Application of SF6 Alternatives for Retro-filling Existing Equipment

Sulphur hexafluoride (SF6) is a gas which has been widely used in gas insulated switchgear (GIS), lines (GIL) and busbars (GIB) as an interruption and insulation medium since the 1960s. The environmental concerns regarding SF6 usage, the most potent greenhouse gas known, have motivated researchers to find a more environmentally sound alternative as a replacement for SF6. While most of current research focuses on developing state-of-the-art high voltage (HV) equipment specifically designed for alternative gases, this work investigates the feasibility of retro-filling existing SF6-designed equipment. This is a more economic and time-saving solution to phase out the usage of SF6 for the power industry. The first step towards the development of a retro-fill solution is to trial various SF6 alternatives with the specific aim of identifying a suitable mixture combination for use in existing gas insulated assets.
A reduced-scale coaxial prototype was developed to determine a technically feasible gas mixture that can match the breakdown performance of SF6 under standard lightning impulse voltage waveform (1.2/50 μs). Breakdown results demonstrate that a 20% C3F7CN and 80% CO2 gas mixture exhibits comparable insulation capability to SF6 in coaxial geometries with similar field uniformity as found in GIL/GIB. A full-scale SF6-designed 420/550 kV rated gas insulated demonstrator was assembled and retro-filled with a 20% C3F7CN and 80% CO2 gas mixture. The demonstrator was type tested under Switching Impulse (SI), Lightning Impulse (LI) and Power Frequency (AC) voltages in accordance with IEC 62271-204. Type test results have shown that the gas mixture can pass the withstand voltage tests at the specified voltage levels for SF6 gas under similar operating conditions. This work has demonstrated that a mixture of 20% C3F7CN and 80% CO2 could be a viable candidate for replacing SF6 in HV insulation applications without the need for changing the operating conditions (i.e. rated pressure) of the equipment. This is an encouraging step towards the potential phase out of SF6 in passive components installed across the transmission and distribution networks in the UK.