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Oscillatory Switching Transients
Occasionally, when a transformer is switched into or out of a system, a transient recovery voltage containing a large component of high frequency voltage will be subjected to the terminals of the transformer. This occurs most often when a vacuum breaker is used and
is a direct result of the characteristic of the vacuum breaker to chop current. These current chops and subsequent re-ignitions produce a transient voltage at the terminals of the transformer that is oscillatory, of high frequency and prolonged duration. When this applied voltage has a frequency component near one of the natural frequencies of the transformer and of sufficient duration, internal damage to the insulation structure of the transformer will result