Qingqing Gao

Calculated rate constants of the chemical reactions involving the main byproducts SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment

SF6 is widely used in electrical equipment as an insulating gas. In the presence of an electric arc, partial discharge (PD) or spark, SF6 dissociation products (such as SF2, SF3 and SF4) react with the unavoidable gas impurities (such as water vapor and oxygen), electrodes and surrounding solid insulation materials, forming several toxic and corrosive byproducts. The main stable decomposition products are SO2F, SO2F2 and SOF2, which have been confirmed experimentally to have a direct relationship with discharge faults, and are thus expected to be useful in the fault diagnosis of power equipment. Various studies have been performed of the main SF6 decomposition species and their concentrations under different types of faults. However, most of the experiments focused on the qualitative analysis of the relationship between the stable products and discharge faults. Although some theoretical research on the formation of main SF6 derivatives have been carried out using chemical kinetics models, the basic data (chemical reactions and their rate constants) adopted in the model are inaccurate and incomplete. The complex chemical reactions of SF6 with the impurities are ignored in most cases. The rate constants of some reactions obtained at ambient temperature or in a narrow temperature range are adopted in the models over a far greater range, for example up to 12 000 K, due to the difficulty in the experimental measurement and theoretical estimation of rate coefficients, particularly at high temperatures. Therefore, improved theoretical models require not only the consideration of additional SF6 decomposition reactions in the presence of impurities but also on improved values of rate constants. This paper is devoted to determining the rate constants of the chemical reactions relating to the main byproducts of SF6 decomposition in SF6 gas-insulated power equipment: SO2F, SOF2 and SO2F2. Quantum chemistry calculations with density functional theory, conventional transition state theory and Wigner’s tunneling effect correction are employed to estimate the rate constants of four important chemical reactions: F + SO2F → SO2F2, F2 + SO2 → SO2F2, SO2F + SF5 → SF6 + SO2 and SOF3 + SF3 → SF4 + SOF2. The results are derived for a large temperature range, from 300 to 12 000 K, and finally fitted by a three-parameter Arrhenius equation. This work lays a basis for the further study of the SF6 decomposition mechanism by means of chemical kinetics modelling. Journal of Physics D: Applied Physics Calculated rate constants of the chemical reactions involving the main byproducts SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment

Dominant particles and reactions in a two-temperature chemical kinetic model of a decaying SF6 arc

This paper is devoted to the computation of the non-equilibrium composition of an SF6 plasma, and determination of the dominant particles and reactions, at conditions relevant to high-voltage circuit breakers after current zero (temperatures from 12 000 K to 1000 K and a pressure of 4 atm). The non-equilibrium composition is characterized by departures from both thermal and chemical equilibrium. In thermal non-equilibrium process, the electron temperature (T e) is not equal to the heavy-particle temperature (T h), while for chemical non-equilibrium, a chemical kinetic model is adopted. In order to evaluate the reasonableness and reliability of the non-equilibrium composition, calculation methods for equilibrium composition based on Gibbs free energy minimization and kinetic composition in a one-temperature kinetic model are first considered. Based on the one-temperature kinetic model, a two-temperature kinetic model with the ratio T e/T h varying as a function of the logarithm of electron density ratio (n e/) was established. In this model, T* is introduced to allow a smooth transition between T h and T e and to determine the temperatures for the rate constants. The initial composition in the kinetic models is obtained from the asymptotic composition as infinite time is approached at 12 000 K. The molar fractions of neutral particles and ions in the two-temperature kinetic model are consistent with the equilibrium composition and the composition obtained from the one-temperature kinetic model above 10 000 K, while significant differences appear below 10 000 K. Based on the dependence of the particle distributions on temperature in the two-temperature kinetic model, three temperature ranges, and the dominant particles and reactions in the respective ranges, are determined. The full model is then simplified into three models and the accuracy of the simplified models is assessed. The simplified models reduce the number of species and reactions by a factor of about 2, while providing results that agree closely with the full model. Thus, the physicochemical processes of SF6 arc can be characterized by relatively few species and reactions in each temperature range. It is noted that the simplified models can also be applied to a wide range of pressures, 1–16 atm, conditions which cover most circuit breaker applications. The simplified species and reactions will allow the computing time of multi-dimensional models, taking into account departures from both thermal and chemical equilibrium, to be decreased dramatically while capturing the main physicochemical processes in SF6 arcs.

