SF6 Arc Research Articles

Research on Arcing Characteristics of Sulfur Hexafluoride Alternative Gases in Rotating Arc Ring Main Unit

Abstract As international environmental constraints such as low carbon emissions and reduction requirements increase, there is an urgent need to seek environmentally friendly alternatives to SF6 gas. The SF6 ring main unit is widely used in the power distribution network system, highlighting the urgent need for research on environmentally friendly gas-insulated ring main units. This study simulates and compares the arc characteristics of 12 kV rotating arc ring main unit with different environmentally friendly gases, such as N2, 20%SF6/80%N2, 40%SF6/60%N2, 10%C4F7N/90%CO2. It is concluded that the arc burns most violently in N2, and the SF6 arc voltage is the highest. The specific heat at constant pressure and thermal conductivity of the gas affect the intensity of arc combustion and thus affect the arc voltage. In this paper, the causes of different gas arc characteristics are explained by analyzing the physical parameters of gas. Specifically, the isobaric specific heat and thermal conductivity of gases affect the severity of arc combustion, thereby impacting arc voltage. Moreover, viscosity plays a role in determining the rotation speed of the arc. The conclusion of this paper provides theoretical guidance on optimizing the gas component ratio of environmentally friendly gases in ring main units.

Voltage–Current Characteristic of Free Burning Arcs in SF₆ Alternative Gas Mixtures

Voltage–current characteristics of free burning arcs in SF6 and air have been known for decades. As the demand for an SF6-free solution is increasing, there is an accompanying need to determine arc parameters in the alternative gases. An unblown arc experiment has been established to determine the voltage–current characteristics of SF6 alternative gases, which have not yet been thoroughly studied. In this experiment, free burning arc measurements were performed in a number of gases under consideration of SF6 alternatives, including CO2 and mixtures of CO2/O2 with and without C4F7N or C5F10O additives at the concentrations of up to 10 %. Measurements were also performed in air and SF6 for comparison. Arc voltage was measured in each gas at pressures ranging from 1- to 5-bar absolute, and electrode separations ranging from 20 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$95 \mathrm {~mm}$ </tex-math></inline-formula> . Voltage–current characteristic measurements for air and SF6 show good agreement with previously published results. A linear relationship of the arc voltage to the arc length is shown as well as the fourth root dependence of the arc voltage on the gas pressure. It was shown that neither the O2 nor the fluorinated additives to CO2 have any significant influence on the voltage–current characteristic. The minimum arc voltage in all measured gases was slightly higher than in SF6, but the arc in SF6 was the least stable and had the highest elongations resulting in high-voltage peaks. The arc voltage in air had a similar minimum value to the CO2-based gases, but the arc was much more stable, resulting in lower effective voltage, especially at low currents.

SF6 Arc Extinction Sensor Design for Substation Mechanical Equipment in Smart Grid

SF6 Arc Extinction Sensor Design for Substation Mechanical Equipment in Smart Grid

Computations on the Thermal Characteristics of SF6 and CO₂ Switching Arcs on a Microscopic Scale.

Pure sulfur hexafluoride is a colorless, odorless, non-toxic, inflammable, chemically inert and thermally stable gas and has proven its worth as an excellent interruption and dielectric medium. SF6 has been successfully used for interruption and insulation purposes as interrupters and circuit-breakers in gas-insulated substations. Due to its long lifetime and high global warming potential, this gas was put on the list of fluorinated greenhouse gases in the Kyoto Protocol aimed at controlling the emission of man-made greenhouse gases. This factor makes the search for an environmentally friendly alternative to SF6 all the more urgent. In this paper, we conducted computations on the thermal and aerodynamic behaviors of SF6 and an alternative CO₂ switching arcs in a self-blast chamber in order to compare the switching phenomena and the thermal reignition from an engineering point of view. Through the complete work, the 3,000 K isotherm of the remnant arc column within microseconds after a current zero was used to evaluate the thermal reignition of SF6 and CO₂ switching arcs with the slope of the tangential line of the transient recovery voltage on a microscopic scale.

