DC Breakdown Characteristics of C3F7CN Gas Mixtures for Potential Insulation Applications

Abstract

To improve power transmission efficiency to satisfy the increasing demand, alternating current gas insulated line (AC-GIL) technology is used. However, there are three issues associated with this transmission system which are the cost is higher in compare with others transmission systems. Also, AC transmission systems require reactive power compensation by using voltage-ampere reactive (VAR) controls which can be overcome by moving to DC transmission systems. Thus, direct current gas insulated line (DC-GIL) technology is under development which has lower transmission losses than AC-GIL and does not require any reactive power compensation. In addition, currently all designed GIL uses sulphur hexafluoride (SF6) gas as an insulating medium. This gas is widely used in gas insulated equipment due to the excellent insulation and arc quenching properties. However, this gas has a high global warming potential (GWP) that is 23,500 higher than CO2. There is an ongoing investigation to replace this gas with an environmentally friendly alternative. One potential candidate is C3F7CN, developed by 3MTM and commercially known as NovecTM 4710, in its pure form it has doubled the dielectric strength than the SF6 when tested using uniform gaps at atmospheric pressure. Both C3F7CN and SF6 possess similarities in terms of the physical and chemical properties. Also, this new alternative can provide 90% reduction in the GWP. However, due to its high boiling point it is used as a mixture with non-condensable gases to reduce the overall liquefaction temperature. This project aims to investigate the potential of adopting C3F7CN as the main insulating medium in DC-GIL, which could lead to a new generation of environmentally friendly high-power long-distance transmission systems. This research focuses on investigating the DC polarity effect of C3F7CN in compare to SF6 which is a key equipment design consideration. Breakdown characteristics will be experimentally examined under non-uniform field using successive discharge method where the voltage will rise in a rate of 5 kV/s until breakdown. Tests will be conducted in 3 bar absolute pressure and varying gap distance from 3 to 20 mm. Results show, positive breakdown voltage for 20% C3F7CN and 80% CO2 is always higher than SF6 gas. However, in the negative breakdown voltage, both gases started at the same breakdown voltage then, 20% C3F7CN and 80% CO2 gas mixture is lower than SF6 while the gap distance is increasing. Moreover, both gases showed a polarity effect where the negative breakdown voltage is higher the positive one after a certain gap distance between the used electrode.