Evaluation of distinctive PD parameters in SF6 under ac voltage

Discharge measurements have been carried out in SF₆ at pressures of 1 and 5 bars in a laboratory test cell with a needle plane electrode configuration. The plane electrode was subjected to an ac high voltage. The discharge sequences were recorded by a combination of phase resolved partial discharge analysis (PRPD) and fast digitizing oscilloscope. It was found that each of those techniques brought distinctive information, while none of them would give all the information about a discharge sequence. Cross correlation between the different results was used to validate the information. The PRPD analysis was found to give a more global information that allowed recognition of characteristic patterns with regard to the ac voltage. This information was lost during the recording of a 1-millisecond digitized frame. The use of the fast digitizer made it possible to evaluate the dependency of successive pulses within a discharge sequence. The measurements performed at low and high pressure presented sequences completely different in nature. This paper shows that some parameters, such as the time between pulses within a discharge sequence need to be used in addition to the PRPD patterns to fully characterize a discharge source or to monitor it over time