Time-Resolved Analysis of SF6 Arc Plasmas in a Laboratory Model Chamber for Circuit Breaker

This article presents time-resolved analysis of SF6 arc plasma generated in a laboratory-scale model chamber prepared for studying the characteristics of a circuit breaker. During the arc discharge, the optical emission spectra are measured by a high-speed spectrometer under various operating conditions. The time-resolved electron temperature and density values are determined using the well-known Boltzmann plot and Stark broadening methods, respectively, for the measured Cu I, S II, and F I emission lines. Moreover, the temporal evolution of the electrical conductivity of the arc plasma is deduced from the measured electron temperature and density values, depending on the operating conditions. In our experimental conditions, it is revealed that the electrical conductivity of arc plasmas is mainly determined by Coulomb (electron–ion) collisions as opposed to electron–neutral collisions due to the much higher Coulomb collision frequency compared to the electron–neutral collision frequency. Both the electron temperature and density were found to increase when the discharge current is increased at a fixed gas pressure, resulting in an increase of the electrical conductivity. When the gas pressure inside the model chamber is increased, the electron density is increased as well, but the electrical conductivity does not change significantly due to the slight change in the electron temperature. The present study will be helpful for investigating the performance capabilities of circuit breakers because the electrical conductivity, the most important parameter to guarantee successful circuit breaking at points near the current-zero, is determined by the temperature and density of the electrons.

Numerical Simulation of Turbulent-like Behavior of Air and SF6 Arc Plasmas in a Model Circuit Breaker

Numerical Simulation of Turbulent-like Behavior of Air and SF6 Arc Plasmas in a Model Circuit Breaker

Theoretical study of the chemical reaction mechanisms and reaction rates of CFx + SFy (where x = 1–3 and y = 1–6) in SF6–polytetrafluoroethylene arc plasma

The microscopic properties of nonequilibrium arc plasma have great significance for researching the breaking capacity of SF6 circuit breakers and constructing arc plasma simulation models. The polytetrafluoroethylene (PTFE) vapor generated by the ablation of the nozzle material significantly affects the SF6 arc plasma particles and thus affects the microscopic properties of the nonequilibrium arc plasma. At present, the calculation of the microscopic properties of nonequilibrium SF6–PTFE arc plasma has not been reported, mainly because the basic data for the chemical reaction mechanism and chemical reaction rate in the plasma cannot be obtained effectively. In this paper, quantum chemistry was used to theoretically research the reaction mechanisms and rates between CF3, CF2, and CF radicals and SF6, SF5, SF4, SF3, SF3, SF2, and SF in SF6–PTFE arc plasma at 298–10 000 K. A total of 18 chemical reactions are included. Structural optimizations, vibrational frequency calculations, and zero-point energy calculations for the reactants, products, and transition states were performed at the B3LYP/6-311++G(d,p) level of theory. The single-point energies of all species were obtained at the CCSD(T)/aug-cc-PVTZ level. The reaction rates of all reactions between 298 and 10 000 K were obtained by using transition state theory. The reaction rates were successfully fitted using the Arrhenius equation and the three-parameter Arrhenius equation. It has found that reactions between CF2 and fluoro sulfides have higher potential energies. Reactions R13, R7, R14, and R8 exhibit higher reaction rates at 298–10 000 K, whereas R3, R4, R5, and R6 have lower reaction rates. This work can provide theoretical guidance for research on nonequilibrium SF6–PTFE and SF6–CF4 arc plasma microscopic properties.

An improved method for fast evaluating arc quenching performance of a gas based on 1D arc decaying model

The evaluation of an arc quenching medium in circuit breakers usually requires the experimental investigation or the 2D or 3D magnetohydrodynamics simulation of switching arcs, which are expensive and time-consuming. In this work, a fast method is proposed for evaluating the arc quenching performance of gases. In this method, the arc decaying process is divided into three stages based on the results of 1D arc decaying modeling, including the thermal recovery stage, the predielectric recovery stage, and the postdielectric recovery stage. Compared to the previous method, the present method is improved mainly in the three aspects: the thermal recovery stage is featured by the average radial temperature instead of the axial temperature; the criterion of dividing the dielectric recovery stage into the pre- and postdielectric recovery stages is validated by the average electron number density instead of choosing arbitrarily; and the postdielectric recovery stage is characterized by the critical electric field strength Ecr instead of the reduced critical electric field strength (E/N)cr. The case study of SF6, CO2, CF4, and air arcs indicates that the present method yields the same descending order of the thermal recovery rate and the predielectric recovery rate for the four gases as the previous method. Moreover, the present method can avoid negative postdielectric recovery rates, which means that it is more reasonable than the previous method.

