Optimization of High Voltage Arc Assist Interrupters

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1 International Journal of Scientific & Engineering Research Volume 4, Issue3, March Optimization of High Voltage Arc Assist Interrupters Himanshu Joshi, Anjani Pandharkar, Ghanashyam Patil Abstract -- The circuit breaker is one of the most important unit in the electrical power system. The protection, stability and continuity of the system depend on the circuit breaker's ability to switch line, load and exciting currents and to interrupt fault currents. High voltage circuit breakers, have to operate with extremely high reliability in the power system to ensure the economical and reliable power distribution. It is observed over the years that most of the failures in circuit breakers are of mechanical nature. Consequently the development efforts over the last decade has been focused towards thedevelopment of high efficiency interrupters which requires low energy mechanism. The first step towards the high efficiency interrupter is the development of Arc Assist Interrupters. In the conventional Puffer type interrupters of a high voltage SF6 circuit breakers, a considerable amount of mechanical energy is used to compress the gas in order to create a gas flow to remove the arc energy. In arc assist Interrupters, arc energy is used to increase the gas pressure required for interruption. This paper discusses the results of detailed flow field and electric field computations. Effect of nozzle shape and thermal volume on the pressure rise is explained. Also capacitive current interrupting performance is estimated based on electric field and gas flow analysis. This paper helps to conclude simulation study using ga flow calculation program which is very useful to understand the mechanism of pressure generation, and it is also very useful to find the data of the optimum design of the interrupter. Index Terms Arc Assist Interrupters, Thermal Puffer, Nozzle, Gas Circuit Breaker (GCB), Capacitive Current Switching, Gas Density, Electrostatic Stress, Breakdown Voltage (BDV) 1 INTRODUCTION The development of SF6 gas circuit breaker started around 1950's, since then it has undergone many technological changes. Several types of SF6 circuit breakers have been developed by various manufacturers in the world during last twenty years, for rated voltages from 6.6kV to 1100kV. During every development phase the aim was to design a new compact, highly reliable and low energy circuit breaker. Figure 1 shows the cross section of typical puffer type gas circuit breaker. When breaker is fully closed, the pressure in the puffer cylinder is equal to that outside the cylinder. During opening stroke puffer-cylinder and moving contact tube start moving against the fixed piston, and there is relative movement, as a result gas gets Himanshu Joshi is currently pursuing masters degree program in electric power systems in Pune University, India. He is working with Crompton Greaves Limited since He is having an experience in the field of Type Testing of Gas Circuit Breakers, Dielectric Design and Gas Flow Design of Interrupters PH himanshu.joshi@cgglobal.com Anjani Pandharkar and Ghanashyam Patil is currently pursuing masters degree program in electric power systems in Pune University, India. Both have them have an experience with R & D-Gas circuit Breakers with Crompton Greaves Limited, Nasik anjani.pandharkar@cgglobal.com and ghanashyam.patil@cgglobal.com compresses in the cavity between puffer cylinder and piston. After certain travel contact separates and arc is drawn between the arcing contacts. During the arcing period, compressed SF6 gas is blown axially along the arc through a convergent divergent nozzle. As a result the arc gets extinguished. The above principle utilizes mechanical energy to compress the gas required for quenching the arc. The development of SF6 gas circuit breaker started around 1950's, since then it has undergone many technological changes. Several types of SF6 circuit breakers have been developed by various manufacturers in the world during last twenty years, for rated voltages from 6.6kV to 1100kV. During every development phase the aim was to design a new compact, highly reliable and low energy circuit breaker. Figure 1 shows the cross section of typical puffer type gas circuit breaker. When breaker is fully closed, the pressure in the puffer cylinder is equal to that outside the cylinder. During opening stroke puffer-cylinder and moving contact tube start moving against the fixed piston, and there is relative movement, as a result gas gets compresses in the cavity between puffer cylinder and piston. After certain travel contact separates and arc is drawn between the arcing

International Journal of Scientific & Engineering Research Volume 4, Issue3, March-2013 2 contacts.

The above principle utilizes mechanical energy to compress the gas required for quenching the arc.

