The most commonly used criteria for PDIV (partial-discharge inception voltage) determination are examined using three synthetic transformer oils as test fluids. It is shown that the PD amplitude distribution changes both with increasing applied voltage and with test duration. The use of a multichannel analyzer was shown to be most helpful in rendering the PDIV statistically more meaningful. It is noted that both the number and amplitude of PDs are random phenomena the occurrence of which is hard to predict. Close to the breakdown voltage the PD activity increases and its amplitude spectrum approaches a quasi-exponential shape.
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