Characteristics of C3F7CN/CO2 as an alternative to SF6 in HVDC-GIL systems

Abstract

The application of environmentally friendly gases to insulated transmission pipelines could potentially improve the flexibility of transmission line corridors and reduce the environmental impact of such lines. Recently, C 3 F 7 CN has been considered as a potential replacement for SF 6 . The present work assesses the feasibility of applying C 3 F 7 CN to high voltage DC transmission lines to attain a lower global warming potential (GWP). The GWP is estimated through calculation of C 3 F 7 CN mixtures with other gases and based on the weighted average method. The liquefaction temperature of these mixtures is between 0.4 and 1.2 MPa as determined by combining the Peng- Robinson equation of state with the van der Waals mixing rule. The insulating characteristics of C 3 F 7 CN/CO 2 mixtures in conjunction with DC voltage in a uniform electric field was also studied. Based on the present research, the viability of C 3 F 7 CN/CO 2 mixtures was evaluated by examining their heat transfer properties. The results show that, at operating temperatures of -20 and -30 °C, and at a pressure of 0.7 MPa, the maximum allowable mole fractions of C 3 F 7 CN in the mixture to avoid condensation is 7.8 and 5.1%, respectively. The insulation characteristics of 4% C 3 F 7 CN/96% CO 2 and 8% C 3 F 7 CN/92% CO 2 mixtures at 0.7 MPa is equivalent to that of SF 6 gas at 0.5 MPa. The heat-transfer capabilities of 4% C 3 F 7 CN/96% CO 2 and 8% C 3 F 7 CN/92% CO 2 mixtures meet the requirements of the GIL temperature rise standard. It is concluded that C 3 F 7 CN/CO 2 represents an environmentally friendly insulation gas suitable for use in HVDC-GIL equipment.