2 International Journal of Scientific & Engineering Research Volume 4, Issue3, March contacts. During the arcing period, compressed SF6 gas is blown axially along the arc through a convergent divergent nozzle. As a result the arc gets extinguished. The above principle utilizes mechanical energy to compress the gas required for quenching the arc. 2 THE FLOW SOLVER The Kernel of the solver is based on the solution of the unsteady Euler equations of gas dynamics. The solver computes the solution by integration of the Euler equations on a moving and deforming Figure 1 Puffer Type Interrupter unstructured triangular grid using a finite volume method. Two modes of operation of the circuit breaker can be simulated viz. Cold mode and Arc mode. In Cold mode, simulation of operation of circuit breaker is done without electric arc. This allows the simulation of the transient flow field occurring during the operation. In addition, a solution of electric field can be obtained inside the, allowing the computation of the ratio (E/N) which governs the dielectric strength of the. The emporal variation of the dielectric The recent time the design efforts has been shifted to develop the arc assist type gas circuit breakers. wherein the arc energy is used to increase the gas pressure required for quenching the arc. But during the low current interruption the arc energy is not sufficient to increase the gas pressure, an additional gas flow required by the mechanical compression. Figure 2 shows the typical cross section of arc assist interrupter. strength which depends on both the inter electrode spacing and the fluid density distribution resulting from the flow can thus be obtained. In the Arc mode i.e analysis with the real current, an arc model is added to the Cold mode which includes the computation of Ohmic heating, the Lorentz forces and the radiation transfer. Nozzle ablation can also be simulated based on the incident radiative flux. [5], [8] 3 RESULTS AND DISCUSSION Many computer simulations done to see the effect of various parameter on the pressure rise in thermal. Figure 3 below shows gas pressure in thermal and compression during current interruption. The pressure in the thermal increases by temperature rise of gas and the temperature increase occurs by a hot gas flow from arcing zone. [2] Figure 2 Arc Assist Interrupter One of the major limitations of the arc assist interrupters is that the higher gas pressure is associated with higher gas temperature, which reduces the dynamic voltage withstand capability between contact space. Also breaking of low currents wherein arc energy is not sufficient is a major challenge to the designers. There are many parameters which influence interrupting performance such as volume ratio of two s, nozzle structure, breaking current, moving speed etc. Therefore design work is very complicated and development of Figure 3 Pressure rise during current interruption this type of GCB need very long time. But optimization is possible through the detailed computations of flow field and the electric field distribution in various phases of interrupting process and under different stress conditions. Firstly the effect of current magnitude is analyzed Figure 4 shows the influence of magnitude of interrupting current on the pressure rise in thermal. The interrupting current varied from 0.5 ka to 40 ka. The result shows that pressure in thermal increases with

International Journal of Scientific & Engineering Research Volume 4, Issue3, March-2013 3 increasing interrupting current.

3 International Journal of Scientific & Engineering Research Volume 4, Issue3, March increasing interrupting current. Figure 4 Effect of interrupting current on pressure rise in thermal Figure 5 shows the influence of arcing time on the pressure rise in thermal. The arcing time is varied from 10ms to 22ms. The result shows that pressure rise in thermal changes by arcing time. Figure 6 Effect of nozzle throat length on pressure rise in thermal Figure 7 shows the influence of nozzle throat diameter on the pressure rise in thermal. Three different throat diameters are considered for analysis viz. 1pu, 2pu and 3pu. The result shows that smaller the throat diameter higher the pressure rise in the thermal. [3] Figure 5 Effect of arcing time on pressure rise inthermal Figure 6 shows the influence of nozzle throat length on the pressure rise in thermal. Three different throat lengths are considered for analysis viz. 1pu =10mm, 2pu=20mm and 3pu=30mm. The result clearly shows that long throat nozzle increases the pressure in the thermal considerably. [3] Figure 7 Effect of nozzle throat diameter on pressure rise in thermal Figure 8 shows the influence of volumes of thermal and compression on pressure rise in thermal. Three cases are analyzed here, wherein the volume ratios of thermal to compression are considered as 1:1, 1:0.5 and 0.5:1. The result shows that the effect of volume of thermal is bigger than that of the compression. [1]

International Journal of Scientific & Engineering Research Volume 4, Issue3, March-2013 4 Figure 8 Effect of thermal and compression volume on pressure rise in thermal Figure 10 Electric stress on