Fault simulation and diagnosis of high speed hydraulic operating mechanism of high voltage circuit breakers

With the rapid development of economy and substantial growth of the scale of power network, the safe operation of power system becomes particularly important. While in field of high voltage, hydraulic operating mechanism gradually plays an important role recently. Thus, monitoring and identifying the most common failures to improve the reliability and stability of operating mechanism are of great significance. In this paper, theoretical analysis is carried out for 252kV hydraulic operating mechanism of high voltage circuit breakers and simulation models are established with the software AMESim and Ansoft. The displacement and velocity of the cylinder are verified by experimental results. Then four faults including pressure fluctuations, variation of oil viscosity, fraction stuck and the addition of air content are simulated. With the characteristics of the velocity curves under different conditions, the fault can be directly identified.

Relationship between time-frequency representation of PD-induced UHF signal and PD current pulse

Partial discharge (PD) detection is significant to the insulation diagnosis in gas insulated switchgear (GIS). The ultra-high frequency (UHF) method was proposed for PD detection, and has been proved to be effective and applicable. However, it is difficult to calibrate the apparent charge quantity in partial discharge detection. This paper deal with the relationship between time-frequency representation of PD-induced UHF signal and PD current pulse. A simulation model of GIS is built based on Finite Difference Time Domain (FDTD) method. The time-frequency representation of different UHF signal induced by Gaussian pulse with different discharge length, pulse current amplitude and pulse width are compared. It is effective to detect the charge quantity in partial discharge.

Comparison of different time-frequency analysis methods for sparse representation of PD-induced UHF signal

Partial discharge (PD) detection and signal processing are of great significance in power grid system. This paper investigates results of PD-induced ultra-high frequency (UHF) signal in GIS based on different time-frequency analysis methods, and compares sparse representation of different methods, as well as the time cost. A processing algorithm based on the threshold is proposed to reduce the size of the analysis result. The volume of the analysis result is reduced significantly, and the most appropriate method is determined.

Optimization design on step-shape cushioning for hydraulic operating mechanism of high voltage circuit breakers

This paper is devoted to doing a simulation research on the cushioning process of hydraulic operating mechanism applied on 252kV high voltage circuit breakers and suggesting an effective design method to improve the performance of buffer device. According to the working principle of cushioning process, parametric analysis and optimization are carried out for the buffer process with considering step gap, step length, buffer area and the cross-sectional area of long hole. The results provide an effective method for the optimization design on hydraulic operating mechanism of high voltage circuit breakers.

Research on modeling and fault analysis of permanent magnet operating mechanism of vacuum circuit breaker

In this paper, a kind of permanent magnet operating mechanism was modeled and studied by using ADAMS and Ansoft software, and the model was verified by experiments. Then, six common faults were chosen to do simulated analysis, and opening and closing process was divided into three stages according to its force analysis, respectively. The angle displacement of main axis, which related to the moving contacts displacement, was treated as the objective to study the performance of permanent magnet operating mechanism. When the research is combined with on-line monitoring technology, it can be used to guide fault diagnosis and condition prediction of vacuum circuit breaker.

Simulation on failure analysis of vacuum circuit breaker permanent magnet operating mechanism based on three-parameter method

The working performance of high voltage circuit breakers is with great importance for the security of power grid. In this paper, a kind of permanent magnet operating mechanism was modeled and studied. The simulation model was verified by experiments. The opening and closing process is divided into three stages according to its force analysis, respectively. The angle displacement of main axis is related to the displacement of moving contact, and treated as the objective to study the performance of permanent magnet operating mechanism. Angular velocity of main axis in three stages is used as three-parameter to analyze mechanical faults. By comparing parameters in fault and normal conditions, three-parameter database for every failure mode is obtained. It proposes assessment methods and judgment basis to evaluate mechanical conditions for intelligent on-line monitoring, failure diagnosis and condition prediction of permanent magnet vacuum circuit breakers.