G.V.Nagesh Kumar 1,*, Amarnath Jinka 2 Regular paper Assessing the Risk of Failure due to Particle Contamination in GIS under various Coefficient of Restitutions JES Journal of Electrical Systems The development of compressed gas insulated switchgear (GIS) equipment has progressed rapidly. Conducting particles in transmission and switching equipment insulated by compressed sulphur hexafluoride (SF 6 ) can result in loss of as much as 90% of the gas dielectric strength. These particles may be free to move in the electric field or may be fixed on the conductors, thus enhancing local surface fields. In a horizontal coaxial system with particles resting on the inside surface of the enclosure, the motion of such particles is random but the randomness depends on the coefficient of restitution and angle of incidence when approaching the coaxial conductors. The power industry has utilized several methods to control and minimize the effect of particle contamination in GIS. One such technique is to apply a dielectric (high resistivity) coating to the inside surface of the outer GIS enclosure. The electric field necessary to lift a particle resting on the inside surface of a GIS enclosure is much increased due to the coating. The simulation of the particle movement was carried under different Coefficient of Restitutions for coated busduct. The results have been presented and analyzed. Keywords: Electric Field, Gas Insulated Substations, Metallic Particles, Particle Contamination 1. Introduction Demand for electrical power has become one of the major challenges faced by the developing countries. Considering the relatively low per capita power consumption, there is a constant need for power capacity addition and technological upgradation whereas nonconventional energy systems have proved to be good alternative sources for energy. In developing countries like India most of the additional power has been met by conventional electric sources. Hence, the emphasis has shifted towards improving the reliability of transmission and distribution systems and ensuring that the innovations are not harmful to the environment. Air insulated power transmission and distribution substations suffer variations in the dielectric capability of air to withstand varying ambient conditions and deterioration of the exposed components due to oxidization and the corrosive nature of the environment. The size of the sub-station is also substantial due to the poor dielectric strength of air. In order to enhance the life and reliability of a power transmission and distribution substation, it is desirable to protect the sub-station components from a corrosive and oxidising environment. Metal encapsulation of the sub-station elements provides a simple and effective solution to the problem of durability of the substations. The use of a bus duct, with pressurized nitrogen gas, is a good example of devices with metal encapsulation used in power sub-stations. The size of the container is a direct function of the dielectric strength of * Corresponding author: G.V.Nagesh Kumar, Associate Professor, Department of EEE, GITAM University, Visakhapatnam, 530045, Andhra Pradesh, India E-mail: gundavarapu_kumar@yahoo.com 1 G.V.Nagesh Kumar, Associate Professor, Department of EEE, GITAM University 2 J. Amarnath, Professor, Department of EEE, Jawaharlal Nehru Technological University Kukatpally, Hyderabad, 500085, Andhra Pradesh, INDIA Copyright JES 2014 on-line : journal/esrgroups.org/jes
G.V.Nagesh Kumar et al: Assessing the Risk of Failure in GIS... the insulating medium. The container/enclosure sizes are thus large with a poor insulation like air or nitrogen. The electric power industry s preferred gaseous dielectric sulfur hexafluoride (SF 6 ), has been shown to be a greenhouse gas. SF 6 gas is mainly used for electrical insulation and, especially, for arc quenching current interruption equipment used in the transmission and distribution of electrical energy. Gas insulated sub-station systems offer a compact, costeffective, reliable and maintenance-free alternative to the conventional air insulated substation systems. Their compact size offers a practical solution to vertically upgrade the existing sub-station and to meet the ever-increasing power demand in developing countries. Compressed Gas Insulated Substations (CGIS) consist basically of a conductor supported on insulators inside an enclosure, which is filled with sulfur hexafluoride gas (SF 6 ). SF 6 gas provides phase to ground insulation so that space required is very much less. The voltage withstands capability of SF 6 bus duct is strongly dependent on field perturbations, such as those caused by conductor surface imperfections and by conducting particle contaminants. The contaminants can be produced by abrasion between components during assembly or operations. As one is aware of the attractive features of a Gas Insulated Substation (GIS), they also suffer from certain drawbacks. One of them is the outage due to seemingly innocuous conducting particles, which accounts for nearly 50% of the GIS failures. Flash over in a GIS is, in general, associated with longer outage times and greater costs than in a conventional air insulated substation. A conducting particle can short-circuit a part of the insulation distance, and thereby initiate a breakdown, especially if electrostatic forces cause the particle to bounce into the high field region near the high voltage conductor. Generally the allowable design levels are used because SF 6 is very sensitive to field perturbations such as those caused by conductor surface imperfections and by conducting particle contaminants. A study of CIGRE group suggests that 20% of failure in GIS is due to the existence of various metallic contaminations in the form of loose particles. These particles may exist on the surface of support insulator, enclosure or high voltage conductor. Under the influence of high voltage, they can acquire sufficient charge and randomly move in the gap due to the variable electric field. Several authors [1-6] have reported the movement of particles with reference to a few parameters. The presence of contamination can therefore be a problem with gas-insulated substations operating at high fields. If the effects of these particles could be eliminated, then this would improve the reliability of compressed gas insulated substation. It would also offer the possibility of operating at higher fields to affect a potential reduction in the GIS size with subsequent savings in the cost of manufacture and installation. Some of the methods of conducting particle control and de-activation are Electrostatic trapping, Use of adhesive coatings to immobilize the particles, Discharging of conducting particles through radiation and Coating conducting particles with insulating films. The purpose of this work is to develop techniques, which will formulate the basic equations that will govern the movement of metallic particles like aluminum, copper and silver and also the maximum movement is to be reduced for increasing the dielectric strength of gaseous Insulation. In This Paper dielectric performance is improved by coating the inner surface of the enclosure with a light shade with micron thickness epoxy resin. The Coating thickness is also varied and the results are presented and analyzed. The specific work reported deals with the charge acquired by the particle due to macroscopic field at the tip of the particle, 332
the force exerted by the field i.e., electric field on the particle, drag due to viscosity of the gas and random behavior during the movement. Wire like particles of aluminum and copper of 10 mm in length and 0.25 mm as radius on a 1-phase bus duct enclosure have been considered. The movement pattern for higher voltages class has been also obtained. 2. Modelling Technique of Busduct Figure 1 shows a typical horizontal busduct comprising of an inner conductor and an outer enclosure, filled with SF 6 gas is considered for the study. A particle is assumed to be at rest at the enclosure surface, until a voltage sufficient enough to lift the particle and move in the field is applied. After acquiring an appropriate charge in the field, the particle lifts and begins to move in the direction of the field after overcoming the forces due to its own weight and drag. The simulation considers several parameters like the macroscopic field at the location of the particle, its weight, viscosity of the gas, Reynold s number and coefficient of restitution on its impact to the enclosure. Fig 1: Typical Single Phase Gas Insulated Bus duct 2.1 Metallic Particles on Bare Electrodes in GIS For particles on bare electrodes, several authors have suggested expressions for the estimation of charge on both vertical/horizontal wires and spherical particles. The equations are primarily based on the work of Felici [7]. The lift-off field for a particle on the surface of an electrode can be estimated by solving the following equations. The gravitational force acting on a particle of mass m is given by F g = mg (1) The expression of the electrostatic force can be expressed as F e = QE; (2) where Q is the particle charge and E is the ambient electric field. The motion of the particle is simulated by using the equation: 2 d y m = F 2 e - mg - Fd (3) dt 333
