Insulation characteristics of epoxy insulator with internal crack-shaped micro-defects - study on the equivalence of accelerated degradation by frequency acceleration test

Abstract

Generally, insulating spacers of gas insulated switchgear (GIS) have outstanding durability and are not considered prone to insulation failure within a design life of about 30 years, provided the products conform and have passed the partial discharge (PD) test. However, to assume operation over 30 years, degradation characteristics are important in cases where an extremely microscopic defect below the detection level in the PD test is present or produced inside the insulator. Accordingly, to date, the authors have obtained the breakdown voltagetime (V-t) characteristics using epoxy insulators provided with three types of micro-defect (crack, void, and delamination); the apparent PD of which is equivalent to 1 pC for an actual 550 kV-GIS spacer. It emerged based on these V-t characteristics that the breakdown risk peaked for a crack defect among those three types of shape and that it was essential to obtain data on long-term breakdown characteristics at the actual operating electric field level to rigorously evaluate the breakdown risk under long-term operation. Therefore, this paper includes a study on the equivalence of accelerated degradation by frequency acceleration as a means of determining the long-term breakdown characteristics. In the study, the dependency of various PD characteristics on frequency was initially examined by changing the frequency of the applied voltage within the range 60 to 3000 Hz. Consequently, the increase in the number of PDs per unit time proportional to frequency could be confirmed without any abnormal phenomena, such as suspended discharge, observed within the frequency range this time. In other words, it was clarified that the degradation due to PD could be accelerated by increasing the frequency. Subsequently, breakdown tests were conducted with frequency as a parameter to evaluate the frequency up to which the equivalence of accelerated degradation could be maintained based on breakdown times. As a result, it was found that the equivalence of acceleration degradation by frequency acceleration was valid in acceleration at up to 1500 Hz because the breakdown time was shortened in reverse proportion to the frequency.