Design and testing of UHF sensors for partial discharge detection in transformers

Fifteen years since the first application of the UHF method for partial discharge in gas insulated switchgear, this has become widely used and the preferred method for PD diagnostics in GIS. Same technique is currently starting to be applied for onsite PD testing of Power Transformers. Significant advantages of this UHF method include the reduction of background noise and the possibility of on-line testing. It does provide a relief of background noise by selecting a working frequency band where interference is minimal or absent . In order to apply this technique to power transformers in service, a specific UHF sensor which can be fitted in a transformer without the need of a power outage has been developed. The process followed to design several suitable sensors and the experimental tests chosen to select and validate the best of these designs are presented in the paper.