Measurements of partial discharges (PDs) are commonly used as diagnostic indicators for assessing the quality of high voltage equipment. In many cases, such PD measurements are of practical reasons either performed at online service stresses or at very low frequency (VLF) off-line tests. This paper aims at providing some insight into how PD parameters determined during VLF off-line testing relate to PD activity occurring at 50 Hz service conditions. The theoretical analysis is based on the classical abc-model applied on glass fibre reinforced epoxy-mica samples including void enclosure with diameter of 10 mm. The PD-activity was examined at test frequencies from 0.1 Hz to 300 Hz, and the temperature was varied in the range from 20°C to 90°C. The PD inception voltage was found to be frequency independent, while the maximum apparent charge magnitudes were found to increase with increasing voltage frequency. A high number of low magnitude PDs were observed at low frequencies, which strongly indicate that just a fraction of the void surface is involved in each discharge. It is shown that the total accumulated apparent charge per voltage period is a rather frequency independent parameter, strongly correlated to the average dissipated partial discharge energy.
Keywords— Diagnostic testing, partial discharges, cavities,
variable voltage frequency