Numerical modeling of post current-zero dielectric breakdown in a low voltage circuit breaker

Abstract

The gas mixture in the contact gap of a low-voltage circuit breaker post current-zero is characterized by a high degree of non-equilibrium. This partially-ionized mixture, which includes air, metal vapor and plastic vapor, is subjected to a transient recovery voltage post current-zero. This gas mixture, however, does not retain its dielectric capability instantaneously and as a consequence, is susceptible to dielectric breakdown post current-zero. Dielectric breakdown strength is characterized by a positive value of net ionization coefficient, which is the difference between ionization and attachment coefficients. A significant effort is directed in this work towards addressing effects of kinetic and chemical non-equilibrium on net ionization coefficient including the effects of a non-Maxwellian distribution of electron energies. The generalized distribution function is obtained from a Boltzmann equation solver. The effects of metallic and plastic vapors on dielectric breakdown strength at atmospheric pressure after current-zero for low-voltage circuit breaker applications have been investigated in this work. 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. The overall objective of this proposed research is analyzing the important factors contributing to the dielectric breakdown in a low-voltage circuit breaker post current-zero, which is a complex electro-hydrodynamic problem. Working towards this objective, a simplified and generalized framework is developed to predict the possibility of dielectric breakdown in a realistic low-voltage circuit breaker. It is expected that the current approach will provide quantitative comparisons between different metallic and plastic vapor combinations observed in realistic circuit breakers. The numerical results have been observed to predict reasonably the possibility of breakdown with available experimental data, particularly for air and metal vapor mixtures. This approach is the first step towards addressing realistic non-equilibrium 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.