Evaluation Methods of Contamination Flashover Voltage Performance of Semi-conducting Glaze Insulators

Superior insulation performance of semi-conducting glaze porcelain insulators under contaminated and wet conditions is attributable mainly to its drying effect by the leakage current flowing in the glaze. Contamination flashover voltages of semi-conducting glaze insulators have been evaluated by the same test methods for normal porcelain and glass insulators. In the case of the clean fog test, 3 to 5 g/m
³
of fog density has been used also for semi-conducting glaze insulators. Now in view of the importance of drying effect on the contamination flashover voltages of semi-conducting glaze insulators, we considered that heavier fog density might give lower flashover voltages. In this paper, firstly, contamination flashover voltage test results under the heavier fog density of 13 g/m
³
, compared with the results under the conventional fog density of 3 to 5 g/m
³
, shall be presented. Conventional contamination design criterion of this type insulator under cold-wet switch-on conditions is based on the results by the test procedure where a contaminated and dried specimen insulator is wetted in artificial fog before applying a test voltage. However, in most cases of the cold-wet switch-on conditions, time durations without energization are very short as is the case of lightning, and so drying effect by the energization before de-energization may be remaining to some extent at the time of re-energization. We had made tests simulating such conditions. Of course, enhanced surface resistances at the time of re-enegization could be confirmed, but, unexpectedly, lower flashover voltages were obtained compared with the values estimated from the conventional relationship between the surface resistance and the minimum flashover voltage. Such results may be explained as follows; By the pre-energization, dry bands are formed and voltage distribution along the insulator surface becomes very non-uniform, resulting in lower flashover voltages. In this paper, secondly, additional investigation results of the effect of de-erergized time duration on the flashover voltage of a contaminated semi-conducting glaze insulator by both the clean fog and the salt fog tests shall be presented.