ARC MOTION IN LOW VOLTAGE CIRCUITBREAKER (LVCB) EXPERIMENTAL ANDTHEORETICAL APPROACHES

International audienceAbstract This paper is related to the study of the arc motion in simple low voltage circuit breaker geometry.Experimental and theoretical approaches are investigated respectively by fast camera and by a magneto hydrodynamicmodel. Two theoretical methods have been developed to characterize the arc movement called MECM (Mean ElectricalConductivity Method) and GCRM (Global Current Resolution Method). The results obtained by the two models are ingood agreement with the experimental observations. The MECM allows obtaining faster results but the stagnation phasesare well represented with the GRCM and this last method is easier to implement in more complex geometry. The resultsshow also the importance of the exhaust description on the arc behavior

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Modelization and analysis of the electric arc in low voltage circuit breakers

246 p.Tesis doctoral que presenta un nuevo modelo de arco eléctrico para interruptores de baja tensión mediante simulación FV y validación experimental

Modelling of High Voltage Circuit Breakers Considering Interaction between the Driving Mechanisms and Switching Arcs

With the increase of voltage and current ratings, design and test of circuit breakers have become more demanding. This is a consequence of the technical difficulties and costs in performing development tests as well as the complexity resulting from the coupling of different physical mechanisms in shaping the interruption performance of a breaker. It is well known that the performance of a circuit breaker is determined by many design and operational parameters among which the motion characteristic of the contact is a key factor. Since the motion characteristic of the contacts has a direct impact on breaker performance, it plays a critical role in the design of a circuit breaker. Difficulties in performing tests or experiments under high voltage and power have rendered the traditional ‘cut and try’ design approach impractical. Computer simulation on the other hand, with its cost-effective and easy-to-implement nature, has become the favored approach to achieve design optimization and attracted a great amount of attention in the field of electrical engineering. An arc simulation model (Liverpool arc model) has been developed in this research group together with a circuit breaker simulation interface, featuring PHEONICS as the differential equation solver. Valuable information can be extracted from arc simulation results and help design better optimized prototypes. However, the absence of a driving mechanism model in the current arc simulation model limits its effectiveness and functionality. The travel of the contact could affect the flow cross section in the arc chamber as well as the length of the arc, and ultimately the performance of the circuit breaker. Additionally, the interaction between the arc chamber and driving mechanism, which has a significant impact on the result (most prominent under high current and long arc duration), has also been overlooked in the existing model. Previously, the absence of a driving mechanism model is dealt with by providing the simulation with a user-defined text file containing the travel profile of the moving contact. However, a user defined file may not reflect the true motion of the moving contact due to the unaccounted interaction between the arc chamber and driving mechanism. Consequently, a truly coupled simulation model, which is capable of calculating the travel of all moving components in real time and eliminating any inaccuracies in the predefined travel curves, is needed. In the current research, an approach to quantify the interaction between the arc and driving mechanism has been proposed. Collectively, the resistive force (known as reaction force) imposed on the moving components in the arc chamber by the high-pressure gas can be calculated by a newly developed integral method. The existing arc model has been expanded to incorporate the calculation of reaction force. In addition, a functional mathematical model for the ZF-11-252 (L)/CYTA hydraulic driving mechanism has also been developed, based on which a number of sensitivity studies have been carried out and the key design parameters that affect the dynamic characteristics of the driving mechanism identified. Considering both the driving mechanism model and the improved arc model (which can now calculate reaction force based on the pressure distribution in the arc chamber) a coupled circuit breaker simulation procedure has been established together with an interface which facilitates the information exchange between the driving mechanism model and the arc model. Based on this coupled model, the interaction between the arc and the driving mechanism is studied under different arcing conditions and nozzle geometries. In particular, two important factors affecting the accuracy of the predicted travel characteristics of the moving components have been identified through the studies. The first one is the need to consider the variable contact surface area between the piston rod in the hydraulic cylinder and the oil as a result of the motion of the piston. The second factor is the prediction accuracy of the pressure field around the moving components in the arcing chamber, especially when there are strong pressure waves propagating in the arcing gas. These aspects have not been studied so far

Investigation of the dynamic characteristics and decaying behaviour of SF6 Arcs in switching applications

