On-Line Partial Discharge (PD) Test Services
PD Testing of Solid Insulated Voltage Transformers
IPEC HV have recently completed a On-line PD Testing, fleetwide survey of over 150x in-service 25kV Outdoor Voltage Transformers (VTs) for a large electromotive traction customer. This was carried out further to the catastrophic insulation failure of two in-service, cast-resin 25kV VTs mounted on the top of their electric locomotive trains. The On-line PD Test Survey provided a very good indication of the insulation condition of the VTs identified any VTs that were likely to fail in the near future.
The testing was carried out by IPEC HV Test Engineers using the OSM-Longshot© PD Test Unit with an HFCT sensor (connected around the primary winding earth cable of the VT) and the TEV sensor (placed on the earthed surface below the VT). The internal PDs measured in this equipment have similar characteristics to those observed in other solid-insulated equipment such as switchgear and cable sealing ends and so can be detected in similar ways.
Ch1: HFCT, f=12.5MHz, risetime=54ns
Ch2: TEV f=230MHz, risetime=11ns
By using the On-line PD test method the customer was able to quickly screen and test all 150 of their in-service VTs. Further to the detection of high PD levels (of up to 20,000pC) in a number VTs tested, the suspect VTs were taken out of service and tested off-line in the laboratory. These off-line PD tests verified and confirmed the results from the On-line testing and, due to concerns about the service reliability, the decision was made to replace the VTs on the entire fleet of trains.