Influence of Dielectric Window on the Performance of Window-type UHF PD Sensors

The quality of ultra-high frequency (UHF) partial discharge (PD) measurements depends, to a large extent, on the sensitivity of the UHF sensors. Since it is not always possible to conduct experiments on large power transformers, it is desirable to design a simulation model that takes into account as many influencing factors on measurements as possible. Two major types of UHF sensors are used for PD measurements in power transformers, namely, drain valve sensors and window-type sensors. The former is directly introduced in the oil space of the transformer tank whereas the latter is installed in a dielectric window. Therefore, to reach the sensor head in a window-type sensor, the UHF signal has to travel through more dielectric media than in the case of drain valve sensors. The influence of the propagation path on the performance of the window-type sensors is studied in this contribution. Three-dimensional models of the dielectric window and the UHF sensors are created in CST Microwave Studio, and the antenna parameters are studied. Thereafter, the models are incorporated in a validated simulation model of a 420 kV power transformer, and the performance of the various sensor types are compared to determine if there is a significant influence of the propagation path on the sensitivity of the sensors.