On-site PD measurements on high voltage cables have to concentrate on the cable accessories because there is a remaining risk for assembling faults on site. PD sensors with an appropriate coupling behavior to accessory-internal PD give sensitivities of a few pC or even better. Unfortunately, two main reasons prevent the general use of PD sensors in cable accessories. First of all, the costs for PD sensors have to be balanced with the costs of the accessories, importance of the cable link, consequential costs for outage etc. This is the reason why PD sensors were mainly used EHV cable systems. The second reason is limited accessibility: the PD sensor cable at the accessory has to be connected to a PD detection unit. Accessibility is much more difficult for direct buried cable systems than for cable terminations and tunnel-laid cable systems: the senor cable must pass the ground and the end up in a box on the surface to provide access. This solution causes additional costs and new problems like sealing the sensor cable against humidity, capability to withstand sheath testing etc. By looking for alternative access to PD signals from cable joints of long cable systems, a very simple solution proved suitable: detecting PD at cross-bonding links. To investigate the high frequency propagation of PD pulses in cross-bonding links, computer simulations and laboratory measurements were done.
Device for online monitoring of insulation faults in high-voltage switchgears