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When attempting to locate partial discharge (Pd) sources in operating transformers, it was found that the speed of the acoustic waves propagating in the transformer oil differed from the value suggested in the literature. Subsequent investigations showed that the sonic velocity in transformer oil was not constant but depended upon a complex relationship which included the temperature of the oil, its gas and moisture content as well as the frequency content of the propagating signal.
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Howells et al. (1984) studied this question.
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