Effect of test voltage frequency on PD inception in service aged cable joints

Condition assessment of cable systems can today be performed at different voltage frequencies. This includes VLF testing at 0.1 Hz, 50 Hz (also on-line) and also at higher frequencies in the range of 200-500 Hz by using oscillating wave test techniques. An important parameter used during condition assessment is the inception voltage of partial discharges. If this voltage is well above the service stress the cable system is likely not aged by partial discharges during service. The underlying assumption during testing is that this voltage is independent of frequency. The main purpose of this study has been to measure the inception voltage and characteristics of partial discharges in service-aged XLPE cold-shrink cable joints. A set-up with possibilities to measure partial discharges in a broad frequency range from 1 mHz to 100 Hz with a voltage up to 20 kV was used. Especial attention has been paid to the frequency dependency of the magnitude and number of the discharges. The results show that the inception voltage can be frequency dependent. It is therefore important to take this into consideration when assessing the condition of cable systems.