Effect of Dielectric Coating on Breakdown Strength in High Pressure SF6

With the development of high voltage devices, various methods have been taken to avoid the insulation failure. Ceramic coating, as one of the methods, can be used as electrical insulation in demanding conditions, like high-temperature ambience or other harsh environment. However, the breakdown characteristics of ceramic coating switch have been little discussed, especially under pulse voltages. In this paper, to study the effect of alumina coating on breakdown characteristics, the high voltage experiments have been carried out in 0.3 MPa SF6 under nanosecond pulses. The electrode substrates were treated by a method of plasma electrolytic oxidation (PEO) to produce a layer of alumina coating. Four pairs of electrodes with varied thickness of alumina coating (20$\mu$m, 60$\mu$m, 100$\mu$m and 120$\mu$m), as well as one pair of electrodes without coating, were employed in this experiment. The applied voltage was produced by a compact Marx generator, and the rise time of it was about 40 ns. The breakdown voltage and breakdown time delay have been measured. The results showed that the thickness of coating layer have little effect on breakdown strength. And as the pulse steepness increased, the effect of coating will get strong at first and then go into a gently-changed period. Comparing the secondary electron emission coefficients of steel and alumina and the U-t curves of the four pairs of electrodes, the different process of discharge initiation and time delay might be the main causes for this phenomenon. Since the discharge initiates differently from cathode and anode, the coating electrode was utilized respectively as cathode and anode to investigate the effect of coating layer on initial process.