Linlin Zhong

Dielectric breakdown properties of hot SF6-CO2 mixtures at temperatures of 300–3500 K and pressures of 0.01–1.0 MPa

Recently, much attention has been paid to SF6-CO2 mixtures as one of substitutes for pure SF6 gas. In this paper, the dielectric breakdown properties of hot SF6-CO2 mixtures are investigated at temperatures of 300–3500 K and pressures of 0.01–1.0 MPa. Under the assumptions of local thermodynamic equilibrium and local chemical equilibrium, the equilibrium compositions of hot SF6-CO2 mixtures with different CO2 proportions are obtained based on Gibbs free energy minimization. The cross sections for interactions between electrons and neutral species are presented. Some unknown ionization cross sections are determined theoretically using Deutsch–Mark (DM) formalism based on quantum chemistry. Two-term Boltzmann equation is adopted to calculate the electron energy distribution function, reduced ionization coefficient, reduced attachment coefficient, and reduced effective ionization coefficient. Then the reduced critical electric field strength of mixtures, corresponding to dielectric breakdown performances, is...

Thermophysical properties of SF6?Cu mixtures at temperatures of 300?30?000?K and pressures of 0.01?1.0?MPa: part 1. Equilibrium compositions and thermodynamic properties considering condensed phases

The equilibrium compositions and thermodynamic properties of SF6–Cu mixtures with copper proportions up to 50% are calculated as a function of temperature from 300 to 30 000 K and pressure from 0.01 to 1.0 MPa. The condensed phases and Debye–Huckel corrections are both taken into account. The influences of condensed phases, Debye–Huckel corrections, copper proportions and gas pressures on the composition and/or thermodynamic properties are discussed in detail under various conditions. Some results are tabulated for the modelling of SF6 arc plasmas contaminated by Cu.

Calculation of combined diffusion coefficients in SF6-Cu mixtures

Diffusion coefficients play an important role in the description of the transport of metal vapours in gas mixtures. This paper is devoted to the calculation of four combined diffusion coefficients, namely, the combined ordinary diffusion coefficient, combined electric field diffusion coefficient, combined temperature diffusion coefficient, and combined pressure diffusion coefficient in SF6-Cu mixtures at temperatures up to 30 000 K. These four coefficients describe diffusion due to composition gradients, applied electric fields, temperature gradients, and pressure gradients, respectively. The influence of copper fluoride and sulfide species on the diffusion coefficients is shown to be negligible. The effect of copper proportion and gas pressures on these diffusion coefficients is investigated. It is shown that increasing the proportion of copper generally increases the magnitude of the four diffusion coefficients, except for copper mole fractions of 90% or more. It is further found that increasing the pre...

Dielectric breakdown properties of hot SF6 gas contaminated by copper at temperatures of 300–3500 K

The dielectric breakdown properties of hot SF6 gas during the dielectric recovery phase play an important role in understanding gas breakdown occurring in high-voltage circuit breakers. This paper is devoted to the theoretical investigation of dielectric breakdown properties of hot SF6 gas contaminated by copper at temperatures of 300–3500 K and pressures of 0.01–1.6 MPa. The equilibrium compositions of SF6–Cu mixtures are obtained with the consideration of condensed species. The unknown ionization cross sections for CuS, CuF and CuF2 are calculated using a Deutsch–Mark (DM) formalism based on quantum chemistry. The two-term Boltzmann equation is adopted to numerically calculate the electron energy distribution function, collision ionization coefficient and electron attachment coefficient. Then the reduced critical electric field strength is determined when the effective ionization coefficient equals to zero. The influences of the Cu proportion and gas pressure on the dielectric breakdown properties are investigated. It is shown that the existence of copper compounds increases the concentration of high-energy electrons significantly, even for the case with a very low percentage (e.g. 1% Cu). With the increase of copper content, the value of (E/N)cr is reduced remarkably at temperatures below 3000 K, but enhanced slightly above 3000 K. It is also found that the increase of pressure can improve the dielectric breakdown performance of hot SF6–Cu mixtures.

Thermophysical properties of SF6-Cu mixtures at temperatures of 300?30,000?K and pressures of 0.01?1.0?MPa: part 2. Collision integrals and transport coefficients

The transport coefficients (including electrical conductivity, viscosity and thermal conductivity) of SF6-Cu mixtures with copper proportions up to 50% are calculated as a function of temperature from 300 to 30,000 K and pressure from 0.01 to 1.0 MPa. The Lennard–Jones like phenomenological potential and some recently updated transport cross sections are adopted to obtain collision integrals. The influence of copper proportion and gas pressure on transport coefficients under various conditions are discussed in detail. Some results are tabulated for the modeling of SF6 arc plasmas contaminated by Cu.