Quasi-Three-Dimensional Computations of the Microscopic Thermal and Dielectric Characteristics of an SF6 Rotating Switching Arc.

Pure sulfur hexafluoride (SF6) is chemically inert, non-flammable, non-toxic and thermally stable, and it has excellent dielectric strength and arc-quenching and control properties. The switching-off process of SF6 arc discharges occurs at the region between the contacts during the opening sequence to interrupt the flow of excessive current in a faulty network. The arc is tolerated in a controlled manner until a natural current zero when the arc discharge is rapidly quenched to restrict the thermal and dielectric reignition to the interruption. An SF6 self-blast switching chamber combines two advantages of blowing by heat expansion of the SF6 and arc rotation by electromagnetic effect of coil to improve the switching performance on thermal and dielectric reignition. The thermal and aerodynamic behaviors of an SF6 rotating switching arc in the chamber physically are complex and difficult to understand only by measurement due to their three-dimensional effects. Since the late nineteen-eighties, significant progress has been made in the method of computational fluid dynamics describing the physical processes occurring in the switching arc. The final goal of computer simulation technology on the arc switching is to predict the switching phenomena on the thermal and dielectric reignition from an engineering point of view. In this paper, we have conducted quasi-three dimensional computations to predict the thermal and dielectric reignition of SF6 rotating arcs occurring after a current zero in the self-blast switching chamber. Through the complete work, the microscopic thermal and aerodynamic behaviors of the remnant arc column after a current zero should be good criteria to predict the thermal and dielectric reignition of the rotating switching arc in the chamber.

Ablated mass of PTFE nozzle due to high‐current SF6 arc exposure: Formulation of ablated mass

AbstractCharacteristics of PTFE ablation due to SF6 arc exposure were studied to predict the ablated mass of PTFE nozzle in high‐voltage circuit breaker. From the measurement of the ablated mass, the predominant factor of the PTFE ablation was revealed as a radiation power of SF6 arc. Subsequently, the radiation power absorbed in PTFE was formulated on the basis of the radiation transfer property and the arc physics. The formulation for the prediction of the PTFE ablated mass was established with the formulated radiation power and the measured masses. Finally, the validity of the PTFE ablated mass prediction was shown from the results of the error verification with the measured masses.

Study of the Arc Interruption Performance of CO2 Gas in High-Voltage Circuit Breaker

CO2 is a possible alternative to SF6 as the interruption medium in gas circuit breakers. The arc interruption performance of CO2 is investigated in this paper, by combining the computational fluid dynamics (CFD) analysis and the Mayr arc model. The interruption processes for a high-voltage circuit breaker filled with CO2 at 0.6, 0.8, and 1.0 MPa, as well as SF6 at 0.6 MPa, are investigated, and the influence of the filling pressure on the gas flow field characteristics is analyzed. It is found that the SF6 arc has a higher arc core temperature and a smaller arc radius than CO2 arc. The arc core temperature and arc radius both decrease with the increasing the filling pressure. The mass fluxes along the axial and radial directions and toward both the nozzle side and the hollow contact side are presented and analyzed. The time constant and arc cooling coefficient at the current-zero point are calculated based on the Mayr equation. Finally, the critical values of the rising rate of the recovery voltage (RRRV) and relative di/dt are obtained. The interruption capabilities of CO2 at 0.6, 0.8, and 1.0 MPa are estimated to be about 47%, 76%, and 95% compared with that of SF6 at 0.6 MPa, respectively, with the assumption that the Mayr equation is applicable to analyze the interruption capability of different gases.

