Breakdown performance of vacuum circuit breakers using alternative CF3I-CO2 insulation gas mixture

Sulphur hexafluoride (SF₆) insulated vacuum interrupter circuit breakers have been used to explore replacing SF₆ gas with a trifluoroiodomethane and carbon dioxide (CF₃I-CO₂) gas mixture. The search for an alternative insulation gas is driven by the well known extreme global warming potential of SF₆. For this purpose, the circuit breaker gas compartment of a piece of gas insulated switchgear (GIS) was filled with a CF₃I gas mixture and then tested using lightning impulses up to the rated withstand strength. The tested ring main unit was initially designed to be insulated with SF₆ gas. The unit is a three-phase switchgear containing two switches per phase; a selector interlock and a vacuum bottle circuit breaker per phase. The test programme performed in this investigation demonstrated the breakdown performance and insulation strength of the new gas mixture as well as the vacuum circuit breakers behavior when insulated with a new insulation medium. Data on the dielectric properties of the proposed gas mixture is presented, and the performance of the tested vacuum circuit breaker is discussed. Promising results are obtained which indicate the suitability of this more-environmentally friendly gas for high voltage insulation purposes.