Zhexin Chen

Two-temperature transport coefficients of SF6–N2 plasma

Sulfur hexafluoride (SF6) is widely adopted in electric power industry, especially in high-voltage circuit breakers and gas-insulated switchgear. However, the use of SF6 is limited by its high liquidation temperature and high global warming potential. Recently, research shows SF6–N2 mixture, which shows environmental friendliness and good electrical properties, may be a feasible substitute for pure SF6. This paper is devoted to the calculation of and transport coefficients of SF6–N2 mixture under both LTE (local thermodynamic equilibrium) and non-LTE condition. The two–temperature mass action law was used to determine the composition. The transport coefficients were calculated by classical Chapman–Enskog method simplified by Devoto. The thermophysical properties are presented for electron temperatures of 300–40 000 K, ratios of electron to heavy species temperature of 1–10 and N2 mole fraction of 0%–100% at atmospheric pressure. The ionization processes under both LTE and non-LTE have been discussed. The ...

Investigation on the Arc Phenomenon of Air DC Circuit Breaker

Comparison analysis on the arc phenomenon of an air dc circuit breaker both in the resistive and inductive circuit was conducted in this paper. It is an interesting phenomenon that severe restrikes occurred in the resistive circuit. According to the results, the reason is that the arc current falls sharply witbreak the rise of arc voltage in the resistive circuit so that the lack of the Lorentz force and the pressure difference before and behind the arc made the hot gas move back toward the entrance of the arc chamber.

Influence of O2 on the dielectric properties of CO2 at the elevated temperatures

SF6 gas is widely used in the high voltage circuit breakers but considering its high global warming potential other substitutes are being sought. Among them CO2 was investigated and even has been used in some practical products. However, at room temperature, the dielectric properties of CO2 are relatively lower than SF6 and air. The goal of this work is to investigate a CO2-based gas to improve the performance of the pure CO2. In this paper, the dielectric properties of hot CO2/O2 mixtures related to the dielectric recovery phase of the circuit breaker were investigated in the temperature range from 300 K to 4000 K and in the pressure range from 0.01 MPa to 1.0 MPa. The species compositions of hot CO2/O2 were obtained based on Gibbs free energy minimization under the assumptions of local thermodynamic equilibrium and local chemical equilibrium. The reduced critical electric field strength of CO2/O2 was determined by balancing electron generation and loss. These were calculated using the electron energy di...

Calculation of 2-temperature plasma thermo-physical properties considering condensed phases: application to CO2–CH4 plasma: part 2. Transport coefficients

The transport coefficients, namely thermal conductivity, viscosity and electrical conductivity, of CO2–CH4 mixture in and out of LTE are calculated in this paper. The calculation was based on local chemical equilibrium (LCE) and local phase equilibrium assumption. The 2-temperature composition results obtained with consideration of condensed phase in the previous paper (Part I) of this series were used in this calculation. The transport coefficients were calculated by classical Chapman–Enskog method simplified by Devoto. The results are presented for different temperatures (300–30 000 K), pressures (0.1–10 atm), non-equilibrium degrees (1–5), and CH4 molar proportions (0–100%). The influence of condensed graphite, non-LTE effect, mixture ratio and pressure on the composition and thermodynamic properties has been discussed. The results will serve as reliable reference data for computational simulation of CO2–CH4 plasmas.

Calculation of 2-temperature plasma thermo-physical properties considering condensed phases: application to CO2-CH4 plasma: part 1. Composition and thermodynamic properties

As the first part of this series of papers, a new calculation method for composition and thermodynamic properties of 2-temperature plasma considering condensed species under local chemical equilibrium (LCE) and local phase equilibrium assumption is presented. The 2-T mass action law and chemical potential are used to determine the composition of multiphase system. The thermo-physical properties of CO2–CH4 mixture, which may be a possible substitution for SF6, are calculated by this method as an example. The influence of condensed graphite, non-LTE effect, mixture ratio and pressure on the thermo-physical properties has been discussed. The results will serve as reliable reference data for computational simulation of CO2–CH4 plasmas.

