Condition assessment of medium voltage underground cables based on tangent delta and partial discharge measurements

The suitability of 0.1 Hz VLF tangent delta (TD) and three different partial discharge (PD) measurement methods for condition assessment and commissioning measurements of MV underground cable systems were studied. On-site measurements were conducted in MV networks of 12 Finnish distribution network operators (DNOs) on a total of 100 three phase cable systems of different ages and 5 different cable types. Based on the TD measurement results the limits given in IEEE Std400.2-2013 seem too high for Finnish cable systems. No clear correlation between the PD and TD measurement results or the age of the cable and TD was observed. It could be concluded that the measurements complement each other revealing different degradation mechanisms. TD measurement does not reliably reveal the presence of PD and thus, for cable commissioning, PD measurement is recommended.