Calibration of UHF Partial Discharge Measurement for Power Transformers And a Comparison to the Calibration of Electrical PD Measurement

The reliability of electrical energy networks depends on the quality and reliability of its electrical equipment, e.g. power transformers. Local failures inside their insulation may lead to catastrophic breakdowns and may cause high outage and penalty costs. To prevent these destroying events, power transformers can be tested on partial discharge (PD) activity before commissioning and monitored during service. In this contribution, a calibration procedure for the ultra-high frequency (UHF) method is proposed as it is necessary to ensure reproducibility and comparability of UHF measurements. Afterwards, a calibrated UHF method can be introduced supplementary to IEC60270 in acceptance tests of power transformers. This contribution compares the calibration procedures of the conventional electric method (IEC60270) and the electromagnetic method. A characterization of UHF sensors by the antenna factor (AF) is a precondition for the UHF calibration procedure. To provide profound knowledge of the equipment, the AF of the UHF sensor is determined under inside transformer conditions. To meet these conditions, an oil-filled GTEM cell is used for correct permittivity. Additional to the calibration procedure, the performance of the installed sensor has to be determined. The evaluation is based on the idea of transmitting electromagnetic waves through the transformer tank from one UHF sensor to another which is called performance check procedure. Keywords— Power Transformers; Partial Discharge; IEC60270; UHF; Electromagnetic; Monitoring; Calibration.