ML19345G163
| ML19345G163 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 05/13/1974 |
| From: | Graves R CONNECTICUT YANKEE ATOMIC POWER CO. |
| To: | |
| References | |
| AO-50-213-74-8, NUDOCS 8103090783 | |
| Download: ML19345G163 (2) | |
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ABil0RMAL OCCURREilCE REPORT 50-213-74-8 Report flumber:
Report Date:
May 13, 1974 Occurrence Date:
May 3, 1974 Facility:
Connecticut Yankee Atomic Power Company Identification of Occurrence:
Failure of Auxiliary Transformer for Emergency Diesel Generator 2B Conditions Prior to Occurrence:
Steady State' Power; conducting routine monthly diesel generator surveillance test.
Description of Occurrence:
Connected to the 4.16KV emergency diesel generator main leads is a 3 phase auxiliary transformer rated 45KVA total..used to provide a 240V AC input to the main generator field circuit. This auxiliary transformer is comp-rised of 3 single phase transformers, each rated 2400/240V 15KVA, connected in Wye on high voltage side and Delta on the low voltage side.
Dur ng a routine monthly load test of 2B Emergency Diesel Generator, smoke was observed coming from the associated remote excitation cabinet.
The rea panel of the excitation cabinet was removed and one of the three field excitation transformers was found smoldering.
The unit was removed from service and the smoldering transformer was extinguished. An inspection revealed a hole in tne nign voltage winding. Transformer wiring suffered consi derable damage and the transformer itself was badly charred.
A second exciter transformer in the same cabinet also exhibited evidence of overheating.
Designa' ion of Apparent Cause of Occurrence:
An improper grounding scheme caused a circulating current, resulting in an over loadeJ transfonner and subseauent failure.
Analysis of Occurrence:
Failure of the exciter power transformer for Emergency Diesel Generator 2B did not degrade the emergency generation capability since a second completely redundant emergency diesel generator was available and operable.
During the outage of 2B Emergency Diesel Generator, 2A Emergency Diesel Generator was started daily and all equipment associated with it was operable in accordance with technical specification requirements.
Corrective Action:
Two of the three excitation transformers and associated wiring were repl aced. After a series of pre-operational checks the 2B Diesel Generator was started, phased and loaded up to 2850lN. Observed line current readings NM M$$M 27030907P3 eege i of 2 1
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"I
z Connecticut Yankee Atomic Power Company Abnormal Occurrence 50-213-74-8 The indicated that the excitation power transformer was heavily overloaded.
readings were considered to be indicative of a circulating current in the low side Delta. The vendor's representative recommended that the auxiliary transformer or ground (.ig connection be lifted and the line currents measured once again. Lifting this auxiliary transformer grounding connection resulted in normal transformer current measurements..It has been concluded that removing this grounding connection is a satisfactory interim solution, since it reduced the loading of the auxiliary transformer to a satisfactory level. lle are presently investigating an improved grounding scheme that should prevent the flow of circulating current.
Since Emergency Diesel Generator 2A has similar excitation transformer equipment and a similar grounding scheme, a similar te>t was performed on this unit. The diesel generator was started, phased and lodded to 2850KV and line currents were measured. A similar overloaded condition was observed. The current readings returned to normal after the grounding connection was removed.
With the installation of an improved grounding scheme, it is not expected that this situation will recur.
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