Characteristics of C3F7CN/CO2 as an Alternative to SF6 in HVDC-GIL Systems

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, C3F7CN has been considered as a potential replacement for SF6. The present work assesses the feasibility of applying C3F7CN to high voltage DC transmission lines to attain a lower global warming potential (GWP). The GWP is estimated through calculation of C3F7CN 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 C3F7CN/CO2 mixtures in conjunction with DC voltage in a uniform electric field was also studied. Based on the present research, the viability of C3F7CN/CO2 mixtures was evaluated by examining their heat transfer properties. The results show that, at operating temperatures of -20 and -30 degrees C, and at a pressure of 0.7 MPa, the maximum allowable mole fractions of C3F7CN in the mixture to avoid condensation is 7.8 and 5.1%, respectively. The insulation characteristics of 4% C3F7CN/96% CO2 and 8% C3F7CN/92% CO2 mixtures at 0.7 MPa is equivalent to that of SF6 gas at 0.5 MPa. The heat-transfer capabilities of 4% C3F7CN/96% CO2 and 8% C3F7CN/92% CO2 mixtures meet the requirements of the GIL temperature rise standard. It is concluded that C3F7CN/CO2 represents an environmentally friendly insulation gas suitable for use in HVDC-GIL equipment.