Application of the UHF technology to detect and locate partial discharges in liquid immersed transformer

The paper explains theoretical aspects of PD signal propagation in the UHF range inside a transformer. The influences of inner structures inside the transformer and the sensor characteristics on UHF measurement results are discussed. Based on practical examples measured on transformers ranging from 100 MVA to more than 1000 MVA the sensitivity of UHF measurements is described with respect to the measurement frequency used and the propagation path between PD spot and UHF sensor. Another discussed aspect is a comparison of PD pattern recorded with a UHF system and pattern of the same fault acquired by conventional PD equipment. The application of UHF signals triggering an acoustical PD localization system is shown. Found limitations of the UHF method are discussed, too. Finally, an outlook on foreseeable evolutions and developments in the field of UHF PD detection, measurement and monitoring is given.