INFLUENCE OF COATING POLISHED AND ROUGH ELECTRODES ON THE IMPULSE AND AC BREAKDOWN OF SF6

Abstract

Thin coatings adhering to the surface of the inner electrode of 30/70 mm coaxial cylinder electrodes, in the range of pressures examined (0.1-0.7 MP a), improved the standard lightning impulse (1.2/50 μs) and the AC (50 Hz) breakdown voltage of SF6 both in the case of polished and of rough electrodes. The results obtained show that (a) the 1 % impulse breakdown voltage with polished electrodes is higher for positive polarity and that the improvement of the breakdown voltage due to electrode coating is less than the improvement for negative polarity for which polarity at high pressures it may reach a figure of 25 %. However, the 1 % impulse breakdown voltage remains slightly higher for the positive polarity even for coated electrodes; (b) the 1 % impulse breakdown voltages with rough electrodes is for both positive and negative polarity lower than the breakdown voltages with polished electrodes. The improvement of the 1 % impulse breakdown voltage due to the coating of rough electrodes is greater than the improvement in the case of polished electrodes. The electrode coating improves the 1 % impulse breakdown voltage to values which are higher than those obtained with bare polished electrodes; (c) the AC 1 min. test breakdown voltage with rough electrodes is lower than the breakdown voltage with polished electrodes. Electrode coating improves the AC breakdown voltage to values which are greater than those obtained with polished bare electrodes.