title: Investigation of the dynamic characteristics and decaying behaviour of SF6 Arcs in switching applications
Creators: name: Wang, Weizong
Publisher: name: University of Liverpool Repository
Year: 2013

description: 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.