On-line PD Testing of HV Transformers

As HV transformers are oil-filled it is not usual for them to discharge in the main tank or windings as the oil 'quenches' any PD activity here.

PD activity does occur in the HV bushings of transformers which can be detected using internal bushing 'taps' or, in some cases with external TEV sensors placed at the bottom of the bushings. PD 'TEV' signals will only come out of the transformer if it has insulated gaskets, either at the bushings or the input/output points. If there are no insulated gaskets and the transformer is fully metal enclosed, this acts as a Faraday Cage with all PD signals held within the Transformer.

Location of the origin of a PD site can be made with distributed TEV probes on the LV and HV side, if there are suitable 'pathways' for the PD signals to come out of the transformer. It is now becoming popular to include internal Capacitive Coupler Bushing 'Taps' in HV Transformer bushings and also UHF Coupler 'Windows' in the main tank to 'look into' the transformer to detect PD in the both the bushings and main tank.

Recommendations for HV Transformer Partial Discharge Monitoring

1. Regular PD testing and monitoring of the HV transformers should be made to ensure trouble free service operation of the high voltage components. Monthly testing seems as frequent as operationally convenient, but the risks of service trouble is always higher after commissioning, so perhaps a sliding scale could be applied, with more frequent measurements at the start, becoming less frequent as the system beds down without incident.
2. There is a real challenge with making On-line PD measurements at HV voltages of 110kV+ as we need to measure very small events in generally large noise. Any new capability which will help look for PD activity which is small (i.e. below 100pC) should be used. Transformer owners should make sure that new methods of analysing PD data and removing noise are implemented. Training in PD measurements for the staff is a vital part in keeping the high voltage systems discharge free.
3. Some thought could be given to fitting permanent PD sensors inside the transformers. This would have the advantage that they could be calibrated, and could be inside the faraday cage of the transformer, and hence be free of interfering noise external to the transformer. Such systems in the form of internal UHF Capacitive Couplers are routinely fitted into GIS switchgear at manufacture and this technology has recently been applied successfully to HV transformers.