Cross-equipment study on charging phenomena of solid insulators in high voltage equipment

This paper reports on a new cross-equipment study on the charging phenomena of solid insulators, evaluating the influence of charging on high-voltage equipment such as gas insulated switchgear (GIS), oil-immersed transformers, gas-insulated transformers and high-voltage vacuum-insulated equipment. New parameters of normalized charge density and surface resistance are introduced to consistently explain all the characteristics of the surface charge density on solid insulators and the decay time constant of the accumulated charges between the equipment. The results show that the surface charge density and the decay constant decrease as the surface resistance of the insulators is lowered. Combinations of insulating media of solid insulators and fluids (gas or liquids) are discussed for GIS and oil-immersed transformers in terms of the relation between the relative permittivity and the volume resistivity. The decay time constant of the accumulated charges is generally dominated by the insulating media of a fluid like oil which has lower volume resistivity than solid insulators in cases like transformers. However, the time constant is determined by the resistivity of the solid insulator rather than the fluid (gas) for GIS. Furthermore, it is suggested that low resistivity coatings on solid insulators are effective in suppressing the charge accumulation on the insulators. These new cross-equipment studies on the charging phenomena are an effective approach to reliable insulation design especially when downsizing high-voltage equipment.