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 SF ₆ . 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, homo-polar charge injection from the high voltage conductor, and bulk polarization of insulator, all contribute to the surface potential distribution. Hetero-polar surface potential dominates at both voltage polarities. Besides, the hetero-polar potential becomes more pronounced at a temperature gradient of 40°C. It indicates that free charges generated by gas-side ionization is the primary potential source. Under temperature gradient, the gas ionization is enhanced due to the dramatically increased electric field, from 2.5 kV/mm 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.