Investigation into the dynamic characteristics and decaying behaviour of SF6 arcs during the current-zero period is of great significance to improving the interruption performance of high voltage circuit breakers and ensuring their reliable operation. The present research was conducted by means of modeling and experiment to provide a better knowledge of the switching process to help the design and optimization of high voltage SF6 circuit breakers. The first part of the work concerns the determination of thermophysical properties of SF6 plasmas under local thermodynamic equilibrium state (LTE). A systematic comparison with transport coefficients obtained using an old data set and experimental test has been performed to check the reliability of the proposed phenomenological approach in evaluating transport cross sections. Properties especially transport coefficients become sensitive to the choice of Debye length definition predominantly due to the different collision integrals affected by the different screening distance. Pressures increase can also influence thermophysical properties due to the inhibited chemical reactions. Moreover, the thermophysical properties under non-equilibrium conditions have been investigated using a two-temperature model. It was noted that the special case with equal electrons and heavy species temperatures produces results agreeing excellently with those obtained by the LTE model. The forms of mass action laws as well as the choice of reaction excitation temperature for molecular ionization used in the calculation can significantly modify the species composition and plasma properties. This model lays the micro-theoretical foundation for a deeper understanding of SF6 plasma formation and evolution mechanism and provides a reliable properties input for non-equilibrium arc behaviour simulation. Following a traditional approach of arc modelling assuming LTE, considerable effort has been devoted to study the arc-shock interaction and its influence on the dynamic characteristics and current zero behaviour of SF6 arcs in a supersonic nozzle with a hollow contact. It was found that the close coupling between the shock region and its surrounding gas flow greatly influences the aerodynamic and electrical behaviour of a nozzle arc and hence the thermal recovery process. In addition, deceleration of gas flow caused by the shocks and enhanced turbulent cooling brought by the sucked gas interacting with the arc both play a significant role in the determination of the thermal interruption capability. Possible departure from LTE in a decaying SF6 arc was studied using a two-temperature hydrodynamic model in a supersonic nozzle under well-controlled conditions. The predicted radial temperature variation presents quite good agreement with test result using emission spectroscopy. It is demonstrated that the electron and heavy-particle temperature diverge in cases where the collision energy exchange is ineffective. For the arc decay phase, the two-temperature model gives a lower cooling rate than the LTE model, and hence a higher conductance of the discharge passage at current zero showing the necessity of using a two-temperature model to accurately predict the current interruption capability of SF6 gas-blast circuit breakers. For thermal recovery phase, considering the chemically non-equilibrium effect, a global kinetic model of decaying SF6 arcs was established to study the electrons elimination mechanisms around current zero. For dielectric recovery phase, the critical dielectric strength of hot SF6 are investigated based on a two-term Boltzmann equation solution of electrons energy distribution function. It is noted that the main mechanisms of electrons elimination are the dissociative attachment from 3500K to 7500K, electron-molecular ion recombination in the temperature lower than 3500K. The temperature increase, pressure decrease and departure from chemically non-equilibrium can all contribute to the dielectric strength reduction. The entrainment of PTFE ablation vapour can enhance the dielectric strengh of SF6 above 2500K. Finally, this research manufactures a model SF6 gas blast interrupter and investigates the dynamic characteristics of electrical, light radiation, pressure along together with electrode movement. Arc dynamic characteristics and decaying behaviour of CO2 and N2 is compared with that of SF6 arcs in order to obtain the dominant properties influencing arc quenching. It is noted that the extinction voltage, which decreases with increase in the interrupting current, is related to the conductance decay during current zero period and can be considered as an evaluation of interruption capability. Gas blast can bring a much more rapid variation of arc resistance and a much higher cut-off current before its extinction. SF6 has a superior interruption capability possibly due to its high thermal conductivity and specific heat

Study on Cyber-physical System for Direct Current Circuit Breaker Based on Atmospheric Arc Plasma Movement

東京都市大学博士（工学）2022年度(令和4年)doctoral thesi

Computer simulation of fundamental processes in high voltage circuit breakers based on an automated modelling platform

