Predominant reaction products and dielectric breakdown properties of gas mixtures consisting of SF₆ and ablation products of C₂F₄/BN in the temperature range of 300–3000 K

Abstract

For a high voltage SF6 circuit breaker equipped with a boron nitride (BN)-filled polytetrafluoroethylene ((C2F4) n ) nozzle, the predominant reaction products in a high-temperature SF6 mixture with vapor decomposition of C2F4 and BN at temperatures of 300–3000 K are determined based on calculations of equilibrium composition. The calculation is performed by considering the molecular products obtained by chemical reactions with boron atoms (e.g. BN, BS, BC, BF, BF2 and BF3), nitrogen atoms (e.g. N2, N3, CN, NS, NF, NF2 and NF3) and carbon atoms (e.g. CS, CS2, C2F4, CF2, CF3 and CF4). The results obtained for the C2F4–BN molar fraction X C F B N of 20%–80% reveal that BF3, N2 and CF4, the molecules originating from the nozzle material, are produced at high molar fractions over a range of 300–3000 K. Such an evaluation of the gas composition allows the derivation of a reduced collision ionization coefficient α/N and a reduced electron attachment coefficient η/N of the gas, considering various electron impact processes such as BF3 + e → BF2 + F− and CF4 + e → CF3 + F−. The evaluation subsequently enables us to determine a critical reduced electric field strength E c r / N causing ( α − η ) / N = 0 . The results suggest that the decomposed C2F4–BN mixture at X C F B N of 80% causes the reduction of E c r / N to 110 Td in the range of 1000–2000 K at a gas pressure of 1.0 MPa. This strength E c r / N is 20 Td lower than that determined separately for the SF6 gas mixture with only 80% C2F4 vapor decomposition, because of the CF4 molar fraction reduction in the SF6/C2F4–BN mixture.

High-temperature properties of dry air used as arc quenching medium and its mixture with vaporized C2F4 : reaction products and critical reduced electric field strength at 300–3000 K

“…(a) Component chemical species of the arc-quenching gas: N 2 and O 2 . (b) Molecules, atoms and ions that originate from N 2 and O By applying a chemical composition calculation method [9,16] to the mixture containing N 2 of molar number X N2 [mol], O 2 of molar number X O2 [mol] and C 2 F 4 of molar number X C2F4 [mol] as an initial state, the following equations expressed in terms of molar number x j [mol] of the chemical species j and gas volume V g [m 3 ] were formulated under specified conditions of gas pressure P and temperature T: (i) four conserved elemental equations for N, O, C and F; (ii) the equation of state; (iii) a charge neutrality equation; and (iv) 86 equations representing chemical reactions under chemical equilibrium. For instance, the conservation of elementary O is expressed as…”

“…for a gas comprising different gaseous species j of molar fraction y j [9,17]. Herein, N denotes the total number density of all gaseous component species, E represents the electric field strength applied to the gas, µ e denotes electron mobility, ε denotes electron energy, σ j, i presents the collision ionization cross section for an individual component species j, and F 0 (ε) denotes an electron energy distribution function.…”

“…The aforementioned high-temperature gas comprises various reaction products formed through arc extinction processes. The gas shows dielectric breakdown properties such as (i) a reduced collision ionization coefficient α/N of the gas as a function of a reduced electric field strength E/N [2] (E: an electric field strength applied to the gas, N: the total number density of all gaseous component species), (ii) a reduced electron attachment coefficient η/N as a function of E/N, and (iii) a critical reduced electric field strength E cr /N that results in (α − η)/N = 0 [3][4][5][6][7][8][9][10].…”

“…Several studies have calculated equilibrium compositions to investigate reaction products considering chemical species dissociated from SF 6 , such as SF 4 , SF 2 and F [3][4][5][6][7][8]. Other research has revealed the equilibrium compositions of a high-temperature gas mixture consisting of SF 6 and C 2 F 4 vapor decomposition, considering that a nozzle made of polymer PTFE (chemical formula C 2 F 4 ) is installed in the arc-quenching chamber [9][10][11]. Additionally, the studies determined dielectric properties at 300-3000 K based on the derived chemical composition [9][10][11].…”

Decomposition of SF6 in the plasma medium of an electron beam

“…Sulfur fluoride is a stable chemical compound which possesses a very high electric strength [1]. This fact allows using it in the high-voltage energy setups and thermal nuclear reactors as isolating gas.…”