Intrinsic hetero-polar surface charge phenomenon in environmental friendly C <sub>3</sub> F <sub>7</sub> CN/CO <sub>2</sub> gas mixture
Geng Chen, Youping Tu, Shaocong Wu, Chuanjie Lin, Sichen Qin, Chuanyang Li, Jinliang He
Abstract
Abstract 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 SF 6 and C 3 F 7 CN/CO 2 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 CO 2 gas, followed by 4% C 3 F 7 CN/96% CO 2 gas mixtures. While in the 10% C 3 F 7 CN/90% CO 2 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 SF 6 . 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.