Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - experimental study using winding models in the air

To establish clear control criteria for a partial discharge (PD) test of long duration ac withstand voltage test of a gas-filled power transformer, the authors have conducted a study on the harmful PD level of materials themselves using the insulating material- and structural-models of an actual gas-filled power transformer. In the PD test on an actual transformer, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors, such as the position of occurrence, measurement position, and internal structure of the equipment. In particular, in the event of the PD between sections or turns, which might actually occur, significant damping of the signal is anticipated in certain cases due to the influence of the position of occurrence and the internal structure of the equipment, which is a propagation path. Therefore, this damping must be taken into consideration to establish control criteria for the PD. In this paper, to understand the characteristics of this damping, such as its magnitude, two types of winding, interleaved disk and continuous disk, were produced as transformer models to investigate the propagation characteristics of PD signals with the position of occurrence, measurement position, and wavefront duration of the PD as parameters. Consequently, it emerged that, where the PD occurs between sections or turns, the transmission rate at the measurement position was generally low because the PD signal circulates within the PD occurrence parts and does not come outside. Assuming measurement at the bushing test terminal, the lowest transmission rate was 2.8% and 2.0% between sections and turns, respectively. A study was also conducted on the occurrence of the PD against ground, despite the low anticipated potential of actual occurrence, and the lowest transmission rate was 4.0%. Accordingly, the PD having occurred inside the winding of the transformer can be detected with a magnitude of 2% or more; however, from a reverse perspective, the signal is detected as one significantly damped to a level of about 1/50.