Mass spectrometry of arcs in SF6 circuit breakers

Today, SF₆ is used to a great extent as insulating and arc-quenching medium in high-voltage gas-blast circuit breakers. The arcing in SF₆ during current interruption forms decomposition products. These can influence the arc-quenching properties of the circuit breaker. Furthermore, they can cause corrosion of the circuit breaker housing. In this comprehensive study we present results obtained for the first time from a direct mass spectrometric investigation of the exhaust gases of a high pressure SF₆ arc in a model circuit breaker. Our mass spectrometric system consists of a time-of-flight mass spectrometer (TOFMS) equipped with a molecular beam sampling systems. This device allows us to measure mass spectra of high pressure sources with a time resolution of up to 10,000 spectra per second. We have determined the formation rate of the most abundant decomposition products in a SF₆ arc at 1 bar. These products are SF₄, CF₄, WF₆, SOF₂, SO₂, CS₂ S₂F₂ and HF. The fast detection time inherent to our system permits also the determination of the formation of SF₄, which is 0.45–0.50 Vol. %/(kJ/1SF₆). In addition, we have studied the influence of water and oxygen impurities which are responsible for the production of highly corrosive HF. Finally, we have considered the influence of the thermal degradation of teflon (P.T.F.E.), which is used as nozzle and insulating material in circuit breakers. On this occasion we have demonstrated that CF₄, which exhibits dielectric properties similar to SF₆, is the main decomposition product formed from teflon. However, we have found that besides CF₄ also excess carbon is formed, which is deposited on insulators of the model circuit breaker.Our time-resolved mass spectra reveal that the CF₄ production from teflon is delayed by a few milliseconds with respect to the SF₆ dissociation in the arc. This delay can influence the interrupting process of the circuit breaker by changing the plasma composition during the arcing period. Although our experiments have been performed on a model circuit breaker we claim that the results presented in this study can be applied to real circuit breakers, since the arc current density and the energy dissipated per liter SF₆ are of the same order of magnitude in both devices.     

The Local Electrical Properties of Gas Blast Arcs near Current Zero

Results are reported of experiments performed on a model air-blast circuit breaker to determine the distribution of voltage along the axis of the arc gap during the current zero period, following half sinusoidal current pulses of frequency 85 Hz. The air-blast was sustained by a reservoir pressure of 6.89 × 105 N/m2. Measurements were made following peak currents of 3 and 8 kA, with copper and carbon upstream cathodes, different separations of upstream cathode from nozzle inlet and different conditions downstream of the nozzle throat. The results have enabled the conductance decays at various axial positions to be determined and these have been related to the improved circuit breaking performance observed by other authors when the nozzle and arc gap geometries are optimized. Although a detailed theoretical analysis awaits the measurement of other fundamental plasma properties, an approximate evaluation of the role played by various basic processes has been made.     

Experimental Investigation on Arc Phenomena in SF6 Puffer Circuit Breakers

A detailed observation of an arc in a model puffer-type SF6 gas circuit breaker in the current range between 10 and 50 kA (rms) has been carried out. It was found that the arc column remained stable on the center axis during the high-current region, then became turbulent near current zero. It was found that the time interval during which the turbulent arc was observed decreased with increasing values of the peak current. These phenomena indicated that the thermal effects of high-current arcs remain even at current zero. It also was observed that the arc diameter at the nozzle throat outlet was smaller than that at the throat (29 mm), even at a current as high as 70 kA (instantaneous), and that the boundary of gas flow at a downstream region had a very large diameter when the arcs were present. However, around current zero the boundary diameter became as small as that without arc.     