Auto-expansion circuit breakers utilize the arc’s energy to generate the flow conditions required for current interruption. The operation of this type of circuit breaker is extremely complex and its interruption capability depends on the whole arcing history as well as a number of geometric factors. On the other hand, circuit breaker development based on test is extremely expensive and time consuming. The accumulated understanding of the underlying physical processes so far enables arc models be used as a tool for optimum design of switchgear product such as high voltage circuit breakers. For academic research, there is often a need to study the performance of a newly developed arc model by inspecting the distribution of relevant physical quantities during a simulation and their sensitivity to model parameters in an efficient and convenient approach. However the effective use of computer simulation by design engineers has been hindered by the complexity encountered in model implementation. This thesis presents the development and structure of an automated simulation tool, the Integrated Simulation and Evaluation Environment (ISEE), for the arcing process in gas-blast circuit breakers. The functionalities of ISEE are identified and developed based on the experience in real product design, which include visual creation and definition of components, automatic setup of arc models based on a commercial CFD software package as equation solver, simulation task management, and visualization of computational results in “real-time” mode. This is the first automated simulation platform in the community of switching arc simulation. Using ISEE as the simulation tool, different designs of auto-expansion circuit breakers have been investigated to reveal the fundamental characteristics of the arcing process under different test duties. Before attempting to investigate the capability of an auto-expansion circuit breaker, the fundamental issue of determining the turbulence parameter of the Prandtl mixing length model is addressed. Previous studies on turbulence arcs were mostly concerned with simple converging-diverging nozzles. There has been little work on real circuit breaker nozzles. In order to calibrate the turbulence parameter, real arcing conditions including interrupting currents, contact travels, and transient recovery voltages of two commercial circuit breakers, with rated voltage of 145 kV and 245 kV, have been used together with the geometry of the circuit breakers to calibrate the range of the turbulence parameter. The effect of nozzle ablation has been considered. All together 6 cases have been used for three circuit breakers with each pair of cases corresponding to a success and failure in its thermal recovery process. It has been found that a single parameter of 0.35 is applicable to all three circuit breakers with an auxiliary nozzle and a main nozzle of converge-flat throat-diverge shape. It must be noted that this value is obtained with the definition of thermal radius introduced in Chapter 3 and the assumption that the parameter linearly changes with the interrupting current from 0.05 at 15 kA to 0.35 at current zero. Using the calibrated turbulence model, a computational study of the thermal interruption performance of a 145 kV, 60 Hz auto-expansion circuit breaker with different arc durations has been carried out in Chapter 4. The relation between pressure peak and current peak in the auto-expansion circuit breaker is discussed. It has been found that a larger average mass flux in the main nozzle indicates a better interruption environment, enabling the circuit breaker to withstand a larger rate of rise of recovery voltage after current zero. Another important finding is that the auxiliary nozzle plays an important role in an auto-expansion circuit breaker both at the high current phase and during the current zero period. Therefore, the proper design and use of an auxiliary nozzle is a key factor to enhance the thermal interruption capability of high voltage auto-expansion circuit breakers. In Chapter 5 of the thesis, the transient pressure variation in auto-expansion circuit breakers was studied. The pressure variation has an extremely complex pattern and the pressure changes in different ways depending on the location in the arcing chamber. It is shown, for the first time, that the time lag between the current peak and pressure peak in the expansion volume can be explained by using an energy flow rate balance method, that is flow reversal occurs when the enthalpy exhaustion rate from the contact space equals the electrical power input. Following the flow reversal, a high enthalpy flow rate from the expansion volume into the contact gap first occurs for a short while (1 ms), which is followed by a high mass flow rate of relatively cool gas at less than 2000 K. This high mass flow rate causes a surplus in mass flow rate into the contact gap and results in the last temporary pressure peak in the contact space before the pressure and flow field finally settle down for arc quenching at current zero. The pressure change under different conditions, i.e. different arc durations, different current levels and different length of the heating channel, has also been studied in details. In summary the present research leads to original findings in three aspects of the operation of auto-expansion circuit breakers, i.e. the calibration of the turbulence parameter for the Prandtl mixing length model, interruption performance with different arc durations, and the transient pressure variation in the arcing process. The results are expected to provide useful information for the optimum design of auto-expansion circuit breakers