Study for external propagation characteristics of the UHF signal emitted by partial discharge in transformers

UHF technique for the detection of partial discharge signal coming from the power transformer has been used for many years. However the UHF sensors were usually installed in the transformers through the oil drain valves or the observation holes on the transformer which is not suitable for the transformers in use. If we want to detect the PD signal onsite at anytime, external detection may be a good choice. As we all know, the steel walls of power transformers can lead to the attenuation of the amplitudes of internal PD signals, and the signal can propagate to the outside through the outlet bushings or the tank seams. In order to value the validity of external detection, it´s necessary to have a detailed study for the external propagation characteristic. In this paper, the propagation characteristic of the electromagnetic waves spreading through the bushings will be further studied, including the influence of the bushings dimension and materials. What´s more, the propagation characteristic research for different frequency electromagnetic waves will also be conducted. The simulation results show that the oscillation characteristics of the high frequency electromagnetic signal is more obvious than the lower one, if we detect the signal near the bushing. The catadioptric influence caused by the bushing material can weaken the oscillation characteristics. This paper can provide some guidance to the research for the installation location of the external UHF sensors, and may be useful for the study of propagation characteristics of the PD signal coming from other electrical equipments.