Time-Resolved Laser Doppler Measurements in the Underexpanded Jet of a Model Gas-Blast Circuit Breaker under Arcing Conditions

A differential laser Doppler system has been used in conjunction with a self-triggered oscillographic raster display system for measuring particle concentrations and flow velocities in a model gas-blast circuit breaker. The flow velocities have been correlated with particle size to enable extrapolation to be made to determine the true plasma flow velocity. The results have been deconvoluted to take account of lateral movements of the arc column. The measurements have been taken with air as the host arcing medium and a flow through a 25-mm-diameter orifice sustained by an upstream pressure of about 7 bar (downstream pressure 1 bar). The arc was sustained by sinusoidal cuffents of peak values 3 and 8 kA and a frequency of 85 Hz. These two currents corresponded to conditions when the model circuit breaker successfully and unsuccessfully interrupted the arc-sustaining current, respectively. The results show that in the case of the 8-kA arc, the flow pattern is severely distorted from that under nonarcing conditions, suggesting that this is a contributory reason for the poorer perfonnance of the model circuit breaker at this higher current level.     

Imaging and spectrum diagnostics of air arc plasma characteristics

This paper is devoted to the study of the air arc plasma characteristics and consists of two parts. An optical high-speed imaging system is integrated with pressure sensing, arc current, and voltage measurement to investigate high-speed and high-temperature unsteady arc plasma motion in a molded case circuit breaker (MCCB) model, especially with the influence of polyoxymethylene (POM), polycarbonate (PBT), nylon, and ceramic arc chamber wall materials. It demonstrates that the performance can be improved effectively by adopting POM and nylon gassing materials. Also, spectrum diagnostics is introduced to analyze the influence of polymethylmethacrylate (PMMA) gassing material on the temperature of arc in a simple model. It shows that material added adjacent to cathode is more effective to cool the arc, compared with the case of anode.     

Correlation of the Radially Integrated Properties of Gas Blast Arc Discharges

Extensive experimental results for the properties of the arc thermal region and local electrical conductances have been reported in the literature for both peak current and current zero phases of orifice air flow arcs. The present paper is concerned with interpreting these results in terms of boundary layer integral analysis concepts. "Characteristic area" and "shape factor" values are presented for orifice flow arcs and methods of correlating these parameters for different arcing conditions are examined. The results show that within experimental accuracy unique relationships exist between various shape factors and correlation parameters, over a wide range of arcing conditions including the current zero period of an ac waveform. The resulting correlation curves, therefore, form the basis for predictive calculations of arc behavior under different gas blast conditions. Finally, an illustration is presented of the manner in which the shape factor correlations may be used for determining the relative importance of various fundamental processes during the current zero period of a gas blast circuit breaker arc.     

Mass-spectroscopic study of the influence of nozzle material on high-pressure SF6 arcs

The interrupting capability of a gas-blast high-voltage circuit breaker (CB) is mainly determined by the self-induced pressure rise caused by the thermal arc energy, the composition of the arc plasma and the chemical reactions occuring during and after current interruption. We have studied the nozzle materials boron nitride (BN), quartz (SiO₂), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene (ETFE), polyethylene (PE) and epoxy resin (ER) with respect to their influence on these processes with the aid of a model circuit breaker (MCB). Direct measurements of the arc-induced pressure rise reveal that the portion of the arc energy available for the pressure rise varies greatly (20%–65%) with the properties of the nozzle material. Nozzle erosion is significantly higher for materials with high values (e.g. polymers). Therefore, the lifetime of polymer nozzles is considerably shorter than that of ceramic nozzles. We have investigated the influence of the nozzle material on the decomposition products formed in the arc discharge of our MCB by studying the composition and time dependence of these products. The MCB was directly attached to the time-of-flight mass spectrometer (TOFMS) with the aid of a molecular-beam sampling system, which allowed real-time measurements of the arced gas during and after current interruption, thus providing information on the ablation mechanism and on the reaction kinetics of vaporised nozzle material with dissociated SF₆. The most abundant long-lived reaction products are SF₄, SOF₂, C₂H₂, CO, and CS₂. Their formation rates have been determined as functions of the nozzle material. With respect to quantities and properties of decomposition products, ceramics are superior to polymers since they form only small concentrations of corrosive and toxic products.     

