Flashover characteristics of vacuum interrupters in liquid nitrogen and its comparison with air and transformer oil for the superconducting switchgear applications

Abstract Liquid Nitrogen Insulated metal enclosed Switchgear (LNIS) is a new type of power switchgear system. It uses liquid nitrogen (LN2) as coolant, insulation and arc quenching medium. LNIS mainly consists of resistive type superconducting fault current limiter (R-SFCL), LN2 isolator switch and Vacuum circuit breaker (VCB). Vacuum interrupters (VIs) are used in combination with magnetic actuator to operate as VCB in LNIS. This paper deals with the surface flashover investigations of VI in LNIS. For fair comparisons, two types of standard manufacturing, 12 kV, VI modules, namely; Smooth type and Umbrella type, were selected and tested in open air, transformer oil (also known as Hydro-treated Light Naphthenic Distillate) and LN2 environments under the standard lightning impulses, i-e positive and negative. Therefore, a total of 12 comparisons have been reported in this paper. Up and down method has been followed and Statistical analysis have been done for all types of tests. Weibull and Box & Whiskers plots reveal some important results with accumulated probabilities from 0% to 100% Flashover voltages (FOVs), the most important be the critical FOV, also known as 50% Flashover voltages (U50). The results reveal that, for Air tests, the 12 kV smooth type VI has a U50 range of 109–121 kV and Umbrella type is 98–123 kV. In transformer oil tests, the 12 kV smooth type VI has a U50 range of 161–164 kV and Umbrella type is 161–168 kV. In addition, For LN2 tests, the 12 kV smooth type VI has a U50 range of 232.5–282 kV while Umbrella type is 256–260 kV respectively. The 200 shoots survival tests were also done on three smooth type and 3 umbrella type VI bottles. Electrical distribution simulations were carried out to compare the experimental results with the simulation results. All types of tests were done at least 200 times or when the results became stable. It was also observed that the FOVs gets higher after 100 tests due to conditioning process. Lastly, it was observed that Umbrella type VIs are more suitable for LNIS applications because of its mechanical toughness and overall higher range of FOVs. The results documented in this paper will play an important role in designing the overall structure of LNIS. VI manufacturing companies are expected to take advantage of these findings for their design of VIs in Air, Transformer Oil and LN2 environments because the literature lacks such results about VIs until now.