Theoretical study of the decomposition mechanism of SF6/Cu gas mixtures

The compositions of SF6 arc plasma are contaminated by Cu vapor through electrode erosion and thus affect the performance of the high-voltage SF6 circuit breaker. But the essential data for chemical kinetic models to study the non-equilibrium compositions of SF6/Cu mixtures, namely the decomposition mechanism of SF6/Cu mixtures, is still not clear. Besides, SF6 decomposition products can be used to diagnose insulation faults of GIS and to evaluate the electric life of the SF6 circuit breaker, but the uncertainty of the decomposition mechanism of SF6/Cu mixtures hinders the application of the analysis method of SF6 decomposition products. Therefore, this work is devoted to investigating the decomposition mechanism of SF6/Cu mixtures by means of density functional theory in conjunction with 6-311G(d,p) basis set (for S and F atoms) and Lanl2DZ basis set (for the Cu atom). The optimized molecular structures, harmonic vibrational frequencies and energetic information of reactants, intermediate products (IM), transition states (TS) and products are thoroughly studied. And the complete decomposition pathways of SF6/Cu mixtures involving 15 reactions are derived. Detailed potential energy surfaces of SF6 + Cu decomposition reactions are also depicted as a result. Among all the decomposition pathways, the favorable decomposition reactions of SF6 + Cu mixtures are found: reaction R1 contributes more than reaction R2 in the decomposition of SF6 + Cu mixtures due to its lower energy released; reaction R3 is dominant in the decomposition of SF5 + Cu other than multi-step reactions; the chemical processes of SF4 + Cu → IMx → TSy (where x stands for 9, 10 and 13, and y denotes 7, 8 and 9) may play the same role in SF4 + Cu decomposition, but the reaction rate of R6 is higher than other reactions; reaction R9 and R11 are relative favorable among SF3 + Cu decomposition reactions; reaction R13 is dominant in the decomposition of SF2 + Cu because of its high potential well. This work hopes to outline a theoretical basis to investigate the non-equilibrium compositions of SF6/Cu arc plasma and the SF6 decomposition products analysis method.

Spatial-filter-installed Shack–Hartmann sensor for two-dimensional electron density visualization of SF6 arc discharge under strong turbulent flow

By using the conventional Shack–Hartmann sensors, it has been impossible to measure the two-dimensional electron density distributions over the -gas-blast decaying arcs exposed to the strong turbulent flow with high spatial frequency variations in the gas density. In order to remove the high spatial frequency components, spatial filters were implemented into the Shack–Hartmann sensors. The novel sensing system was successfully applied to the decaying arc plasmas under current-zero phases generated in a 50 mm-long interelectrode gap confined by a gas flow nozzle. Our experimental results showed that the decaying arcs had large shot-to-shot variations in the electron densities and arc diameters, and these parameters were not always smallest in the upstream nozzle region with the highest blasting gas speed. Such irreproducible arc behaviour was quite different from the previously reported air and arc plasmas and it was predominantly caused by the spatiotemporally irreproducible strong turbulent flow.

Thermal re-ignition processes of switching arcs with various gas-blast using voltage application highly controlled by powersemiconductors

This paper focuses on a fundamental experimental approach to thermal arc re-ignition processes in a variety of gas flows in a nozzle. Using power semiconductor switches in the experimental system, the arc current and the voltage applied to the arc were controlled with precise timing. With this system, residual arcs were created in decaying phase under free recovery conditions; arc re-ignition was then intentionally instigated by application of artificial voltage—i.e. quasi-transient recovery voltage—to study the arc behaviour in both decaying and re-ignition phases. In this study, SF6, CO2, N2, O2, air and Ar arcs were intentionally re-ignited by quasi-TRV application at 20 μs delay time from initiation of free recovery condition. Through these experiments, the electron density at the nozzle throat was measured using a laser Thomson scattering method together with high speed video camera observation during the re-ignition process. Temporal variations in the electron density from the arc decaying to re-ignition phases were successfully obtained for each gas-blast arc at the nozzle throat. In addition, initial dielectric recovery properties of SF6, CO2, air and Ar arcs were measured under the same conditions. These data will be useful in the fundamental elucidation of thermal arc re-ignition processes.