The Dynamic Behavior of Nozzle Arcs

The dynamic behavior of ac arcs in a supersonic nozzle has been investigated. The close coupling between the arc and its external flow determines the arc characteristics as well as the flow transients within the nozzle in a gas-blast circuit breaker. For affinely related nozzles, the dynamic behavior of the system is found to be fully determined by two nondimensional parameters: the nozzle coefficient N [17] and the nondimensional frequency ? which is the product of the angular frequency of the current and the time scale of the system. Of these two parameters, ? controls the qualitative behavior of the arc and its external flow. When ? is of the order of 10-2 or less, a quasi-steady period of arcing exists. Arc modeling can then be conveniently divided into a quasi-steady-state (hence dc) phase and a current-zero period.     

Study of a circuit-breaker arc with self-generated flow. I. Energytransfer in the high-current phase

This study deals with the first phase of operation, corresponding to the energy transfer between arc and gas and to the pressure rise. The experimental study is devoted to measurements of current, arc voltage, and pressure variations in N₂ and SF₆. For currents of the order of 10 kA the mean measured electric field was about 32 V/cm in SF₆ and 36 V/cm in N₂. Through a bibliographical study and a modeling approach of the interruption arc, an analysis of the role of the different mechanisms of energy transfer between the arc and SF₆ was conducted. With a 10 kA pulse, about 80% of the transfer is found to be due to convection and the rest to radiation from the arc. This transfer results in an overpressure of nearly 1 bar in SF₆ and 2 bar in N₂. It appears that the operation of this type of circuit breaker is limited to high currents: for currents below 7 kA the overpressure is lower than 0.5 bar, which does not provide efficient blowing at current zero     

Study of the Arc and of the Interaction between Arc and Operating Mechanism in SF6 Puffer Breakers

The article deals with studies of the arcing performance of SF6 puffer breakers at currents of up to 80 kA and voltages of up to 200 kV, using an interrupter unit fitted with viewing windows. The studies are mainly concerned with the behavior of the arc at current zero and with restrike phenomena under various conditions. In addition to measurements of the electrical variables and the pressure variation, the tests cover the use of high-speed cameras, spectroscopic diagnostic methods, and schlieren methods. The density fields and flow fields derived from the latter are used to optimize the gas flow. On the basis of the breaker data and those of its operating mechanism a method for computing the essential variables, e.g., pressure, gas flow, back pressure, and contact travel, is developed. The variation of these parameters as a function of the interrupter data for various currents and with due allowance for the interaction between the arc and the operating mechanism must be known if the breaker design is to be optimized. The method uses simplified equations for the gas flow and the arc. The relationship between the gas data (e. g., density, enthalpy, speed of sound, electrical conductivity, and dielectric strength) and the temperature is taken into account by using approximation functions. The temperature of the arc core is matched to the test results.     

Current Chopping Introduced by Arc Collapse

The intensive cooling by a moving gas can cause violent elongations and curls of the circuit breaker arc, especially when small currents are interrupted. During the elongation the arc voltage increases rapidly. This introduces a breakdown across a smaller distance by short circuiting a part of the arc. Such a breakdown is called here arc colapse. The abrupt decrease in resistance and arc voltage may give rise to an oscillating discharge of the circuit breaker parallel capacitance into the arc path. This oscillation can force the main current to zero and thus cause current chopping. In this paper this kind of current chopping is studied and compared with chopping by instability oscillation. It is theoretically explained why these independent origins for chopping produce the same chopping levels.     

Visualization of arcing process in an auto-expansion circuitbreaker

The arcing process in an auto-expansion circuit breaker, a new generation of high voltage circuit breakers, has been simulated and visualized by implementing an arc model into a general purpose computation fluid dynamics (CFD) package, PHOENICS. The model takes account of radiation transport, arc radiation induced inner wall ablation, turbulence enhanced momentum and energy transport and the moving parts of the breaker. Details of the temperature, pressure, velocity and electric fields within the breaker can be visualized thus helping the optimization of the geometry and operation characteristics of a breaker     

