The Conduction Current Characteristics of C4F7N/CO2 Gas Mixture With Different Influencing Factors

The accumulation of charge on insulator surface is a key problem that restricts the development of high voltage direct current (HVDC) gas-insulated transmission lines (GILs) and gas-insulated switchgears (GIS), the ionization of insulating gas is an important source of hetero-polar charges accumulated on insulator surface. The value of gas conduction current is one of the key factors that determine the surface charge accumulation characteristics of insulators. In this article, the conduction current characteristic of C4F7N/CO2 gas mixture is experimentally studied with a coaxial electrode system like GIL structure, and the source of gas ions under different electric fields is analyzed. The results show that the average ion pair generation rate caused by natural radiation ionization of C4F7N/CO2 gas mixture during saturation current regime increases with the increasing C4F7N content, and the growth rate of the average ion pair generation rate of gas mixture decreases with the increasing C4F7N content. The increase of gas pressure shows a significant positive correlation with the average ion pair generation rate, but the growth rate decreases gradually. The critical electric field when the C4F7N conduction current increases exponentially is significantly higher than that of SF6 and CO2, and the critical electric field of C4F7N/CO2 gas mixture increases with the increasing C4F7N content. In addition, with an increase in the gas pressure, the higher C4F7N content in C4F7N/CO2 gas mixture is, the more obvious the inhibition effect on gas ionization is within the range of the electric field in surge regime. The critical electric field with exponential growth of C4F7N/CO2, CO2, and SF6 conduction current decreases with the increase of temperature, and the current in surge regime increases rapidly with the increasing electric field. The results of this article provide a reference for the theoretical study of surface charge accumulation at the solid-gas interface.