Surface and interface charge behaviour in multi-layer oil-paper insulation under DC voltage application

Oil-paper insulation system is an extraordinarily essential system of converter transformers as well as HVDC oil-impregnated paper capacitance graded bushings. Due to a few researches in surface and interface charge behaviour of supporting materials, there are still some problems in charge accumulation, thus affecting the application of HVDC bushings and the safety of entire grid. This paper studies the surface and interface charge behaviour of oil-paper insulation system under DC voltage and inducing electrification was used to charge the oil-paper compound. The amplitudes of the DC voltage varied from 1 to 4 kV and the paper was 0.13 mm in thickness. From the research of different voltage amplitudes and charging times of the paper, conclusions can be drawn that the decay is fast initially, and then becomes slower with the time. At last the dissipation trend tends to be flat. With the increase of the voltage amplitudes, the surface potential rises more rapidly, and higher voltage would also cause the charge to dissipate more quickly. As for another influence factor, longer charging times would cause the charge to dissipate a bit more slowly. The decay rate of the negative voltage is a little smaller than that of the positive charge. Moreover, when it comes to the effect on the interface charge of the paper layers, the surface potential is more proportional to the increase of paper layers. When the amplitude of the voltage applied in the test is small or the charging time is short enough, the bottom layer has hardly any interface charge accumulating.