Computational Investigation of Arc Behavior in an Auto-Expansion Circuit Breaker with Gases of SF 6 and N 2 **

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1 Computationa Investigation of Arc Behavior in an Auto-Expansion Circuit Breaker with Gases of SF 6 and N 2 ** Jining Zhang Abstract An eectric arc behavior in a fu-scae 245 kv autoexpansion circuit breaker with a moving contact has been carried out for two fiing materias, SF 6 and N 2. The computer simuation invoves the simutaneous soution of the three conservation equations (mass, momentum and energy) and Maxwe s equations. The Lorentz force due to the interaction between the arcing current and its own magnetic fied, radiation mode and Prandt mixing ength turbuence mode have been taken into account. The features of temperature during the whoe arcing period are obtained. Resuts show that the arc coumn does not shrink propery in radia direction with N 2 fiing. Keywords Computationa simuation, arc behavior, autoexpansion circuit breakers, SF 6-free gas C I. INTRODUCTION IRCUIT breakers are used in the transmission and distribution network to prevent damage to generators, transformers, cabes and end user equipment from faut currents induced by accidents, or to satisfy the requirement of switching on/off. The breakers are designed to stop the current fow and to withstand votage transients which arise on the eectrica network when the current is interrupted. High-votage circuit breakers are excusivey fied with Suphur Hexafouride (SF 6 ) since it is an exceence medium for extinguish the arc and has exceent insuation properties to withstand the induced votage transients after arc extinction. However, the gas SF 6 is a very strong greenhouse gas which has potency 22,800 times more than that Carbon Dioxide [1] and has a ife time of about 4,000 years in the upper atmosphere. There is no other technoogy that is avaiabe to repace these circuit breakers. Therefore the repacement of SF 6 with a more environmentay friendy gas is becoming an increasingy interesting research topic [2] and practicay important issue for power equipment manufacturers and network operators [3]. Due to the compexity of the arc process in the circuit breakers, the Jining Zhang is with the Department of Eectrica and Eectronic Engineering, Xi an Jiaotong Liverpoo University, , Suzhou, China. **This project is supported by the Centre for Smart Grid and Information Convergence of Xi an Jiaotong-Liverpoo University, Suzhou, China performance of the circuit breakers depends on a number of design parameters and the whoe arcing history. It is difficut or the cost woud be intoeraby high if the investigation soey depends on short circuit tests. It is therefore highy desirabe to deveop computer program to aid the circuit research and design, which woud at east reduce the number of short circuit tests required. An arc modeing based on the basic physica processes occurring in a circuit-breaker arc has been carried out for modern circuit breakers. In genera, computer simuation of a modern circuit breaker arc invoves the simutaneous soution of the three conservation equations (mass, momentum and energy) and Maxwe s equations with moving boundaries. The Lorentz force due to the interaction between the arcing current and its own magnetic fied needs to be taken into account. The conservation equations and the Maxwe s equations are soved by the PC based software PHOENICS [4], which is sufficienty powerfu for the simuation of the operation of auto-expansion and rotary/sef-pressurizing circuit breakers. The behavior of eectric arc burning in pure SF 6 and in pure N 2 is investigated in a high votage autoexpansion circuit breaker. II. THE ARC MODEL Since the fow in a supersonic nozze is axis-symmetric, the governing equations that describe the arc behaviours in the nozze can be written in cyindrica poar coordinates (r,z) system: ( ) V S (1) t where φ, Γ φ and S φ are, respectivey, the dependent variabe, the diffusion coefficient, and the source terms, which are isted in Tabe 1 for the mass, momentum, and energy equations, where a notations have their conventiona meaning [5]. The subscript denotes the aminar part of the diffusion coefficient and t the turbuent part. q in the energy equation represents the net radiation oss per unit voume and time. It is cacuated with the approximate mode of Zhang et a [6]. Equation (1) assumes oca thermodynamic equiibrium. Thus, the therma and transport properties 374

are functions of pressure and TABLE I SOURCE TERMS AND DIFFUSION COEFFICIENTS FOR GOVERNING EQUATIONS Equation φ Γ φ S φ Continuity 1 0 0 Momentum in z direction Momentum in r direction Energy

