Simulation and Measurement of Pressure Rise in GIS 145 kV due to Internal Arcing

Abstract

Internal arc testing of metal-enclosed, SF6 gas insulated switchgear (GIS) is defined by IEC 62271-203 and is not a part of mandatory type tests.

However, due to the increasing demands on the safety of personnel, more often the implementation of this test is required in the tender documentation. According to IEC, the duration of the electric arc is related to the performance of the protective system determined by the first and second stage of protection. For the rated short-circuit current equal or higher than 40 kA, during the first stage of protection (0.1 s), no external effects on enclosure other than the operation of pressure relief device is permitted. During the second stage of protection (≤0.3 s) no fragmentation is permitted, but burn-through is acceptable. The test should be carried out on the GIS compartment with the smallest volume at nominal gas pressure. Since a newly developed GIS 145 kV is designed as a three-phase encapsulated, arc initiation is achieved by short connecting of all three phase conductors in the vicinity of a partition by means of a thin metal wire. This ensures that two electric arcs burn simultaneously commutating between the phases, so the possibility of enclosure burn-through in this type of GIS is minimized. In order to prevent the release of SF6 gas in the atmosphere during the testing, a test enclosure should be placed in a protective gastight enclosure filled with air or more often SF6 gas at pressure of 0.1 MPa. This test object configuration significantly complicates the pressure rise calculation and increases the testing cost. In order to prevent enclosure fragmentation, the pressure difference between the test enclosure and the protective enclosure during the test should always be less than the bursting pressure of test enclosure. Also, the protective enclosure should be designed to withstand the maximum pressure rise that may occur after pressure relief device opens. In order to assess the likelihood of passing the upcoming type test for newly developed GIS, a computer program for calculation of pressure and temperature in the test enclosure and protective enclosure was developed. The mathematical model is based on the paper of the working group CIGRE A3.24, published in 2014. The basic model shown in the paper is enhanced by the real properties of the SF6 gas/plasma, evaporation of the electrode material and the insulator ablation. The contribution of exothermic/endothermic reactions between the gas and the electrode material on the pressure and temperature rise was also considered. At the same time, the measurements of pressure rise in GIS enclosure and protective enclosure were carried out in Končar High Power Laboratory. The experiments performed on a copper and aluminum electrodes in SF6 gas confirmed significantly higher contribution of aluminum electrodes to the pressure and temperature rise compared to the copper electrodes. The computer program is verified by measurement results.