Mathematical modeling of SF6 puffer circuit breakers.II. Current zero region

For pt.I see ibid., vol.24, p.490 (1996). A mathematical model of arc behavior in an SF₆ puffer gas-blast circuit breaker in the high current phase was reported in a previous paper. This model is extended to the current zero region by solving the full partial differential are conservation equations taking account of both turbulent and radiation effects. The critical RRRV for the breaker can therefore be calculated based on the whole arcing history. The predicted values are compared with the experimental results of Noeske et al. (1983), and good agreement is found providing that the free parameter in the turbulence model is set appropriately. Results for temperature, electric field, and velocity are also presented and analyzed     

脉冲电流作用下TIG电弧的数值分析

建立了脉冲电流下自由燃烧的TIG电弧的二维轴对称数学模型,利用FLUENT软件,通过选择合适的边界条件和强烈耦合控制方程组对脉冲TIG电弧进行了数值模拟,得到了在焊接电流周期性变化下电弧形态、电弧温度场、电弧轴线方向上的温度和速度及焊接工件表面电弧压力的变化情况;针对电弧压力,得到了不同峰值电流、占空比、脉冲频率作用下的分布情况,并分析了它们在脉冲电流作用下的周期性变化规律.分析结果表明:当脉冲电流发生突变时,它们的变化滞后于脉冲电流的变化,且从基值电流向峰值电流变化时的响应速度更快,并最终达到一个相对稳 关键词： 脉冲TIG焊 电弧 数值模拟 FLUENT     

Axially blown SF6-arcs around current zero

The increase of the interrupting capability of modern SF6 puffer breakers demands a better knowledge of the interaction of the arc with the gas flow. During the current interruption in an SF6 breaker the arc temperature in the stagnation zone is of decisive importance. The temporal evolution of the arc temperature and the diameter is studied by means of interferometry and emission spectroscopy. Experimental results are presented which show the influence of the current slope and the gas pressure on the arc decay. These results are compared with a theoretical model describing the temperature decay after current interruption.     

On Current Distribution through the Cross-Section of an Arc Moving in a Narrow Insulating Slot

Current distribution across the arc plasma cross-section constructed in a narrow slot between isolating walls and other factors affecting the distribution are considered. Experimental data on current density distribution of the arc moving at subsonic velocity are analyzed. Data available on the mean current density in an arc moving at supersonic velocity and under phase transition conditions on an active surface of the walls (gas-gen erating) are summarized. Possible influence of conditions of heat transfer with walls and pressure inside the column on the extent of non-uniformity of current distribution on the arc cross-section are shown. The problems under consideration are of practical application for avoiding thermal overload of arc chambers of magnetic-blast breakers.     

The DC Arc in a Supersonic Nozzle Flow

The boundary layer integral method at its second level of approximation has been used to study the DC arc in a supersonic nozzle flow. It is shown that with the inclusion of the arc momentum balance, the critical point of the flow is, generally, not the sonic point of the external flow. The speed, at which a disturbance propagates relative to the external flow, is in general supersonic and is dependent on the arc conditions. The arc model is capable of predicting the axial electric field, the arc size and the axial pressure distribution as a function of current. For affinely related nozzles, the solution is determined by a parameter N, which is related to zt, the stagnation condition and the nominal current density at the throat (I/At). Numerical results are given for a particular nozzle shape although the method of analysis is general. Practical implications as regards nozzle design for a gas blast circuit breaker are briefly discussed.     

Use of Axial Magnetic Fields to Improve High-Current Vacuum Interrupters

When an axial magnetic field is applied to a vacuum arc, the arc tends to be stabilized in its diffuse mode. A minimum arc voltage is found for a certain magnetic field. In this condition, interrupting current is significantly increased, and it is nearly proportional to the diameter of electrodes. About ten years ago, a practical axial magnetic field electrode was developed for vacuum circuit breakers. Since then, through various improvements in its structure, this electrode has been refined for practical application in vacuum circuit breaker interrupters. The application has successfully covered not only medium-voltage circuit breakers, but also high-voltage (84 kV), dc high-voltage, and high-current circuit breakers. In this paper, ten years experience in this area is described.