Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - study using the equivalent circuit of the winding model

The soundness of a gas-filled power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. To establish clear control criteria for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves used in a gas-filled transformer. In the actual PD test, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors. Since this damping must be taken into consideration to establish control criteria, a preceding study investigated the propagation characteristics of PD signals through actual measurement using transformer winding models. As a result, it was found that the PD having occurred inside the winding is detected as the signal significantly damped depending on the position of occurrence. This paper studies whether PD signal propagation characteristics can be analytically evaluated using the equivalent circuit of the transformer winding. This analytical evaluation, if feasible, can be applied to various winding structures and will make it possible to evaluate the propagation characteristics in further detail as well as the validity of the actual measurement results. The study was conducted on the interleaved and continuous disk windings to make a comparison between the analysis and measurement results of the voltage waveform and the propagation pattern at various locations inside the winding with the position of occurrence and measurement position of the PD as parameters. Consequently, it emerged that the analysis using the equivalent circuit could adequately simulate the propagation pattern and waveform shape of the PD signal. A detailed study using this equivalent circuit also clarified the fact that the PD signal propagation waveform was determined by the winding constant and the terminal conditions. Furthermore, the analytical and measurement results of the transmission rate were almost identical, whereby it was analytically confirmed that the transmission rate significantly decreased depending on the position of occurrence of the PD. It also emerged that, since the frequency characteristics of the PD measurement device may influence the transmission rate, the PD control criteria must be studied taking such influence into consideration.