G.V.Nagesh Kumar et al: Assessing the Risk of Failure in GIS... The motion equation using all forces can therefore be expressed as [8-14]: 2 π 0 l E(t 0 ) V Sin ω t m&& y ( t ) = X - mg 2 l r0 ln( ) 1 [r 0 - y(t)] ln ( ) r r i - y& (t) π r 0.5 ( 6 µ K (y) & + 2.656 [ µ ρ l y& (t)] ) d g (4) So, two initial conditions are necessary for a solution:.. my(t = 0 + ) = Rmy(t = 0) and y (t 0 ) = 0 = + where R is the restitution coefficient given by the ratio of incoming-to-outgoing impulses. The restitution coefficient for copper and aluminium particles seem to be in the range of 0.7 to 0.95: R=0.8 implies that 80% of the incoming impulse of the particle is preserved when it leaves the enclosure. The motion equation is a second order non-linear differential equation and in this paper, the equation is solved by using Runge-Kutta 4 th Order Method. 2.2 Metallic Particle Movement on Coated Electrodes in GIS The main reason for coating an electrode surface with a dielectric material is to increase the breakdown voltage of the system. The dielectric coating decreases the high local fields caused by conductor roughness. It impedes the development of pre-discharges in the gas due to the resistance of the coating. The Charge mechanism of a metallic particle on a dielectric coating is shown in Figure 2. Fig 2. Charge mechanism of a metallic particle on a dielectric coating Pseudo-resonance occurs when the particle impacts the outer electrode with a speed and at a phase angle of electric field which is nearly the same as on previous bounce, it most occurs with bounce of small amplitude. When Rc increases, the maximum height reached by the particle and the amplitude of pseudo-resonance increase, which leads to higher possibility of capturing the particle. Meanwhile, the time between the instant of particle liftoff, and the instant of onset of pseudo-resonance, would increase. The coefficient of restitution, R c, depends on the coating and the angle of impact of the wire particle with the 334
coating. It is very difficult to get an accurate value for the coefficient of restitution, because particle movement is random. 3. Results and Discussions The influence of applied voltage on the maximum height reached by the particle for two different values of the coefficient of restitution (neglecting drag force) is shown in Table I. When the coefficient of restitution increases, the maximum height would increase, because when impact with outer enclosure the particle gets higher inverse velocity with higher coefficient of restitution which enables the particle to overcome more gravitational force. Therefore, the particle may eventually cross the gap due to the large height of the bounce. As the applied voltage increases the maximum radial movement also increases as given in Table I. Further calculations may reveal the limiting voltage to enable the particle to reach the high voltage conductor. The values given in the Table II are the maximum flight of particles in radial and axial directions using Monte Carlo Technique. The main reason for the lower movement in Coated Busduct is the lower charge acquired by the particles even at higher fields. The low magnitude of charge acquired by the particle ensures relatively feeble force on the particles. Thus manufactures of GIS often resort to coating of the busduct to prevent particle movement and consequential break down. For particles (Al, Ag and Cu) the nature of movement is similar. However the maximum movement for aluminum particle is higher than that of the copper particle. This is expected since aluminum particle is lighter. Figures 3 to 5 show the movement pattern for applied sinusoidal voltage of 100 kv. Figure 3 shows the movement of the Al, Cu and Silver particles with the variation of voltage. Figures 4 to 27 shows the axial and radial movements of particle with and without Monte Carlo Technique. TABLE I Maximum radial movement of aluminum, copper and silver particles in a 1- phase coated Gas Insulated Busduct Voltage (kv) 75 100 132 145 Type Maximum Radial Movement (mm) RC = 0.8 RC = 0.95 Al 0.11329 0.34051 Cu 0.00494 0.0125 Ag 0.00301 0.00581 Al 0.42659 0.72828 Cu 0.01473 0.09453 Ag 0.01144 0.05773 Al 0.91454 1.28811 Cu 0.09136 0.30877 Ag 0.04483 0.23238 Al 1.12572 1.53034 Cu 0.16511 0.41098 Ag 0.09917 0.31804 335
G.V.Nagesh Kumar et al: Assessing the Risk of Failure in GIS... TABLE II Axial And Radial Movement Of Aluminum, Copper And Silver Particles in A 1- Phase Coated GIB Using Monte Carlo Technique Voltage (kv) 75 100 132 145 Maximum Radial Movement (mm) Type RC = 0.8 RC = 0.95 Axial Radial Axial Radial Al 2.678 0.1132 7.3267 0.3405 Cu 0.1048 0.0049 0.1794 0.0125 Ag 0.0637 0.003 0.1053 0.0058 Al 13.772 0.4266 17.481 0.7282 Cu 0.3696 0.0147 1.4128 0.0945 Ag 0.273 0.0114 0.7386 0.0577 Al 34.346 0.9145 29.867 1.2881 Cu 2.0877 0.0913 6.482 0.3087 Ag 0.9852 0.0448 4.4998 0.2324 Al 41.388 1.1257 34.45 1.5302 Cu 4.1865 0.1651 9.219 0.411 Ag 2.2955 0.0992 6.8015 0.318 Fig 3. Variation of Maximum Radial Movement to Sinusoidal Voltage 336
