Intrinsic hetero-polar surface charge phenomenon in environmental friendly C3F7CN/CO2 gas mixture

The hetero-polar charges on the spacer surface due to the gas ionization has been verified as one of the hazard charge sources threatening the safe operation of HVDC gas-insulated transmission line (GIL). This letter reports the differences in surface charging properties of spacers in SF6 and C3F7CN/CO2 gas mixtures, and a rigorous re-examination of environmental firendly gas assessment for DC GIL is introduced. The results show that when the maximum value of electrical field strength at the spacer surface is in 0.45 kV mm(-1), homo-polar surface potential in different insulating gases indicates a dominant charge originating from bulk injection. As the maximum value of electrical field strength at the spacer surface reaches 0.68 kV mm(-1), local hetero-polar surface potential areas are detected and the area ratio of these regions is higher in CO2 gas, followed by 4% C3F7CN/96% CO2 gas mixtures. While in the 10% C3F7CN/90% CO2 gas mixtures, homo-polar potential regions are still dominant with very low area ratio of hetero-polar potential regions, which is close to that measured in SF6. An indicating factor is put forward to further clarify the key role of hetero-polar charges plays in potentially triggering surface flashover. This letter provides an important basis for determining contents and components of insulating gases, which serves as one of the key issues in developing HVDC GIL.