Measurements of discharge parameters in C3F7CN/CO2 and C3F7CN/N2 gas mixtures by SST

C 3 F 7 CN/CO 2 and C 3 F 7 CN/N 2 gas mixtures are regarded as the most promising substitutes to replace SF 6 in high-voltage devices. In this study, a measurement system for steady state Townsend (SST) method was built and the effective ionization coefficient (α), density normalized critical electric field (E/N) and the coefficient of synergistic effect (C) of the two gas mixtures were obtained. The effective ionization coefficients were measured in the variety of E/N between 150 and 1050 Td (1 Td = 10−21 V-m2). The density normalized critical electric fields ((E/N) lim ) of gas mixtures shows nonlinear increase with the increase of C 3 F 7 CN concentration, from 0 to 100%. It was found that the (E/N) Um of C 3 F 7 CN/N 2 gas mixture is higher than that of C 3 F 7 CN/CO 2 at a C 3 F 7 CN mole ratio lower than 60.5%, and the difference between two gas mixtures becomes very small when the ratio of C 3 F 7 CN is higher than 60.5%. The synergistic effect rises sharply and then decreases slowly with increasing ratio of C 3 F 7 CN. When the mole ratio is 6.9% and 6.6%, the coefficients of synergistic effect in C 3 F 7 CN/CO 2 and C 3 F 7 CN/N 2 gas mixtures reach to the highest point respectively. In the range where the ratio is less than 20.5%, the synergistic effect of gas mixture using N 2 as the buffer gas is stronger. This paper will supply the basic data for C 3 F 7 CN/CO 2 and C 3 F 7 CN/N 2 gas mixtures measured by SST.