Effect of Conductor Surface Roughness and Coating on Dielectric Breakdown in Dry Air Gas

The use of SF6 (Sulfur hexafluoride) gas as an insulation medium in GIS (Gas Insulated Switchgear) is becoming limited due to its high GWP(Global Warming Potential). CO2, dry air, and Novec mixed gas have been proposed as alternative gases, and relevant research and product development are underway. Among these, dry air is attracting attention as an alternative gas due to its low GWP and application with high pressure VI (Vacuum Interrupter). In this paper, as part of the basic research to understand the dielectric properties of dry air and to establish insulation design criteria, the change in insulation performance due to conductor surface roughness was evaluated by positive and negative Lightning Impulse tests. Under dry air conditions, there was a significant decrease in the dielectric breakdown voltage due to increased surface roughness. In addition, contrary to the general tendency for the breakdown voltage of the negative polarity to be lower than that of the positive polarity, the breakdown voltage of the positive polarity was lower than that of the negative polarity when the surface roughness was high. This could be compensated by applying a dielectric coating to the conductor. In conclusion, it was found that in dry air it is necessary to apply methods to improve the dielectric performance, including more stringent management of conductor surface roughness than in SF6 gas.