Electromagnetic-Mechanical Characteristics Study of a High-Speed Electromagnetic Launcher

The high-speed electromagnetic (EM) rail launcher has a wide range of applications. An inductive shunt device is usually paralleled with the rails of the launcher as an approach to limit the muzzle arc by transferring the extremely high armature current. The shunt parameters will significantly influence the rail current, armature velocity, and device stress conditions, which also makes the physical process more complicated. In this paper, a circuit-EM-mechanical coupled simulation model is developed to analyze the launching process. Based on this model, the influences of shunt impedance, armature resistance, and capacitor energy are investigated numerically. The EM force, stress conditions, and deformation of the shunt are studied as well. In addition, some launcher design suggestions are given according to the conclusion.

Two-temperature thermodynamic and transport properties of SF6?Cu plasmas

SF6 and Cu are widely adopted in electrical equipment as a dielectric medium and for conductive components, respectively. SF6–Cu plasmas are frequently formed, particularly in high-voltage circuit breaker arcs and fault current arcs, due to erosion of the Cu components. In this paper, calculated values of the thermodynamic and transport properties of plasmas in SF6–Cu mixtures are presented for both thermal equilibrium and non-equilibrium conditions. The composition is determined by the two-temperature Saha equation and Guldberg–Waage equation in the form derived by van de Sanden. The composition and the thermodynamic properties are evaluated through a classical statistical mechanics approach. For the transport coefficients, the simplified Chapman–Enskog method developed by Devoto, which decouples the electrons and heavy species, has been applied using the most recent collision integrals. The thermodynamic and transport properties are calculated for different electron temperatures (300–40 000 K), ratios of electron to heavy-species temperature (1–10), pressures (0.1–10 atm) and copper molar proportions (0–50%). It is found that deviations from thermal equilibrium strongly affect the thermodynamic and transport properties of the SF6–Cu plasmas. Further, the presence of copper has different effects on some of the properties for plasmas in and out of thermal equilibrium. The main reason for these changes is that dissociation reactions are delayed for non-thermal equilibrium plasmas, which in turn influences the ionization reactions that occur.

Experimental investigation of arc characteristics in medium-voltage DC circuit breaker

Within this paper, the arc characteristics in a typical air DC circuit breaker (DCCB) are studied through experimental approach. A specially designed experimental setup is employed to observe the dynamic process of arc evolution. The phenomena of arc commutation and arc stagnation under fault currents are revealed according to the high-speed photography and arc voltage curves. The influence of arc currents on the detailed arc behaviors is preliminary discussed. It is found that the arc would burn continuously between the contacts when the arc current decreases to a relatively small value. Assisting measures should be employed to help the switch interrupt the small arc current. Finally, a simple magnetic system is designed to increases the magnetic driving force of arc motion. And the interruption characteristics of the switch with the external magnetic field are investigated.

Simulation of arc characteristics in molded case circuit breaker

This paper focuses on the numerical investigation of arc plasma behavior in a certain type molded circuit breaker. A 3-D simulation model based on the magneto-hydrodynamics theory is built and calculated. The basic arc parameters are described by the coupled equations of gas dynamics and electromagnetic field. By adopting a thin layer of nonlinear resistance elements to represent the voltage drop of plasma sheath, the arc splitting process is included. The detailed arc behaviors are presented and preliminarily discussed according to the calculated temperature and arc current distributions. Moreover, experiments are carried out to validate the simulation results. The arc voltage acquired by the numerical and experimental approaches shows good agreement.

A Strategy for Inspection of Cracks in a Thick Structure Using an ECT Probe

A strategy for enhancing the inspection precision of cracks in a thick structure is proposed. The key problem of inspection of cracks in a thick structure is the penetration depth of eddy currents. Using lower frequency to drive the probe can overcome it, but too low frequency will cause too small signal to noise ratio and inspection sensitivity. To solve these problems, effects of frequency, inspection mode, and probe type on eddy current testing signals were studied. At last, an effective strategy for inspection of cracks in a thick structure was proposed.