Surface partial dischargeand breakdown characteristics of oil-paper insulation under -40℃ condition

In order to analyze the surface discharge characteristics of oil-paper insulation under extremely-low temperature, the paper established a high voltage experimental system which could simulate extremely-low temperature extend to -50℃. On the basis of the system, the partial discharge (PD) and breakdown characteristics of oil-paper insulation between -40℃ and 40℃ were compared with the surface electrode model which could simulate the parallel electric field component. The results show that, the PD initial and extinction voltage under -40℃ is about 18% and 12% higher than the voltage under 40℃ respectively. The PD development process can be divided into three stages. The mean discharge magnitude under -40℃ of each stage are lower than that under 40℃, and partial discharge patterns show obvious asymmetric features under -40℃. The breakdown voltage under -40℃ is about 16% lower than that under 40℃. The water activity has been used to explain the mechanism for the first time, as a low temperature, the transformer oil could separate out unbound water that may become grain of ice which would distort the electric field. Lastly, the suggestions on UHV transformer insulation design and test commissioning were proposed. The results are useful to understand the discharge mechanism of oil-paper insulation under low temperature further.