We follow up on our previous publication on C 4 F 7 N and investigate the ion kinetics influencing C 4 F 7 N discharges with a pulsed Townsend experiment. The measured current signals at pressures between 6 and 69 kPa, in the range of density-reduced electric fields E/N from 500 to 1050 Td, show evidence of three anions: one long-lived anion with a detachment rate coefficient in the order of 10 −20 m 3 s −1 , a presumably non-detaching anion, and a short-lived anion with a detachment rate coefficient in the order of 10 −17 m 3 s −1. We obtain the corresponding rate coefficients for ionization, attachment and detachment in C 4 F 7 N. Using these results, we calculate a corrected value of the effective ionization rate coefficient (ν eff /N) * , taking into account detachment from negative ions. It results in a corrected value of the density-reduced critical electric field strength (E/N) * crit = 785 ± 15 Td, much lower than the previously obtained value of (E/N) crit = 975 ± 15 Td, which is valid only when electron detachment is negligible, i.e. for low pressure and small geometric distance. To find which value is most relevant for electrical insulation, we perform breakdown voltage measurements in homogeneous electric fields in C 4 F 7 N at 5 to 65 kPa. We compare the measured density-reduced breakdown field strength (E/N) bd to that calculated using the streamer criterion with the presently obtained rate coefficients, including electron detachment. We find excellent agreement between the measured and calculated (E/N) bd .

The present article gives an overview of research on SF6 replacement gases for electric power equipment, of existing SF6-free technologies, and of worldwide developments in legislation to support these new technologies.

The electric strength in HFO1234ze(E)/SF6 mixtures is investigated with swarm and breakdown experiments. The density-reduced critical electric field as well as the breakdown voltage measured with both techniques, are found to be higher than that of the pure gases in mixtures with more than 10% SF6. The underlying mechanism for the observed positive synergy is investigated and the explanation proposed by Hunter and Christophorou in [1] is discussed for this mixture. The pressure-dependent attachment rate is found to increase with SF6 ratio thus satisfying the main requirement of the proposed mechanism in [1]. It appears nevertheless that due to the fast saturation with pressure and low rates in the mixtures, the three-body attachment processes account only for a small increase in the electric strength. An alternative hypothesis is proposed which considers the strong reduction of electron energies via inelastic processes in HFO1234ze(E), and is qualitatively demonstrated based on measurements and simulations.

We measured mixtures of SF6 and HFO1234ze(E) (C3H2F4) and determined the densityreduced critical electric field strength (E/N)crit in a pulsed Townsend experiment. HFO1234ze(E) is an environmentally friendly alternative to SF6 since contrary to SF6, its global warming potential is negligible. While SF6 is unsurpassed in its efficiency of attaching thermal electrons, HFO1234ze(E) appears to moderate electron energies very efficiently. Electron attachment to HFO1234ze(E), on the other hand, is much weaker. We find a positive synergism in SF6/HFO1234ze(E) mixtures in the sense that the mixture of both compounds has a higher electric field strength than each gas separately. A maximum (E/N)crit of 425 Td is reached for a mixture of 40% SF6 and 60% HFO1234ze(E). A mixture of 90% HFO1234ze(E) and 10% SF6 has a critical electric field strength of 360 Td, similar to pure SF6. We attribute this effect to the high efficiency of HFO1234ze(E) in moderating down the electron energies, which enhances electron attachment to SF6 and reduces electron impact ionization.

The electron swarm parameters of R1225ye(Z) (cis-1, 2, 3, 3, 3-pentafluoroprop-1-ene) and its mixtures with the buffer gases N 2 and CO 2 from 1% to 50% are experimentally investigated using a pulsed Townsend experiment. The analysis of the electron avalanche displacement current yields the effective ionization rate coefficient, the electron drift velocity and the longitudinal electron diffusion coefficient. To assess the potential of the gas as an insulating medium for medium or high voltage applications, the density-reduced critical electric field (E/N) crit is obtained. In pure R1225ye(Z), (E/N) crit increases with increasing pressure from around 280 Td at 2 kPa to 300 Td at 14 kPa. Mixtures of R1225ye(Z) with N 2 and CO 2 show almost no synergy. Finally a comparison to HFO1234ze(E) and l-C 3 F 6 is given.