Transformer fire delays Watts Bar Unit 2 commissioning

What happened

On 30 August 2016, an electrical fault on the 2B main bank transformer at Tennessee Valley Authority's Watts Bar Unit 2 caused the main turbine to trip. The reactor also tripped by design. The fault led to an oil fire in an exterior switchyard transformer, reported about a minute later and extinguished by the onsite emergency response team. The unit was in power ascension testing and had reached 99% power the previous day.

The operator said the incident was likely to keep commercial operation from the then end-of-summer target, and that testing could not continue until the transformer was restored or another path to the switchyard was established. No timetable was given.

Contributing factors

The source reports an electrical fault on the 2B main bank transformer as the initiating event. It does not state a deeper cause. The installed transformer was not new: it had been on site since the 1990s, had been modified and maintained, and had previously been used on Unit 1. Damage had not been fully diagnosed, and it was not yet decided whether to repair or replace it. A spare transformer was on site.

Lessons learned

1. During commissioning, a single main transformer fault can stop power ascension and push back commercial operation, so spare equipment or an alternative path from the generator to the switchyard should be identified in advance.
2. Do not treat a long-service transformer as proven for a new unit solely because it has been modified, maintained or used elsewhere; diagnose damage before choosing repair or replacement.
3. Practised onsite response can limit a switchyard oil fire before it develops into a wider event.

Source

powermag.com