Compositions, thermodynamic properties, and transport coefficients of high-temperature C5F10O mixed with CO2 and O2 as substitutes for SF6 to reduce global warming potential

C5F10O has recently been found to be a very promising alternative to SF6. This paper is devoted to the investigation of compositions, thermodynamic properties, and transport coefficients of high-temperature C5F10O mixed with CO2 and O2. Firstly, the partition functions and enthalpies of formation for a few molecules (CxFy and CxFyO) which are likely to exist in the mixtures, are calculated based on the G4(MP2) theory. The isomers of the above molecules are selected according to their Gibbs energy. The compositions of C5F10O-CO2-O2 mixtures are then determined using the minimization of the Gibbs free energy. Next, the thermodynamic properties (mass density, specific enthalpy, and specific heat) are derived from the previously calculated compositions. Lastly, the transport coefficients (electrical conductivity, viscosity, and thermal conductivity) are calculated based on Chapman-Enskog method. It is found that, as an arc quenching gas, C5F10O could not recombine into itself with the temperature decreasing down to room temperature after the arc extinction. Besides, the key species at room temperature are always CF4, CO2, and C4F6 if graphite is not considered. When taken into account, graphite will replace C4F6 as one of the dominate particles. The mixing of CO2 with C5F10O plasma significantly affects the thermodynamic properties (e.g. vanishing and/or shifting of the peaks in specific heat) and transport coefficients (e.g. reducing viscosity and changing the number of peaks in thermal conductivity), while the addition of O2 with C5F10O-CO2 mixtures has no remarkable influence on both thermodynamic and transport properties.

Mechanical life prognosis of high voltage circuit breakers based on support vector machine

Mechanical fault is one of the main faults occurring during the life cycle of high-voltage circuit breakers (HVCBs), which has a significant influence on the reliability of the electrical power system. In this paper, the mechanical prediction algorithm for HVCBs based on support vector machine (SVM) was studied. Firstly, we used a sliding time window (STW) method to extract features of the travel curves of the movable contacts and coil current curves of HVCBs. Then the historic data were used to learn a support vector regression machine and finally to predict the new curves. In the end, the mechanical life experiment data of a HVCB were applied to validate the feasibility of the algorithm. The results showed that the proposed algorithm could predict the mechanical condition of HVCBs successfully.

Influence of metallic vapours on thermodynamic and transport properties of two-temperature air plasma

The metallic vapours (i.e., copper, iron, and silver in this paper) resulting from walls and/or electrode surfaces can significantly affect the characteristics of air plasma. Different from the previous works assuming local thermodynamic equilibrium, this paper investigates the influence of metallic vapours on two-temperature (2 T) air plasma. The 2 T compositions of air contaminated by Cu, Fe, and Ag are first determined based on Sahas and Guldberg–Waages laws. The thermodynamic properties (including mass density, specific enthalpy, and specific heat) are then calculated according to their definitions. After determining the collision integrals for each pair of species in air-metal mixtures using the newly published methods and source data, the transport coefficients (including electrical conductivity, viscosity, and thermal conductivity) are calculated for air-Cu, air-Fe, and air-Ag plasmas with different non-equilibrium degree θ (Te/Th). The influences of metallic contamination as well as non-equilibr...

Influence of copper contamination on equilibrium compositions, thermodynamic properties, transport coefficients, and combined diffusion coefficients of high-temperature SF6 gas

The copper vapor resulting from electrode surfaces in arc devices, such as circuit breakers, may modify the characteristics of arc plasmas. The corresponding properties of plasmas contaminated by the metallic vapor are therefore needed to be determined before setting up physical models. In this paper, the equilibrium compositions, thermodynamic properties, transport coefficients, and combined diffusion coefficients of SF6-Cu mixtures are calculated at temperatures of 300-30,000 K and a pressure of 0.6 MPa. The influences of copper proportion on such properties are investigated. It is found that a small quantity of copper has no significant influence on the above thermo-physical properties of SF6-Cu mixtures, while a large quantity of copper can generally change these properties dramatically.

Influence of copper contamination on dielectric breakdown properties of high-temperature SF6 gas based on Boltzmann equation analysis

In order to better understand the gas breakdown occurring in high-voltage circuit breakers (HVCBs), it is necessary to know the dielectric breakdown properties of hot SF6 gas during the dielectric recovery phase. In this paper, the dielectric breakdown properties of hot SF6 gas contaminated by copper vapor are calculated based on the Boltzmann equation analysis, and the influence of copper contamination on such properties is studied in the temperature range of 300-3500 K at 0.6 MPa. The result shows that the existence of copper compounds increases the concentration of high-energy electrons significantly even for very low percentage (e.g. 1% Cu). The copper and its compounds also lead to the considerable rise in ionization coefficient and the decline in electron attachment coefficient of SF6-Cu mixtures. The critical electric field strength is reduced significantly at temperatures below 3000 K whereas enhanced marginally above 3000 K.