1. When measuring the leakage current of oil-paper insulated power equipment, it is easier to find the insulation defects by applying negative polarity of DC high-voltage test voltage. It can be explained by electroosmosis, the movement of a liquid through a porous solid under an applied electric field. Operational experience shows that the insulation of oil-immersed power equipment such as cables or transformers is usually exposed to moisture from the outside or near the housing. According to the phenomenon of electroosmosis, the water in the insulation of cable or transformer is positively charged under the action of electric field. When the cable core or transformer winding is applied with positive polarity voltage, the water in the insulation is rejected by the cable and seeps to the outer skin or shell, so that the water content is relatively reduced and the leakage current is reduced. When negative polarity voltage is applied to the cable core or transformer winding, the moisture in the insulation will be attracted by the cable core or transformer winding, and then move to the cable core or transformer winding through the insulation, so that the moisture in the high-field area of insulation will increase relatively, leading to the increase of leakage current.

2, dc withstand voltage test, in bar - plate electrode composed of asymmetric, highly uniform electric field, the gas gap phase at the same time, due to the polarity effect, negative rods - positive plate of the spark discharge voltage is anode spark discharge voltage is great - more than double, usually, the power equipment of the external insulation level is higher than its insulation level, obviously, applying negative polarity test voltage insulation flashover are less likely to outside, it is helpful to realize the dc withstand voltage test for the purpose of the internal insulation defect. In addition, due to the phenomenon of electroosmosis, the breakdown of internal insulation test voltage of oil-immersed paper insulation power equipment such as cables is about 10% lower than that of positive polarity. That is to say, the sensitivity of detecting defects in negative polarity test voltage of cable core connection is higher, that is, it is easier to find insulation defects.