Thermophysical and radiation properties of high-temperature C4F8-CO2 mixtures to replace SF6 in high-voltage circuit breakers

C4F8-CO2 mixtures are one of the potential substitutes to SF6 in high-voltage circuit breakers. However, the arc quenching ability of C4F8-CO2 mixtures is still unknown. In order to provide the necessary basic data for the further investigation of arc quenching performance, the compositions, thermodynamic properties, transport coefficients, and net emission coefficients (NEC) of various C4F8-CO2 mixtures are calculated at temperatures of 300–30 000 K in this work. The thermodynamic properties are presented as the product of mass density and specific heat, i.e., ρCp. The transport coefficients include electrical conductivity, viscosity, and thermal conductivity. The atomic and molecular radiation are both taken into account in the calculation of NEC. The comparison of the properties between SF6 and C4F8-CO2 mixtures is also discussed to find their differences. The results of compositions show that C4F8-CO2 mixtures have a distinctive advantage over other alternative gases e.g., CF3I and C3F8, because the dissociative product (i.e., C4F6) of C4F8 at low temperatures has a very high dielectric strength. This is good for an arc quenching medium to endure the arc recovery phase. Compared with SF6, C4F8-CO2 mixtures present lower ρCp at temperatures below 2800 K and larger thermal conductivity above 2800 K. Based on the position of peaks in thermal conductivity, we predict that the cooling of C4F8-CO2 arc will be slowed down at higher temperatures than that of SF6 arc. It is also found that the mixing of CO2 shows slight effects on the electrical conductivity and NEC of C4F8-CO2 mixtures.

Pressure oscillation due to arcs in a closed container filled with air and SF6

To develop an alternative method for internal arc testing in SF6-insulated power equipment, it is essential to clarify the conditions that lead to pressure increase due to air and SF6 arcing in the testing. Therefore,a systematical experimental data on air and SF6 arcs have been obtained in order to calculate the pressure rise in power equipment. In this study, experimental evaluation of the pressure increase and pressure oscillation due to air and SF6 arcs in a closed container has been described. The results of this research suggest that it is necessary to consider the rise in pressure phenomena, when performing internal arc tests for replacing SF6 with air.

The modelling of an SF6 arc in a supersonic nozzle: II. Current zero behaviour of the nozzle arc

The present work (part II) forms the second part of an investigation into the behaviour of SF6 nozzle arc. It is concerned with the aerodynamic and electrical behaviour of a transient nozzle arc under a current ramp specified by a rate of current decay (di/dt) before current zero and a voltage ramp (dV/dt) after current zero. The five flow models used in part I [] for cold gas flow and DC nozzle arcs have been applied to study the transient arc at three stagnation pressures (P0) and two values of di/dt for the current ramp, representing a wide range of arcing conditions. An analysis of the physical mechanisms encompassed in each flow model is given with an emphasis on the adequacy of a particular model in describing the rapidly varying arc around current zero. The critical rate of rise of recovery voltage (RRRV) is found computationally and compared with test results of Benenson et al []. For transient nozzle arcs, the RRRV is proportional to the square of P0, rather than to the square root of P0 for DC nozzle arcs. The physical mechanisms responsible for the strong dependence of RRRV on P0 have been investigated. The relative merits of the flow models employed are discussed.

Development of Macroscopic and Microscopic Model of SF6 Arc

SF6断路器是电力系统中的重要开断设备，断路器的稳定性与可靠性事关电力系统的安全运行。但是SF6电弧是电场、磁场、温度场以及气流场等多场耦合互相影响不断变化的复杂过程，因此虽然国内外已有很多研究者对SF6电弧进行了多方位的研究，但是已有的研究都是建立在假设和实验室设定的条件下的。本文基于国内外研究者对SF6的研究现状以及方向进行了整理归纳，介绍了SF6研究中宏观和微观模型，并对SF6电弧的未来研究方向以及亟待解决的问题进行了探讨。 SF6 circuit breaker is important breaking equipment in power system; the operation of the circuit breaker is related to the stability and reliability of the power system. SF6 arc is a complex multi-field coupling process evolving electric field, magnetic field, temperature field and flow field. Although there are a lot of domestic and foreign researchers conducting studies of SF6 arc, those previous studies are all based on the assumptions and under the conditions of the specific laboratory setting. Based on the research status of SF6 arc, this paper introduces the development of macroscopic and microscopic models in SF6 arc and discusses the future research directions and problems to be solved in SF6 arc.

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 (Te) is not equal to the heavy-particle temperature (Th), 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 Te/Th varying as a function of the logarithm of electron density ratio (ne/) was established. In this model, T* is introduced to allow a smooth transition between Th and Te 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.

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–Hückel corrections are both taken into account. The influences of condensed phases, Debye–Hückel 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.