Influence of the arcing time on the interruption behaviour and current zero conditions of vacuum circuit breakers

Interruption tests with different arcing times were carried out with a commercial vacuum circuit breaker in a synthetic test circuit. At given test conditions (i=35 kA, di/dt=18 kA/ms, dv/dt=8.5 kV/μs, u=55 kV), an optimum of the arcing time of 2.5 ms was found where no interruption failures were detected. Longer arcing times were regarded to be critical under these test conditions as restrikes occurred. Arcing voltage measurements show the occurrence of an constricted arc at critical arcing times. Detailed investigations of post-arc currents and post-arc current charges were carried out to describe the conditions of the gap at current zero. With critical arcing times the post-arc current charge scatters from 15 μAs to more than 100 μAs. The restrike correlates with higher post-arc current charges. By use of a modified “continuous transition model” post-arc currents were modelled to describe the behaviour of the ion density around current zero. The ion density and decay time of the ion density are increased with higher post-arc current charges. With steeper transient recovery voltages, the ability of the gap is reduced to interrupt successfully with higher post-arc current charges. It is demonstrated that thermal overloads of the contact system lead to increased post-arc current charges followed by a restrike