Breakdown Voltage and Recovery Rate Estimation of a Supercritical Nitrogen Plasma Switch

Supercritical fluid (SCF), characterized by high pressure and high density, combines the advantages of gas and liquid: high ability of mass transfer and high heat transfer. SCF has a high potential as an electrical switching medium owing to its high breakdown strength and fast dielectric recovery capability after electrical breakdown. One other significant characteristic of SCF as insulating medium is its low impact to the environment, hence it is proposed as an ideal substitute for SF6-an extreme greenhouse gas in power system circuit breakers. In this paper, the thermodynamic characteristics of SCF are introduced by the review of references. An experimental analysis of electrical breakdown phenomena of a supercritical (SC) plasma switch with pressure up to 180 bar is presented, focusing on the breakdown voltage variation as a function of the fluid pressure, gap width of the electrodes, and fluid flow rate through the gap. A thermodynamic model for recovery rate analysis of a gas insulated switch is proposed and its feasibility is discussed. The model is used to predict the recovery rate of an SC switch.

I. Introduction

Supercritical fluid (SCF), which has high pressure and high density, combines the advantage of gas and liquid: high ability of mass transfer and high heat transfer. Substantial amount of research in SCF, typically supercritical (SC) , has been carried out in its application in chemistry fields, as an alternative for traditional solvents [1], [2]. In addition to these chemical applications, also the combination with electrical discharges in SC attracts attention in the area of pollutants destruction [3]–[6]. Research on the discharge behavior of SCF in high electrical fields is motivated by environmental and economical reasons. Nowadays, gases, liquids, and solids, as the widely used switching medium, all have considerable weak points in high-voltage and high-power applications. Bubble formation in liquids limits the breakdown strength [7], [8]. Gas insulators, such as air, nitrogen, and argon, have typical recovery times in the order of 10 ms, which results in a low allowable repetition rate [9]. Solid-state switches are expensive and sensitive to damage by overload, heat, and internal discharges. Therefore, SCF is proposed as an alternative high voltage switching medium, taking advantage of its high breakdown strength (high heat capacity) and fast dielectric recovery after electrical breakdown (high heat conductivity and low viscosity). SC nitrogen is chosen to be the studied medium in this paper because of its low impact to the environment, since nitrogen is the major component of earth's atmosphere.