Application Of Alternative Gases In Retro-Filling SF6 Insulated High Voltage Equipment

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Sulphur hexafluoride (SF6) gas has been widely used in high voltage equipment e.g. Gas Insulated Switchgear (GIS) due to its excellent dielectric strength and arc quenching property. However it is one of the six gases included in the Kyoto Protocol, with a high global warming potential. Although SF6 is sealed in the high voltage equipment, utilities including National Grid are often penalised due to its leakage. Environmentally friendly alternative gases to replace SF6 in high voltage equipment have been proposed in recent years. Most of current research focuses on developing new high voltage equipment filled with those alternative gases. The aim of this research project is, however, to investigate the feasibility of retro-filling the existing SF6 equipment owned by National Grid with alternative gases. One of the promising candidates chosen for the project is Fluoronitrile based NOVEC gas. A test system which can replicate typical fields found in 400 kV SF6 filled equipment will be established. The dielectric strength of the alternative gas or gas mixture with SF6 under AC, lightning impulse (LI) and switching impulse (SI) voltages will be characterised, whereas SF6 only will be tested as benchmark. Both conductor to wall bulk gas insulation and spacer surface insulation will be considered. Different percentages of gas mixture under a range of pressures will be studied. Effect of particle contamination and moisture ingression on the performance of the alternative gas/gas mixture will also be studied. Partial discharge measurement and spectrum analysis of gas components after discharge or breakdown will be carried out. A long term withstand test will finally be deployed to verify the long-term performance of the proposed solution.