Origin and Cause
A Power Loss and a Damaged High Voltage Transformer
A section of a large city unexpectedly experienced an electrical power failure. At the time of the incident, the weather was clear, and there were no reported fires or accidents near electrical equipment. Determination of the cause of the power outage was an issue, but also of concern was the scope of damage to utility equipment that resulted from the incident. This equipment included an expensive high-voltage transformer. On the day of this incident, the displays in the utility control room showed that a device called differential voltage relay sensed a voltage difference between two electrical phases of the output of a distribution transformer. The relay automatically operated an electrical breaker to de-energize the transformer. The result was the loss of electrical service to a section of the city. Oil sample testing, a turns ratio test, internal examination using a boroscope, and examination of the transformer at a repair facility confirmed that electrical arcing (an electrical short) had occurred in the Phase B secondary coil.
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