Influence of humidity on electric strength of CO₂ and C₄-FN/CO₂ gas mixtures

Substantial amounts of moisture can be present in gas insulated systems (GIS) if polymeric insulators are not dried properly or the amount of drying agent is not sufficient. The humidity in insulating gas can influence the insulating performance and can lead to corrosive and toxic decomposition products. Though the addition of desiccant and/or molecular sieves can significantly reduce the side effect of moisture, the influence of gas humidity on the electric strength of CO₂ and C₄-FN/CO₂ gas mixtures is investigated. A pumping device based on a micro annular gear pump and a defined procedure has been developed to inject minuscule quantities of deionized water into a dry GIS compartment. The relative humidity is measured in CO₂ and clean air by a capacitance-type polymer film sensor. The influence of humidity on electric strength of CO₂ and C₄-FN/CO₂ gas mixtures is investigated by breakdown experiments at AC-stress in a weakly non-uniform electrode arrangement. Humidity measurements in humidified clean air at 0.1 MPa and 20 °C shows that only less than 40 % is in the insulating gas. The major part of injected water is adsorbed at inner surfaces of the gas compartment. In addition, it could be demonstrated that humidity measurements by a capacitance type polymer film sensor are influenced by the gas type and the gas pressure. The electric strength and the span of the measured breakdown values at AC-stress in C₄-FN/CO₂ gas mixtures are significantly reduced with increasing amount of injected water. This is attributed to an early provision of initial electrons due to the presence of water molecules.