Experimental Study On Surface Flashover Characteristics Of Nylon And PMMA Under Fast Pulse

In the pulsed power devices, solid-liquid and solid-gas composite insulation structure are universal. The flashover voltage is much less than the body breakdown voltage, and therefore the flashover insulating strength becomes a major factor restricting high-power devices operating voltage level. Domestic and international researches on the surface flashover phenomenon mostly focuses on DC voltage, power frequency AC voltage and lightning impulse, and the study on the flashover under nanosecond fast pulse is few. In this paper, for the actual need of pulsed power technology, the flashover characteristics of two common insulating materials-PMMA and nylon under microsecond and nanosecond pulses are studied, respectively, in nitrogen and dodecylbenzene.This paper establishes the insulating material surface flashover experimental platform, including the development of the pulse generator of different rise time and amplitude, nanosecond pulse measurement system, gas containers, liquid containers, experimental electrode systems. Based on the output voltage requirements of the flashover experiments in gas and oil, the design of pulse generator is different. Ultimately the rise time of steepest pulse we get is 15 ns and the maximum output voltage amplitude is 250 kV. The nanosecond pulse measurement system is a resistive divider with the oscilloscope. The experimental electrodes are planar electrodes, forming slightly nonuniform field in the gap.The influence of the initial electron emission of the triple junction on the insulation material flashover in the gas is analyzed. The theoretical relationship among flashover field strength, gas pressure and gas dielectric constant is derived. The relationship can effectively explain the flashover field strength trends with the gas pressure, gap distance and pulse rise time. Based on the triple junction field electron emission and secondary electron avalanche theory, the flashover development of the insulating material in the oil is describedThe experiment research on the flashover characteristics of PMMA and nylon in nitrogen and dodecylbenzene, under DC voltage, microsecond and nanosecond pulses of different rise times, is carried out. In detail, the research is among the surface flashover voltage/field strength trend with nitrogen pressure, flashover distance and other parameters. The result is shown. When the pressure is small, with the gas pressure increasing, the flashover field strength increases rapidly, and when the pressure is high(greater than 3 atm), with the pressure increasing, the flashover field strength increases slowly and trends to be saturated. Meanwhile, when the gap is small(less than 5mm), increasing gas pressure to improve insulating strength is a very limited way. For a larger gap distance, increasing the gas pressure can be used as an effective way to improve the insulating strength. Under the same pressure, the flashover field strength of PMMA and nylon under fast pulses is a lot bigger than that under the DC voltage. But with the pressure and gap distance increasing, the differences between them trend to be smaller. The flashover voltage increases with the gap distance, but the flashover field strength decreases. The steeper the rising edge of the pulse voltage, the higher the flashover field strength. Whether in gas or oil, the insulating properties of PMMA are superior to that of nylon.