Nuclear Reactor R'egulation- -i

U S Nuclear Regulatory CommissionAttn: Document Control DeskWashington,'00' -20555'

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' PRAIRIE ISLAND NUCLEAR' GENERATING PLANT .I

j'Docket Nos. 50 282 License Nos. DPR-4250-306 DPR-60-

' Unit 2 Reactor Trip from Generator Lockout,

Caused by Faulty Relav in Test Circuitry -

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The Licensee Event Report for this occurrence is attached.

This. event was reported via the Emergency Notification System in accordancewith 10 CFR Part 50, Section 50.72, on March 8, 1990. Please contact us if

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you require additional information related to this event.!

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Thomas M Parker,

ManagerNuclear Support Services

c: Regional Administrator - Region III, NRCNRR Project Manager, NRCSenior Resident Inspector, NRCMPCA

Attn: Dr J V Ferman

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On March 8, 1990, Unit 2 was at 100% power. An Operations Instructor and atrainec ; ore making operator rounds in the Turbine Building. The bus ductcooler local panel test was to be demonstrated to the trainee. When the test

button wasi depressed, a relay failed to operate properly and a generatorlockout signal was produced, resulting in a turbine trip and reactor trip at1104. Plant response to the trip was as expected for asecondary side initiated reactor trip; some water hammer occurred in secondaryside piping. After the circumstances of the trip were reviewed and correctiveaction taken, the unit was restarted.

cause of the event was failure of a test relay to operate properly, allowing a,

| trip signal to be generated. The circuit is designed so that when the testbutton is depressed, relay K3 deenergizes, removing the trip relay (23.X) fromthe circuit prior to simulating a high temperature condition. The K3 relaycontacts failed to open as designed. Thus, when the high temperaturecondition was simulated, a trip signal was generated, producing a generator,

) lockout and resulting in a turbine trip and reactor trip.

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EVENT DESCRIPTION * *

On March 8, 1990, Unit 2 was at 100% power. An Operations Instructor and atrainee were making operator rounds in the Turbine Building. The bus ductcooler local panel test was to be demonstrated to the trainee. When the testbutton was depressed, a relay (EIIS Component Identifier: RLY) failed tooperate properly and a main generator lockout signal was produced, resultingin a turbine trip and reactor trip at 1104. Plant response to the trip was asexpected for a secondary side initiated reactor trip; some water hammeroccurred in secondary side piping. After the circumstances of the trip werereviewed and corrective action taken, the unit was restarted.

The Generator Bus Duct Cooling System is provided to cool the generator outputbusses. Each of these three busses is located inside of a duct which providesprotection, insulation and cooling for the bus. The bus duct cooling system

| consists of an air filter, two blowers and two water cooled heat exchangers.The system filters, cools and circulates the air inside of the bus ducts. One

| of the blowers and its associated cooler is normally in service and the other; in standby. The standby blower / cooler unit will start if the primary unitj trips or if the bus duct temperature exceeds 62 degrees C. If the bus duct'

temperature exceeds 80 degrees C the generator is tripped automatienlly. Alocal control and alarm panel is provided near the bus duct coolers. The j

alarm panel on this local panel is tested occasionally by the Turbine Building |operators as they make their rounds of the plant. The test circuitry is !

designed to test the trip signals without actually causing a trip to occur.

| [AUSE OF THE EVENT |

Cause of the event was failure of a test relay to operate properly, allowing atrip signal to be generated. The circuit is designed so that when the test

| button is depressed, relay K3 deenergizes, removing the trip relay (23 X) from| the circuit prior to simulating a high temperature condition. The K3 relay 1

| contacts failed to open as designed. Thus, when the high temperaturei condition was simulated, a trip signal was generated, producing a generator| lockout'and resulting in a turbine trip and reactor trip.

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ANAINSIS OF THE EVENT - .

Because the plant responded as designed to the generator lockout signal, andbecause all plant safeguards equipment remained available for servicethroughout this event, there was no effect on the health and safety of thepublic.

This event is reportable pursuant to 10CFR50.73(a)(2)(iv) since this was anunplanned actuation of the reactor protection system.

CORRECTIVE ACTION

On line testing of the Unit 1 and Unit 2 bus duct cooler local panel has been:

discontinued. The value of on line testing of this panel is being studied.If on line testing is deemed advisable, modification to the- test circuitrywill be made prior to the resumption of testing.

FAILED COMPONENT IDENTIFICATION

K3 Relay in an OmniGuard System Model 310.

PREVIOUS SIMilAR EVENTS

There have been no previous similar events at Prairie Island,'

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