VLF testing and diagnosis on medium voltage power cables

The advantages on VLF (very low frequency) on site testing compared to conventional testing at power frequency are shown in practical and dielectrical aspects. Furthermore VLF test voltages enable the possibility of diagnosis on site. The two well established diagnosis methods, dissipation factor (DF) measurement and partial discharge (PD) measurement are explained. Where the DF diagnosis gives a tool for judgement of the ageing status especially of plastic cables, the PD diagnosis enables detection of single defects within a cable system. The economic advantages of diagnosis are shown in practical examples. An outlook for application on VLF high voltage on site test systems concludes the paper.