Comparison between Electrical and UHF PD Measurement concerning Calibration and Sensitivity for Power Transformers

The application of regular condition checks and continuous monitoring of power transformers gains in importance. Nowadays, these methods are frequently used to detect damages in the insulation system at early stage and thus to avoid outages. In addition to the electrical partial discharge (PD) measurement according IEC 60270 [1], the electromagnetic measurement method starts to become common practice. It is suitable for diagnostic measurement on-site and also as continuous PD monitoring system because of its lower sensitivity to external interference compared with the electrical measurement. For electrical PD measurement a calibration procedure for the ratio between capacitance of specimen and coupling capacitor is available. Although the actual PD charge still remains unknown, the associated comparability of electrical PD measurement systems has led to an acceptance level at transformer routine tests. A similar calibration of the electromagnetic UHF (ultra-high frequency: 300 MHz – 3 GHz) measurement has not been implemented yet, but is a necessity for further standardization of this method. This contribution compares both methods and takes a critical look at the established PD acceptance level. Therefore, the diagnostic value of the apparent charge measured according to IEC 60270 is discussed with regard to its dependency on the propagation path inside the winding and the frequency range of the measurement. Keywords—Partial Discharge (PD), Power Transformer, Sensitivity, Calibration, UHF, Apparent Charge