Christian Ruempler

Chemical equilibrium composition of air-copper-PA66 mixtures and their effects on dielectric breakdown for low-voltage circuit breaker post-current-zero

Summary form only given. Metallic vapor emanating from the electrodes and plastic vapor from wall ablation on or before current-zero in a low-voltage circuit breaker (LVCB) significantly affect the dielectric recovery characteristics of atmospheric pressure air in the contact gap after current-zero. When the net ionization coefficient becomes positive, dielectric breakdown is said to occur and the reduced electric field (E/N) of this occurrence is termed the critical reduced electric field ((E/N)crit) [1]. In this paper, we analyze the dielectric breakdown behavior for the case of copper being the electrode material and polyamide 6/6 (PA-66) being the plastic wall material. Firstly, the chemical composition is calculated by the minimization of Gibbs free energy and the results are compared for two different methodologies (denoted as M1 [2] and M2 [3], for convenience). Unlike M2, M1 includes the condensed species of copper and carbon (graphite) and it will be shown that below 3500 K, the two methods provide widely different composition results. Secondly, Boltzmanns EEDF equation is solved to obtain the generalized non-Maxwellian electron-energy distribution function (EEDF) [4], with the electron-impact collision cross-sections gathered from literature as input. Using the afore-mentioned inputs, (E/N)crit is calculated and plotted against temperature ranging between 300-6000 K, for different mass-fraction values of air, copper and PA-66. Additionally, it has been observed that the presence of vibrationally- and electronically-excited species enhances the dielectric breakdown by lowering (E/N)crit. This approach is part of an initial attempt towards addressing realistic chemical non-equilibrium conditions involving finite-rate kinetics in an LVCB after current-zero and the numerical results will subsequently be utilized for comparisons with available experimental data.

The Influence of Metallic and Plastic Vapors on Dielectric Breakdown for Low-Voltage Circuit Breaker after Current-Zero

The effects of metallic and plastic vapors on dielectric breakdown strength at atmospheric pressure after current-zero for low-voltage circuit breaker (LVCB) applications have been investigated. Dielectric breakdown is characterized by a positive value of net ionization coefficient, which is the difference between ionization and attachment coefficients. In this paper, preliminary results are presented for the case where the metallic electrode is composed of copper while the ablating plastic wall material is polyamide 6/6 (PA-66). The effects of vibrationally-excited diatomic species and electronically-excited monatomic species on breakdown strength are also presented. The excited species are observed to lower the breakdown strength. It is expected that the current approach will provide quantitative comparisons between different metallic and plastic vapor combinations observed in realistic LVCBs and the numerical results will subsequently be utilized for comparisons with available experimental data. This approach is the first step towards addressing realistic nonequilibrium conditions prevailing in an LVCB after current-zero and the results will be a valuable tool for suggesting improvements in dielectric interruption ability of LVCBs.