4 International Journal of Scientific & Engineering Research Volume 4, Issue3, March Figure 8 Effect of thermal and compression volume on pressure rise in thermal Figure 10 Electric stress on arcing contact tip 4 CAPACITIVE CURRENT SWITCHING Many researches have shown that the dielectric withstand voltage calculated by electric field and static gas pressure is not so accurate because of the formation of shock wave at the tip of arcing contact. The tip of the fixed arcing contact is investigated to get the dielectric recovery strength, as the maximum electric field strength and variation in the gas density occurs due to the formation of shock waves. Here four points are considered on the tip of arcing contact. Figure 9 shows the points of calculation. Figure 11 Gas density variation on arcing contact tip Finally the dynamic BDV is calculated for all four points based on the streamer criterion. Figure 12 shows the dynamic BDV and applied voltage Vs time. From BDV curve, it is clear that in dynamic condition max BDV always shifts on tip of arcing contact. Figure 9 Points of calculation on arcing contact tip Gas flow analysis and electrostatic analysis is carried out during opening operation of breaker. Figure 10 and 11 shows the electrostatic stress and gas density variation on four points. Figure 12 Transient BDV on arcing contact tip Hence this method is more accurate to evaluate the capacitive switching performance. [4], [6], [7]

5 International Journal of Scientific & Engineering Research Volume 4, Issue3, March CONCLUSION - Simulation study using calculation program is very useful to understand the mechanism of pressure generation, and it is also very useful to find the data of the optimum design. - The nozzle throat diameter and the length of nozzle throat influence the pressure rise in the thermal considerably. The influence is as large as the influence of interrupting current. - The thermal effect of pressure rise in the thermal is big in relatively earlier interrupting phase. - The volume of the thermal influences the pressure rise in the thermal very much. The gas in the smaller thermal is easily heated by the back flow of hot gas, and higher temperature in thermal increases the pressure there. - The volume of compression influences the pressure in the thermal also. The larger volume increases the pressure in compression, and it influences the pressure in thermal at the longer arcing time. - To improve the dielectric performance of circuit breaker, it is effective to reduce the maximum electric field strength and the formation of the shock wave near the arcing contact. ACKNOWLEDGMENT The authors wish to thank Crompton Greaves Limited for providing continual guidance to work on Gas Flow Design of Arc-Assist Interrupters. REFERENCES [1] N. Osawa et al. Influence of puffer volume And operating force on puffer pressure built up in thermal puffer type gas circuit breaker taking nozzle ablation into account. IEEE Trans. Power Delivery, [2] Jan Sedlacek et al. Optimization of high Voltage self blast inter rupters by gas flow and electric field computations. IEEE Trans. Power Delivery, Oct [3] J-C Lee et al. Effects of nozzle shape on the interruption per formance of thermal puffer type gas circuit breakers. Elsevier Vacuum 80 (2006) [4] F.Endo et al. Analytical prediction of Transient breakdown characteristics of SF6 gas circuit breakers. IEEE Trans. Power strength of an SF6 circuit breaker, IEEE Trans. Power Delivery, Vol.6, No 2, Apr [6] A. Pedersen, Criteria for Spark Breakdown in Sulfur Hexafluo ride, IEEE Trans. Power Apparatus and Systems, PAS-89(8), pp.2043~2048, Nov [7] H.K.Kim et al. Optimal Design of Gas Circuit Breaker for In creasing the Small Current Interruption Capacity. IEEE Trans. On Magnetics, Vol. 39, No.3, May [8] Lin Xin, Li Junmin, et al. Numerical Calculation of Interrup tion properties of the Self Extinguishing type SF6 circuit breaker During Small Current Interruption. IEEE Trans. On Magnetics, Vol. 37, No.5, Sept BIOGRAPHY [1] Himanshu Joshi was born in 1978 in India. He received the Bachelor degree in Electrical Engineering from Pune University in Currently he is doing Masters Degree in Power Systems from Pune University. Currently, he is working with Crompton Greaves Limited, Nashik, India. Engaged in R & D works on Gas Circuit Breakers. Actively involved in development of SF6 Gas Circuit Breakers and Type Testing of SF6 Gas Circuit Breakers. Research Interest is in Dielectric Design of High Voltage Gas Circuit Breakers, Gas Flow Analysis. Authored 2 Technical Research Papers in Different National and International Conference. He has contributed for Filing 9 Patents and 2 Design Registrations. Delivery, 4(3): , [5] J.Y.Trepanier, M.Reggio, et al, Analysis of The Dielectric

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