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Vacuum circuit-breakers (VCBs) are commonly applied in mechanical type direct current circuit-breakers (DCCBs). A high frequency (HF) current is injected to help create a forced current zero (CZ) and facilitates DC interruption in these DCCBs. This work is focused on investigation of the forced-current-zero vacuum arc and the following post-arc current.
A circuit capable of producing high di/dt and imposing the HF current in two opposite ways onto the breaking current by VCB is first constructed, providing better completeness in terms of interruption duties for investigation.
Study of post-arc current indicates that the peak post-arc current can reach higher than 100 A when di/dt reaches hundreds of A/μs and rate of rise of initial transient interruption voltage (ITIV) reaches kV/μs. However, it lasts only about 1 μs. It is summarized from groups of experiments that post-arc current is only influenced by a memory time no longer than 4.5 μs before the forced CZ. The residual plasma density only depends on current in this short interval. The memory time actually reflects how fast charged particles are escaping out of the gap. Analysis of experiments with different gap distances and opposite polarities of the breaking current suggests that the high post-arc current after forced CZ is nothing more than a rapid movement of residual charges. It has no impact on interruption performance. Besides, failure modes are classified according to features of post-arc current and ITIV. The impact of breaking currents, peak arcing current and distance of gap on post-arc re-ignition or restrike has been further investigated.
This research improved the understanding of electrical properties around the forced CZ. It is possible to enhance DC interruption performance where VCBs are applied.
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