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5. Conclusion It has been observed that metallic particle contamination is often present in GIS and such contamination adversely affects the insulation integrity. One of the techniques used to reduce the particle movement and improve dielectric performance is to coat the inner surface of the enclosure. Coating the inner side of the enclosure increases the electric field necessary to lift a particle resting on the bottom of a GIS enclosure. If the coating thickness is increased, the charge on the particle is reduced. For GIS of a given voltage class, the busbar voltage and the clearance between busbar and enclosure are generally fixed. Hence, the charge on the particle can be reduced by coating the enclosure inner surface by a suitable dielectric material. In this paper epoxy coating of thickness 100 µm is taken. It is also observed that as the coating thickness increases the movement of the particle decreases. When the coefficient of restitution increases, the maximum height would increase, because when impact with outer enclosure the particle gets higher inverse velocity with higher coefficient of restitution which enables the particle to overcome more gravitational force. The Randomness of the particle is also simulated by Monte Carlo Technique to find the motion in Axial and Radial directions and it is observed that the movement in radial direction is same without or with Monte Carlo Technique. It is also inferred that the axial movement is strongly dependent on the random behavior of the particle. References [1] L. G. Christophorou, J. K. Olthoff, R. J. Van Brunt, SF 6 and the Electric Power Industry, IEEE Electrical Insulation Magazine, DEIS, 1997, pp. 20-24. [2] Hand Book of Switch Gears BHEL, Tata McGraw Hill Publishing, 2005. [3] M. M. Morcos, S. Zhang, K. D. Srivastava and S. M. Gubanski, Dynamics of Metallic Particle Contaminants in GIS with Dielectric-Coated Electrodes, IEEE Trans. on Power Delivery, Vol. 15, No. 2, Apr 2000, pp. 455-460. [4] Arora, A.; Koch, H. Design features of GIS, IEEE Power Engineering Society General Meeting, 2005. Issue, 12-16 June 2005 Page(s): 927-929 Vol. 1 [5] J.R. Lahari, A. H. Qureshi, A review of particle contaminated gas breakdown, IEEE Trans, on Electrical Insulation, Vol, EI 16, NO.5, pp 388-398, 1981. [6] G.V.Nagesh Kumar J.Amarnath, B.P.Singh, K.D.Srivatsava., Influence of Power frequency and Impulse voltages on particle movement in Gas Insulated Bus duct (GIB) with epoxy coatings. at National Seminar on Insulating materials for the Power Industry, organized by Central Power Research Institute, Bangalore during 26-27 August, 2004 [7] N.J.Felici; Forces et charges de petits objects en contact avec une electrode affectee d un champ electrique ; Revue generale de I electricite, pp. 1145-1160, October 1966. [8] G.V.Nagesh Kumar, J.Amarnath, B.P.Singh, K.D.Srivatsava Particle movement in a 245 KV, 300 KV and 400 KV Gas Insulated Substations with and without dielectric coating enclosure National Conference APSC 2004 NIT, Rourkela [9] H. Anis, K.D. Srivastava; Free Conducting particles in Compressed Gas Insulation ; IEEE Trans. on Electrical Insulation, Vol E1-16, pp.327-338, August, 1981. [10] K.S. Prakash, K.D. Srivastava, M.M. Morcos, Movement of Particles in Compressed SF 6 GIS with Dielectric Coated Enclosure, IEEE Trans. DEI, Vol. 4 pp. 344-347, 1997. [11] A.Bargigia, W. Koltunowicz, A. Pigini; Detection of Partial Discharges in Gas Insulated Substations ; IEEE Transactions on Power Delivery, Vol 7, No.3, pp.1239-1249, July 1992. [12] G.V. Nagesh Kumar, J. Amarnath, B.P.Singh and K.D.Srivastava Electric field effect on metallic particle contamination in a common enclosure gas insulated busduct, IEEE Transactions on DEI, April 2007, Volume: 14, Issue: 2 Page(s): 334-340, ISSN: 1070-9878. [13] M.Venu Gopala Rao, G.V.Nagesh Kumar, J.Amarnath, S.Kamakshaiah, and K.D.Srivastava, Movement Patterns of Metallic Particles in a Single Phase Gas Insulated Busduct with Superimposed Lighting and Switching Impulses Journal of High Voltage Engineering, China, December 2008, Pages: 2642-2647, Volume: 34, Issue: 12 [14] G.V. Nagesh Kumar, J. Amarnath, B.P.Singh and K.D.Srivastava Particle Initiated Discharges in Gas Insulated Substations by Random Movement of Particles in Electromagnetic Fields, International Journal of Applied Electromagnetics and Mechanics Volume 29, Number 2 / 2009, Pages 117-129. 343