Materials Compatibility Study of C₄F₇N/CO₂Gas Mixture for Medium-Voltage Switchgear

Recent studies suggest that C4F7N/CO2 gas mixture is a promising and eco-friendly sulfur hexafluoride (SF6) alternative with a much lower global warming potential (GWP). In gas-insulated apparatuses such as switchgear, besides the insulating gas and the metal parts, nonmetallic materials also play an important role in insulation, load bearing, sealing, environmental control, and so on. Materials compatibility study thus becomes essential to ensure the successful adoption of SF6-free gas-insulated apparatuses. In this study, to carry out the aging process and evaluate the compatibility of each material, special glass cells with in situ dielectric strength testing capability are designed and employed, with significantly improved gas tightness and process variation control. Five typical materials in direct contact with the insulating gas in medium-voltage switchgear, namely zeolite molecular sieves, polycarbonate (PC), perfluoroalkoxy alkane (PFA), polymethyl methacrylate (PMMA), and nitrile O-ring, are selected for long-term thermal aging with the C4F7N /CO2 gas mixture for up to nine months. In addition, the aged gas mixtures and material samples are analyzed by a combination of characterization techniques. Experimental results show that the C4F7N /CO2 gas mixture remains stable during long-term thermal aging and has the potential to replace SF6 as the insulating gas medium. This study also shows that the concerns about material compatibility in the application of alternative insulating gases cannot be ignored as materials studied exhibit different levels of compatibility after aging. While PC and PFA are proved to be well compatible with the C4F7N /CO2 gas mixture, evidence of incompatibility is observed in the cases of zeolite molecular sieves, PMMA, and nitrile O-ring. Care must be taken in choosing more compatible materials after extensive compatibility and aging studies.