Changes of 50% Flashover Voltage and Compounds Produced on the Cathode Surface by Discharges Repeated in Low Pressure Air

Abstract

A lightning impulse voltage is applied repeatedly 10, 000 times to an electrode system of positive point to plane and two electrode systems of positive point to plane which has installed a protrusion in low pressure air of 8Torr. The flashaver voltages where the rate of arc occurrence times to discharge times is 50% (henceforth, call 50% flashover voltage, V_A50) are examined. Moreover, effects of the increase of the discharge times on changes of the V_A50 in the range of 0.1-30Torr and the protrusion surface forming a part of the cathode surface are examined, and the change of the protrusion surface is observed and analyzed by an electron probe micro-analyzer and a x-ray photoelectron spectroscope.
The main results are as follows.
(1) Even in the case that the 50% flashover voltage of the electrode system rised and saturated in the relatively little discharge times, when the discharge times were increased further, the 50% flashover voltage became higher and again saturated, depending on the electrode system geometry and the oxidation of the cathode surface.
(2) The 50% flashover voltages became higher in the vicinity of 8Torr in which the voltage was applied so many times, and the curves of the V_A50 to the air pressures of 0.1-30Torr before and after the many discharges differed.
(3) After a number of discharges, CuO and Cu₂O were formed and the adsorptive oxygen molecule or moisture appeared on the copper cathode surface, and as a result the 50% flashover voltage became higher.