The influence of coatings and the surface state on the electric strength of vacuum insulated electrodes used with a 50 Hz supply

Abstract

The electric strength of the vacuum insulation of plane electrodes not subject to surface conditioning was studied with a 50 Hz sinusoidal wave in the range up to 100 kV (peak). The electrodes with rounded-off edges of Rogowski’s profile were made of OFHC copper or aluminium and were of 50 mm diameter. The paper shows the influence of fundamental factors defining the conditions of a vacuum insulation system on its electric strength, such as: the presence of a cobalt–tungsten layer coating the surfaces of copper electrodes, the presence of aluminium oxide layer coating the surfaces of aluminium electrodes, the method of preparing the electrodes surfaces (mechanical polishing or electropolishing), the value of air pressure (within the range of 2 mPa–0.2 Pa) and at a constant value pressure of 0.2 mPa, the length of vacuum gap. Experiments showed an insignificant electric strength increase of unconditioned vacuum insulation systems could be obtained by means of electropolishing copper electrode surfaces instead of mechanical polishing. When copper electrode surfaces were coated with a layer of cobalt–tungsten or cobalt–tungsten–molybdenum deposited by electroplating, the investigated systems showed a 30% increase of electric strength at a pressure of 2 mPa. Aluminium electrodes coated with aluminium oxide showed a 100% increase of electric strength of unconditioned vacuum insulation systems at the same pressure.