Surface Charging Physics of the Basin Insulator in ±320-kV Gas-Insulated Transmission Lines

Surface charge accumulation is a key factor that must be considered in the design of dc gas-insulated transmission line (GIL) insulators. In this letter, the insulator surface potential distribution of a ±320-kV GIL is measured under different voltage polarities and temperature gradients in the 0.5-MPa SF6. In order to explain the surface potential distribution, the mechanism of surface potential generation is proposed. We found that physical mechanisms, including micro gas ionization from the grounded enclosure, homopolar charge injection from the high-voltage conductor, and bulk polarization of insulator, all contribute to the surface potential distribution. Heteropolar surface potential dominates at both voltage polarities. Besides, the heteropolar potential becomes more pronounced at a temperature gradient of 40 °C. It indicates that free charges generated by gas-side ionization are the primary potential source. Under temperature gradient, the gas ionization is enhanced due to the dramatically increased electric field, from 2.5 to 4.3 kV/mm, on the surface of the grounded enclosure near the insulator. The findings provide importance reference for the design of dc GIL insulator.