2 are functions of pressure and TABLE I SOURCE TERMS AND DIFFUSION COEFFICIENTS FOR GOVERNING EQUATIONS Equation φ Γ φ S φ Continuity Momentum in z direction Momentum in r direction Energy (Enthapy) w v h P z viscous terms t P r viscous terms t 2 ( k k ) / c E q dp dt viscous dissipatio n t p temperature, which are tabuated and put into the program. The properties of SF 6 from Murphy [7], and N 2 from Yos [8] are used. The gases are assumed to be idea at temperatures beow 500 K. Lorentz force and Ohmic heating resuting from the arcing current are incuded in the source terms of the momentum and energy equations, respectivey. The current density and the eectric fied are cacuated by soving the current continuity equation, which is expressed in terms of eectrica potentia: 0 (2) where σ is eectrica conductivity and ϕ is the eectrica potentia. The Lorentz force is computed based on the axia and radia current densities and the azimutha component of the magnetic fied induced by the arcing current. Turbuenceenhanced momentum and energy transport are modeed by the Prandt mixing ength mode. The turbuent viscosity is cacuated by: 2 w μ ρ (3) t m r where ρ is gas density and m is the ength scae. m, in the arc region is taken as a fraction of the arc therma radius defined as the radia distance from the axis to the position of 4,000 K isotherm. III. RESULTS AND DISCUSSION The governing equations and the current continuity equation are soved by a commercia CFD package caed PHOENICS [4]. The geometry of the 245 kv auto-expansion circuit breaker under investigation is schematicay shown in Fig. 1. The circuit breaker is initiay fied at an absoute pressure of 0.69 MPa at room temperature for both cases of fiing with gases of SF 6 and N 2. The eectric fied is of ong range and requires a cacuation domain bigger than the fow fied, which is confined within the breaker. The domain is constructed by adding an annuar bock in the radia direction to the fow fied domain based on the circuit breaker structure, which is schematicay iustrated in Fig. 1. The radia dimension of this added area is varied unti the cacuated eectric fied in the circuit breaker is no onger sensitive to the radia boundary conditions of the extended domain. Fig. 1 Schematic diagram of the auto-expansion circuit breaker under investigation. The position of the moving soid contact and the instantaneous arcing current at different instants are aso given. The region indicated as a transparent contact coects the current. It has no effect on gas fow inside the hoow contact. The vertica ines correspond to the positions of the moving contact at different time

3 Within the fow fied, body fitted coordinates (BFC) are used to mode the compex geometry of the arcing chamber. Wa friction and energy oss by heat conduction at the nozze and eectrode surfaces are not taken into account because of their negigibe effects on the arc behavior, especiay at the high current period. The physica phenomena of arc rooting inside the hoow contact is extremey compex. The rooting mechanism is not exacty known. To simpify the three-dimensiona probem of the arc root within the hoow contact, a transparent (to the fow) contact in the contact is paced and a circuar spot is defined in the pane fush with the tip of the hoow contact to simuate the cathode spot. Its radius is cacuated by assuming an average current density of 10 8 A/m 2. Fig. 2 Typica resuts of the temperature fieds for the case of fiing gas being SF 6. t represents time, and I stands for the instantaneous current

4 Fig. 3 Typica resuts of the temperature fieds for the case of fiing gas being N 2. t represents time, and I stands for the instantaneous current. The arc is initiated at a current of 2 ka across a gap of 8 mm between the transparent and soid contacts by introducing a conducting pasma coumn across the gap. The coumn has a radius of 3 mm, over which the gas temperature ineary decreases radiay from 20,000 K at the centre to 300 K at the edge. The arcing duration is 18.6 ms with a second peak current of 78 ka at 13 ms (Fig. 1). The behaviour of eectric arc burning in the mixture of SF 6 and poytetrafuoroethyene (PTFE) vapour has been investigated in the 245kV auto-expansion circuit breaker (Fig.1) for the same current waveform shown in the same diagram [7]. The predicted arc votage and pressure variation in the expansion voume are compared with the test resuts and they matched we. The same breaker has been used to investigate the effect of the fiing gases on the arc behaviour here. The aim of the project here is to investigate the arc behaviors with different arc quenching and insuating mediums, SF 6 and N 2, therefore, the abation of PTFE nozze 377

