The effect of electrode surface roughness on the breakdown characteristics of C3F7CN/CO2 gas mixtures

Abstract

C3F7CN/CO2 gas mixtures have been widely studied in recent years. And they are considered a potential alternative for replacing SF6 in high voltage power equipment. In this paper, the DC breakdown characteristics of 6%C3F7CN/94%CO2 mixtures in a uniform electric field were studied at electrode surface roughness levels of 0.4 μm, 1.2 μm, 2.1 μm and 3.8 μm. At the same time, in order to provide reference for the power engineering, the gas pressure was set between 0.3 MPa and 0.7 MPa. Meanwhile, the results at 0.4 MPa gas pressure were compared with it of SF6. The growth rates of positive and negative DC breakdown field strengths of C3F7CN/CO2 mixtures decreased gradually with the increasing of gas pressure, and the decreasing trend of growth was more obvious with the increase of electrode surface roughness. At higher levels of surface roughness, the polarity effect of the C3F7CN/CO2 mixture was more significant. At 0.4 MPa gas pressure, as the surface roughness increasing from 0.4 μm to 3.8 μm, the decline degree of positive and negative DC breakdown field strengths of C3F7CN/CO2 mixtures were significantly lower than that of SF6 gas between 0.4 μm and 3.8 μm. The critical breakdown field strength, (E/N)crit, and coefficient, β, of 6%C3F7CN/94%CO2 mixture and SF6 were obtained by Steady State Townsend method(SST), and (Nh)crit values were calculated to characterize the sensitivity of these gases to surface roughness. The (Nh)crit value of the 6%C3F7CN/94%CO2 mixture was higher than SF6. It showed that the 6%C3F7CN/94%CO2 was less sensitive to roughness than that of SF6.