ML19263B892

Final Rept on Deficient Diesel Generator Hydraulic Actuator Speed Control Circuits on Diesels 2A-A & 2B-B.Electric Drives for Hydraulic Actuators Developed Open Armature Circuit & Relay Contacts Burned.Caused by Voltage Mismatch

ML19263B892
Person / Time
Site:	Sequoyah
Issue date:	12/20/1978
From:	TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML19263B891	List: ML19263B890 ML19263B892
References
NUDOCS 7901250033
Download: ML19263B892 (2)

Text

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ENCLOSURE SEQUOYAll NUCLI:AR PLANT DIESEL GENERATOR AClUATOR 110 TORS NCR 855 FINAL REPORT Description of Deficiency During preoperational testing of the diesel generators, failures occurred in the hydraulic actuator speed control circuits on diesels 2A-A and 23-B. The electric motors used to drive the hydraulic actuators were developing an open armature circuit and relays used to control the motors were found to have burned contacts.

There are four diesel generator sets each consisting of two diesels in tandem and one generator. There is one hydraulic actuator for each diesel. When one of the actuator motors failed, the affected actuator began to reduce the amount of fuel to its diesel. As a result, the actuator on the other diesel began to feed its diesel more fuel in an attempt to maintain diesel rpm. These tests were being run at no-load conditions.

In this condition the second tandem diesel was able to maintain design rpm and generator output. The failure was discovered by an alarm signaling fuel flow mismatch.

Safety Implications These four diesel generator sets are designated 1A-A, IB-B, 2A-A, and 2B-B. In any event requiring onsite emergency power, both generator sets 1A-A and 2A-A or both IB-B and 2B-B are necessary to bring the plant to an orderly shutdown and maintain it. Both diesels in a tandem set are required to operate for that set to be functional.

If the deficiency had gone uncorrected, insufficient emergency onsite power would have been available to mitigate the consequences of a design basis accident. Because the cause of the deficiency is common to each hydraulic actuatot .-*or, it is possible that one or more of the motors in the remaining a: ators could have failed in a similar manner during a postulated acciernt. If more failures had occurred, it is possible that there may not have been any onsite emergency power to mitigate the accident thereby jeopardizing the ability to bring the plant to a safe shutdown.

Corrective Action The actuator failure was originally postulated to be the result of a mismatch between the hydraulic actuator motor rated voltage (115V + 10%) and the diesel generator battery system rated voltage 790125003_3

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, , __ (125V + 107.). Upon investigation of the cause for this, a 100-ohm resistor was found to be wired into the field circuit of the governor motor instead of the armature as should have been done.

This allowed a full 140V DC to be applied to the motor armature winding, exceeding the voltage rating. TVA sent this information to Power Systems, Division of Morrison Knudsen (diesel-generator supplier), and requested them to investigate if this could be the cause of the failure. Power Systems, in turn, requested the verifica-tion tests of Woodward Governor Co. (governor manufacturer). Woodward ran a test of a sample motor with the voltage at both 90 and 140 volts DC for about 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> with the 100-ohm resistor in both the field and armature circuits. No failures were experienced.

Upon learning this, TVA went back for further investigation of the cause of the failure. Before the governor failures, wiring errors were found in which the wiring for the field and armature circuits had been reversed. This was discovered when diesel generator speed changes were opposite than that anticipated when a resistance adjustment was made to the circuit. This condition was corrected and operation of the diesel generators continued.

To summarize, the actuators were evidently damaged due to the combination of incorrect wiring and the misplaced 100-ohm resistor by causing a gradual degradation of the actuator motor until it failed. The wiring errors have been corrected and the failed actuator motors have been replaced.

Since the corrective action was taken, the diesel generators have been taken through their preoperational testing which includes 23 con-secutive starts of each diesel generator set as well as periodic surveillance testing. No failures or abnormalities were observed in the hydraulic actuator circuit. Futther, the remaining unfalled actuator rotors which could have incurred similar degradation damage will also be replaced. Engineering Design is providing guidance to other plant sites to ensure this condition does not exist for other TVA diesel generators with Woodward Governors.