UHF PD Sensor Characterisation using GTEM Cells

In this paper, we compare results of sensor characterizations in air and mineral oil to investigate the influence of the dielectric medium on frequency response and sensitivity of the UHF sensors. The results of this comparison, together with supporting theoretical analysis, lead us to propose an empirical scaling procedure for determining the response of a UHF sensor under oil from its response measured in air. The method considers the effect of dielectric constant of the liquid insulating medium on both the wavelength of electromagnetic waves and the electric flux density which the sensor experiences. This approach will simplify the characterization of UHF sensors for transformers and should also work for synthetic or natural oils having different permittivity values. Through the exchange of monopole UHF sensing sensors with a well-defined frequency response, the method is demonstrated to be effective. The good level of agreement obtained suggests that a monopole sensor response calibrated at an accredited national testing laboratory could serve as a convenient and accurate reference to enable the standardisation of UHF sensor frequency response measurements. The procedure for converting between sensor frequency response characterization as effective height (He) and antenna factor (AF) is also explained. These research findings are timely in view of the increasing use of UHF sensors to enhance both factory and site acceptance testing (FAT, SAT) of power transformers for partial discharge activity.