Investigation of the dielectric recovery in synthetic air in a high voltage circuit breaker

Abstract

The dielectric recovery of an axially blown arc in an experimental set-up based on a conventional HV circuit breaker was investigated both experimentally and theoretically. As a quenching gas, synthetic air was used. The investigated time range was from 10 µs to 10 ms after current zero (CZ). A fast rise in the dielectric strength during the first 100 µs, followed by a plateau and further rise later was observed. The dependences on the breaking current and pressure were determined. The measured dielectric recovery during the first 100 µs after CZ could be reproduced with good accuracy by computational fluid dynamics simulations. From that it could be deduced that the temperature decay in the axis does not depend sensitively on the pressure. The dielectric recovery during the first 100 µs scales therefore mainly with the filling pressure. The plateau in the breakdown characteristic is due to a hot vapour layer from the still evaporating PTFE nozzle surface.

Calculation of the Effective Ionization Rate in Air by Considering Electron Detachment From Negative Ions

“…These equations are not coupled, so the solution for the number density of electrons can be expressed in exponential form (2) with the effective ionization coefficient (3). In the present work, an approach is developed for calculating electron swarm parameters using the solver BOLSIG+, coupled with the package Cantera for calculating the gas composition at chemical equilibrium.…”

“…For instance, it is known that detachment in air can be important at temperatures above 2'000 K due to the presence of atomic oxygen in the gas [3], [4]. On the other hand, it was shown in [5] that another mechanism of detachment from negative ions and eventual negative ion conversion has also significant impact, especially at elevated electric fields.…”

“…Although these detachment mechanisms are fairly well known, they are rarely taken into account for determination of electron swarm parameters. One of the method used, for example, in [3] describes approximately the 3 rate of multiplication of electrons as…”

Ablation-assisted current interruption in medium voltage switchgear – Development and prospect

“…The experiments show that the peak value of the recovery voltage is the most important factor for the outcome of the interruption, and that the rate of rise of recovery voltage is of less importance. This makes sense when considering the findings by Seeger et al [17] and Kurz et al [18] that the withstand voltage increases rapidly to a certain level, then stays at that level for some time before rising again. In [20], no interruptions were successful when the source voltage was raised to 13,9 kV.…”

“…The influence of nozzle ablation on the dielectric properties after current zero is an important topic in high voltage circuit breaker design, even though ablation is not used actively for arc quenching in these breakers. Seeger et al [17] and Kurz et al [18] argue that, in a gas blast circuit breaker, the ablation gas inside the nozzle significantly inhibits the dielectric recovery. Initially, they observe a fast increase in dielectric strength for some 100 µs after current zero, after that the withstand strength stay at a stable level for about 1 ms before it starts to increase again.…”