Partial Discharge Measurements over the Frequency Range 0.1 Hz to 50 Hz

The need for partial discharge (corona) measurements at very low frequencies is explained, and the results of tests on epoxy resin and polyethylene samples are given for the frequency range 0.1 Hz to 50 Hz. As no suitable commercial generator was available, a new type of low-frequency low-noise generator has been developed. The discharge detection system includes a pulse discrimination system for eliminating unwanted pulses from outside the test component. This was particularly useful when measuring discharges in a short length of cross-linked polyethylene cable and in a stator bar sample, in the presence of discharges over the ends. Traditional display methods are not suitable at low-frequency and the paper describes possible alternative recording techniques. One of these is a multi-channel pulse height analyzer, which facilitates the comparison of results over the complete frequency range. Discharge characteristics have also been recorded on a storage oscilloscope. The results obtained show that the inception voltage is virtually constant over the frequency range 0.1 Hz to 50 Hz, and also indicate how the partial discharge characteristics are affected by frequency, void size, temperature and humidity.