On-line partial discharge measurement on instrument transformers

The viability of partial discharge (PD) measurements in the field on current transformers (CTs) is investigated. On-line monitoring of each CT is achieved by making use of the existing DDF link where a HF current transformer is clipped around it. A computer-based system is used for recording and analysing the results. It was found that the typical interference level from external corona discharges in the substation can be in the order of a few thousand pCs. On the other hand, a PD threshold as low as a few hundred pCs in a CT is already considered to be dangerous. Apart from this is the interference associated with RF transmissions from nearby commercial radio stations and also possible PD signals picked up from the other phases. Techniques to overcome some of the interference problems are presented in the paper. These include filtering at the front end detection circuit (hardware) and phase-position windowing (software). Furthermore, to minimise the inter-phase signal coupling, a multichannel monitoring system is required for recording simultaneously the signals from the various phases. Signal discrimination can be implemented with appropriate software algorithms.