On the Particle-Contaminated GIS/GITL Systems with Dielectric Coated Electrodes S. Zhang, M.M. Morcos, S.M. Gubanski, K.D. Srivastava Author Affiliations: Kansas State University, Manhattan, KS; Chalmers University of Technology, Gothenburg, Sweden; The Uni- versity of British Columbia, Vancouver, BC Canada. Abstract: Conducting particles in transmission and switching equipment insulated by compressed sulphur hexafluoride (SF 6 ) can re- sult in loss of as much as 90% of the gas dielectric strength. Particles in practical systems can exist in a wide variety of shapes and sizes, and of materials of different densities. In this letter, the effect of particle size on the probability of SF 6 breakdown is discussed. Theoretical and ex- perimental results are presented. Keywords: Compressed gas insulated systems, dielectric coated electrodes, particle movement, breakdown probability. Introduction: The development of compressed gas insulated switchgear (GIS) and compressed gas insulated transmission line (GITL) equipment has progressed rapidly [1]. The electrical insulation performance of GIS/GITL systems is adversely affected by metallic particle contamination [2], however. Accumulated field experience in- dicates that sources for such contamination are mechanical abrasions, movement of conductors under load cycling, and vibration during ship- ment and service. 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 inci- dence when approaching the coaxial conductors. The coefficient of res- titution is the ratio of incident and rebound velocities and depends on conductor surface roughness. Conductors in a GIS/GITL system may be coated with a dielectric material to restore some of the dielectric strength of the compressed gas, which is lost due to surface roughness and contamination by conducting particles. The improvement in the dielectric strength of the system due to coating can be attributed to several effects. Coating reduces the degree of surface roughness on conductors. Also, the high resistance of the coating impedes the development of predischarges in the gas, thus increasing the breakdown voltage. The electric field necessary to lift a particle resting on the inside surface of a GIS enclosure is much increased due to the coating. Once a particle begins to move in the gas gap under the applied voltage, it may collide with either conductor. With coated conductors the particle will acquire a drastically reduced charge, thus the risk of break- down initiated by a discharge is reduced significantly. Coating thickness has been varied from a few microns to several millimeters and the influ- ence of coated electrodes on the insulation performance has been studied under dc, 60 Hz ac, and lightning impulse voltages. Particle Charging: The purpose of the coating is to decrease the net charge on the particle, thus making the particle less influenced by the electric field. Charging a metallic particle on the surface of a dielectric coating is attributed to two different charging mechanisms: conduction through the dielectric coating and microdischarges between the particle and the coating. Moving particles can also acquire net charge by contact- ing an already charged dielectric surface if the surface resistivity of the material is high enough to enable charges to be trapped on the surface. Particle Motion: A metallic particle resting on a coated enclosure surface may acquire free charge through several physical processes such as charging from an already charged dielectric surface, conduction through the coating, and through partial discharge (PD) between the particle and coating. There is experimental evidence to suggest that the PD mechanism dominates at lower gas pressures and thin coatings. The electric field necessary to lift a particle resting on the bottom of a GIS enclosure is much increased due to coating. Once a particle begins to move in the gas gap under the applied voltage, it may collide with either conductor. If the conductors are coated, the particle will acquire a smaller charge, thus the risk of a breakdown initiated by a discharge is reduced significantly. In general, if the coating thickness is increased the electric field near the particle surface is reduced, and consequently, a higher applied field will be necessary to lift the particle. To study the influence of coating thickness on the magnitude of the electric field along the surface of a wire particle, finite element calculations were performed for a standing particle. A vertical wire particle was chosen since it is possible to fully account for its shape in the finite elements program. Figure 1 shows the lift-off voltage and the corresponding calculated lift-off field at the enclosure surface for an aluminum wire particle (0.27 mm diameter × 6 mm long) with the coated enclosure. Continuous lines show lift-off values. The mean values were calculated using approxi- mately 40 individual lift-off values. No significant pressure dependence of lift-off voltage/field was found. This fact suggests that the microdischarge mechanism (between 0.3 and 0.6 MPa) is not suitable for estimating the charge on the particle as it leaves the coated surface, since the microdischarge mechanism is strongly pressure dependent [3]. Effect of Particle Size: The simulation of the movement was stud- ied under different ac voltage levels for coated and uncoated cases. In the presence of particles, if the GIS is energized below a certain voltage level for a sufficiently long time, the particle position may assume defi- nite nonuniform distribution values. The principle of calculating the SF 6 breakdown probability has been reported elsewhere [4]. If the volt- age is applied for an unspecified but finite period of time, the risk of ex- ceeding the breakdown threshold given by the breakdown voltage profile, hence causing breakdown, is determined by P f x dx x x = ∫ ( ) 1 2 52 0272-1724/01/$10.00©2001 IEEE IEEE Power Engineering Review, August 2001 Figure 1. Lift-off voltage/field as function of gas pressure-coating thickness 62 μm [3] Figure 2. Breakdown probability as function of particle length