A study of charge accumulation and spacer flashover in compressed gas insulation

Compressed gas insulated substation (GIS) and transmission line (GITL) equipment have been developed rapidly throughout the world during the past decades. Compactness is the main advantage of GIS and GITL over conventional air-insulated substations and transmission lines. Sulphur hexafluoride (SF₆) is used as an insulation gas in GIS and GITL because of its excellent insulating properties. Supporting spacers are identified as the most likely places for flashover to occur and they often determine the overall strength of a system. For energized system, surface charges have been observed on spacer surfaces and are considered to play an important role for anomalous flashover of a GIS or GITL system. The purpose of this research is to study the mechanisms and factors governing the magnitude and distribution of surface charges and their influence on flashover voltage. In this investigation, experiments to study surface charge accumulation under different experimental conditions have been conducted, with a rod-spacer-plane electrode system. The parameters varied are applied voltage levels, insulating gases, gas pressures, spacer materials, rod electrode diameters, and the duration of applied voltages. Experiments with impulse voltage pre-charging were also conducted. It was found that the mechanisms of surface charging are corona, gas conduction, and photoionization. Surface charge magnitude and distribution are strongly field dependent and are related to the duration of voltage application. Spacers in SF₆ gas accumulate less charges on the surface than in air and nitrogen , for a given geometry, spacer material, voltage, and gas pressure. In order to determine the effect of surface charges on overall electric field, a surface charge simulation program (the SSM program) was developed to calculate the overall electric field when there are surface charges on a spacer surface. The calculation results show that surface charges significantly distort the overall electric field magnitude, field direction, and distributions, which may explain the anomalous flashover in GIS/GITL systems. It was found that the flashover propagation field on a PTFE spacer surface may be between 6 kV/cm and 10 kV/cm. High speed photographic observations, with an image intensifier, are suggested to examine the propagation of a flashover. It may, therefore, be possible that, a reasonable model to predict flashover in SF₆ gas can be built by comparing the flashover propagation model with the electric field pattern on a spacer surface. The role of photoionization near the spacer surface in charge accumulation should also be studied in greater detail.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat