Negative DC corona characteristics in SF6 under moisture contamination

This paper reports on the effect of moisture contamination related to negative partial-discharge activity in SF₆. Moisture contamination is common in gas-insulated systems and is mainly due to 1) temperature-dependent water desorption from inner surfaces and 2) external sources of pollution that infiltrate during installation and operation of the apparatus. However, few studies have been published on the subject and even they have focused on positive or AC voltages. Little is known about the influence of water vapor on negative partial-discharge phenomena, or its physical reality. The aim of this work, therefore, is to identify the influence of water contamination on a negative partial-discharge regime from electrical measurements and to corroborate our observations with known data. Any observations of negative effects from water contamination and/or methods of detecting water contamination by means of electrical measurements on gas-insulated systems could be of great help to engineers. The experiment was conducted under true-corona conditions; for that particular arrangement, the breakdown occurred in the range of 25 kV for atmospheric SF₆ . The characterization study focused mainly on the corona current and the distribution of recorded pulse amplitudes and frequencies. The breakdown and inception voltages were also measured. The results of all these measurements, taken separately in pure and contaminated SF₆ are compared