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Due to the greenhouse effect of SF6, the research on alternative insulation gases is currently a hot topic in high voltage engineering. The refrigerant gas such as HFC-227ea with low GWP has stable chemical properties, and high dielectric strength because of a large number of fluorine atoms. However, it is necessary to add buffer gases for relatively high liquefaction temperature(-16.7°C) of HFC-227ea. In this article, the dielectric strengths of HFC-227ea and its gas mixtures under different gas pressure, mixture composition and ratio are investigated under DC and AC electric fields. At atmospheric pressure, the results show that the breakdown voltage of HFC-227ea mixed with N2 or CO2 could only account for 60% of SF6 gas. When HFC-227ea is mixed with HFC-125, which has similar chemical composition and structure, their dielectric strength can reach at least 80% of SF6 gas and the gas mixture shows good synergistic effect. At higher gas pressure, the HFC-227ea/HFC-125 gas mixture at pressure of 0.8 MPa and low HFC-227ea content has the best insulating performance and it is suitable for a lower operating temperature, demonstrating the potential of substituting SF6 for gas insulated electrical equipment.