Post-arc Transient Characteristics for DC Vacuum Circuit Breaker With Diode Connected in Series

Experimental investigations have been carried out on the rapid high current commutation interruption (CI) process of a $10\mathrm{k}\mathrm{V}$ DC vacuum circuit breaker (VCB). The VCB arc restriking and breaking failure phenomenon appeared frequently in $23\mathrm{k}\mathrm{A}$ CI tests with arc current fall rate of about $300\mathrm{A}/\mu \mathrm{s}$ for a single VCB, while the VCB CI reliability has been improved a lot under the same conditions with diode connected in series. After a detailed analysis of the vacuum gap (VG) post-arc transient characteristics, it was found that it lasted about $450\mu \mathrm{s}$ before the VG finishing the ion sheath development for the single VCB in the post-arc stage. Meanwhile, the VG electric field intensity (EFI) and power density (PD) would quickly rise to the peak values of $12\mathrm{M}\mathrm{V}/\mathrm{m}$ and $45\mathrm{M}\mathrm{W}/\mathrm{m}^{2}$ separately in about $1\mu \mathrm{s}$ after the arc zero-crossing. As a result, the VG electrical or thermal breakdown were very prone to come out when the plasma concentration was still high in the sheath development stage. With the diode in series, the VG power injection path was cut off effectively by the diode reverse blocking, the initial transient recovery voltage rising very fast would be avoid and the plasma concentration was promoted to decay significantly, by which both the EFI and PD were decreased greatly when the VG began to get insulation recovery in the post-arc phase. Under the most extreme condition that the VG started to build dielectric strength at the end of the diode reverse blocking, the simulation results revealed the EFI and PD peak values were substantially reduced to $130\mathrm{k}\mathrm{V}/\mathrm{m}$ and $1.4\mathrm{k}\mathrm{W}/\mathrm{m}^{2}$ respectively and delayed for about $30\mu \mathrm{s}$, which were very beneficial to reduce the VG breakdown probability and improve the dielectric recovery performance in the post-arc transient.