Numerical Simulation of Thermal Characteristics of Anodes by Pure Metal and CuCr Alloy Material in Vacuum Arc

Anode material seriously influences the characteristics of vacuum arc and further affects the performance of medium-voltage vacuum switches when the interruption current is high. There are many materials used for electrode manufacture, and different materials are selected for different switches. For a pure metal, its performance usually cannot satisfy the actual requirement. To improve switch's performance, an alloy is usually used as an electrode material. In this paper, thermal processes of six kinds of metal anodes (including pure metal and alloy anodes) are simulated and researched. The physical parameters of the pure metals all come from experiment results directly or are fitted by the experimental data. The physical parameters of the CuCr alloys are derived from Cu and Cr parameters. Two kinds of temperature calculation methods are used, which are called melting and solidification model and equivalent specific heat method, respectively. Simulation results show that W and Mo anodes have the higher temperature than Cu, Cr, CuCr25, and CuCr50 anodes. A pure Cr anode has the largest melting width and highest saturated vapor pressure and evaporation energy. A Cu anode has the biggest melting depth. A W anode has the smallest melting width and depth. Axial temperature gradient is related to the thermal conductivity, and the Cr anode has the largest axial temperature gradient. The thermal characteristics of CuCr25 and CuCr50 anodes are located between the pure Cu and Cr anodes. There are two melting points appearing in the results of CuCr alloys, and between the two melting points, the alloy anodes are in solid-liquid mixture state.

I. Introduction

Vacuum arc (VA) essentially is metal vapor arc in vacuum environment. Metal vapor generated by electrodes determines the characteristics of VA. When the current does not exceed the interruption limit, the cathodes spots are the main sources of VA plasmas. Cathode spots can be clearly seen in many experiments with the aid of a high-speed digital camera [1]. Under this kind of arc model, the arc plasma has the characteristics of cathode material. For instance, in the modeling and simulation of VA in [2], many parameters (as average charge, ion temperature, electron temperature, and so on) in the arc plasma takes the value of copper. When the current reaches a certain level, the anode will become active due to the intense heat flux density from arc column. Herberlin and Gorman [3] observed detailed VA modes: diffuse arc, diffuse column, constricted column, jet column, and anode jet. For jet column and anode jet modes, the interaction between anode jet and cathode jet becomes more significant. Toya et al. [4] thought that the anode discharge mode will influence not only current distribution in arc column but also the bunch of cathode spots. Therefore, electrode material has a serious influence on VA. In gas switches (such as trigatrons, low-voltage air circuit breakers, high-voltage SF6 circuit breakers, and so on), electrode material is also very important.