Investigation on dielectric recovery strength for the DC-VCB with multi-breaks
For the 126kV DC vacuum circuit breakers (VCBs) with multi-breaks, the continuous transition model, metal vapor and ion density are introduced to describe the dynamic mathematical model of dielectric recovery with intermediate frequency vacuum arcs. During the breaking process, the numerical simulation is employed to obtain the thickness of the sheath and the curves of the dielectric recovery strength. Based on the numerical simulation, the relationship between the dynamic variation of the sheath and dielectric recovery is obtained. And the statistics can help to analyze the current, transient recovery voltage, electric field strength and dielectric recovery characteristics on the new cathode surface. By comparative analysis, the distribution of transient recovery voltage (TRV) and the factors affecting the dielectric recovery strength can be achieved. The simulation results show that: due to the line impedance between the breaks, the voltage and arc energy distribution of each break is not uniform, and the voltage and arc energy distribution on the break are higher than that of the other breaks, the metal vapor has a high initial density, the drop rate of the post-arc current is larger, the rate of TRV is higher and the development speed of sheath is slower. The surface electric field strength on the new cathode is higher. It could initiate re-strike and approach to breaking failure for the whole CB. And larger capacitor is preferred to ensure the reasonable distribution of TRV and improve the breaking probability of DC VCBs with multi-breaks.