5 is not counted. In fact, when the instantaneous current fas beow 30 ka, the PTFE mass concentration reduces to ess than 5% and the arc starts to burn in a fiing gas-dominated environment. At time 18.6 ms, when the instantaneous current is 2.8 ka, there is ony 2% PTFE vapor by mass in the main nozze [7]. Therefore, the therma recovery of the circuit breaker immediatey after current zero can be regarded as taking pace in a pure fiing gas environment. Two fiing gases, SF 6 and N 2, have been investigated. Figs. 2 and 3 show that the temperature fieds at a few different moments with the fiing gases being SF 6 and N 2 respectivey. Generay, during the high current period, the pictures of the temperature fieds with the two fiings are simiar, but when the current drops beow 30 ka, the arc coumn coud not shrink in radia direction propery with Nitrogen, especiay when the current drops towards its fina zero. This phenomenon aso observed by the investigation on the arc behaviour in a simpe supersonic nozze [9]. By comparing the materia properties of the two gases, the properties of Nitrogen under ow temperature ( 5000 K) shoud be responsibe for it. Further and more detai investigation of the effects of the materia properties on the arc behaviour wi be continuousy studied. [7] J. L. Zhang, J. D. Yan, A. B. Murphy, W. Ha, and M. T. C. Fang, Computationa Investigation of Arc Behaviour in an Auto-Expansion Circuit Breaker Contaminated by Abated Nozze Vapour, IEEE Trans. Pasma Science, Vo. 30, PP , [8] J. M. Yos, Report, Liverpoo University, UK. [9] J. L. Zhang, Computationa Investigation of Effects of SF 6, CO 2 and N 2 on Arc Interruption in a Supersonic Nozze, in Proc. 20th Internationa Conference on Gas discharges and Their Appications (GD2014), Oreans, France, 2014, pp Jining Zhang was born in Shaanxi, China. She received the B.Sc., M.Sc., and Ph.D. degrees from the Department of Energy and Power Engineering, Xi an Jiaotong University, Xi an, China, in 1982, 1987, and 1995, respectivey. She was a Lecturer at Xi an Jiaotong University between 1987 and 1996, and then became an Associate Professor before she joined the Department of Eectrica Engineering and Eectronics, the University of Liverpoo, U.K., in 1997 as a Research Feow working on circuit-breaker arcs. She is a Lecturer in the Department of Eectrica Engineering and Eectronics, Xi'an Jiaotong- Liverpoo University with research interests in therma pasma and computationa fuid dynamics. IV. CONCLUSION The behavior of an eectric arc burning in two fiing materias, SF 6 and N 2, has been investigated in a 245 kv auto-expansion circuit breaker. The resuts show that, with the circuit breaker geometry under investigation, the temperature fieds with the two fiings are simiar during the high current period. However, when the current drops beow 30 ka, the arc coumn coud not shrink in radia direction propery with Nitrogen, especiay when the current drops towards its fina zero. The properties of Nitrogen under ow temperature ( 5000 K) shoud be responsibe for this. REFERENCES [1] A. R. Ravishankara, S. Soomon, A. A. Turnipseed, and R. F. Warren, Atmospheric ifetimes of ong-ived haogenated species, Science, Vo. 259, No. 5092, PP , [2] Kynast, and K. Juhre, N2 and N2-CO2 Mixture in Gas Insuated Compartments under High Pressure, in Proc. 10th Internationa Symposium on Gaseous Dieectrics, Greece, 2004, pp. 18. [3] T. Uchii, Y. Hoshina, T. Mori, H. Kawano, T. Nakamoto, and H. Mizoguchi, Investigations on SF6-free Gas Circuit Breaker Adopting CO2 Gas as an Aternative Arc Quenching and Insuating Medium, in Proc. 10th Internationa Symposium on Gaseous Dieectrics, Greece, 2004, pp [4] CHAM, Phoenics, London, U.K. [5] J. D. Yan, M. T. C. Fang, and W. Ha, The deveopment of PC based CAD toos for auto-expansion circuit breaker design, IEEE Trans. Power Deivery, Vo. 14, PP , Jan [6] J. F. Zhang, M. T. C. Fang, and D. B. Newand, Theoretica Investigation of a 2KA DC Nitrogen Arc in a Supersonic Nozze, J. Phys. D, App. Phys., Vo. 20, PP ,

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