Interface charge accumulation and decay of multi-layer oil-paper insulation under DC and polarity reversal voltages

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil-paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil-paper insulation system under DC and polarity reversal voltages. Induced electrification was used to charge the composite oil-paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. From the research on different layers and thicknesses of paper, with an increase in the number of the paper layers, the interface charge density decreases. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer.