Study of breakdown inside a supercritical fluid plasma switch

Supercritical fluid (SCF), characterized by high pressure and high density, combines the advantage of gas and liquid: high ability of mass transfer (dense, high diffusivity and low viscosity) and high heat transfer (high heat conductivity). Research on discharge behavior of SCF in high electrical field is more recent and less common, and is also motivated by new environmental and economical reasons. SCF has high potential in high power switching area, and is proposed to be the alternative for high voltage switching media, owing to its distinguished advantage of high breakdown strength, fast dielectric recovery, and low impact to environment. In conventional circuit breakers, SCF would be the perfect substitute medium for SF₆, which is a powerful greenhouse gas and after use is highly toxic. In this work, the design of a versatile SC switch up to 200 bar with adjustable heavy duty electrodes, and imbedded current and voltage sensor is briefly introduced. The SCF supply (adjustable flow rate from 5-700 Liter/h) and repetitive pulse voltage source (30 kV peak value, up to 1kHz repetition rate) enable measurement of breakdown voltage and breakdown delay time inside SC nitrogen, corresponding to variations of parameters such as voltage repetition rate, gap width and gas flow rate through the gap. Via a simultaneous triggering system, breakdown inside a SC nitrogen switch is pictured by a sub-nanosecond fast CCD camera, providing space and time parameters for theoretical analysis of breakdown and subsequent dielectric recovery inside a SCF.