Controlled switching technology operates circuit breakers at the minimum arcing time for short-circuit current interruption. However, there is no report to reveal a relationship between the short-circuit current interrupting capability with the minimum arcing time and the opening velocities of vacuum interrupters. The objective of this paper is to reveal a relationship between the short-circuit current interrupting capability with the minimum arcing time and the opening velocities of vacuum interrupters. A 40.5 kV fast vacuum circuit breaker prototype was built up which was equipped with a commercial 40.5 kV-31.5 kA vacuum interrupter and it was driven by an electromagnetic repulsion mechanism. It provided an average velocities ranging from 2.5 to 5.5 m/s. 50 times of 40 kA short-circuit current interrupting tests were carried out in a synthetic circuit. Test results showed that the minimum arcing time of successful interruption decreased with the increasing of the average opening velocity. The minimum arcing gap was determined as 1.8 mm and it kept as a constant as the average opening velocity increased. The experiment results also showed a relationship between the arc energy density and the arc extinguishing gap, which acted a linear boundary between the successful and failure short-current interruptions.

Partial breakdown (PB) has been reported in breakdown of vacuum gaps in series but has not been fully understood. The objective of this article is to investigate the influence of PB on breakdown of vacuum gaps in series under lightening impulse voltages. Two commercial vacuum interrupters (VIs) of the same model were connected in series with various gap distance arrangements. Experimental results show that four breakdown types could be classified, i.e., repeatedly PB without final full breakdown of the series gaps (type I), two PBs before final full breakdown (type II), one PB before final full breakdown (type III), and no PB before final full breakdown (type IV). Breakdown voltage of the vacuum gaps in series decreases with the increasing difference between the two gaps and could be even lower than breakdown voltage of the larger gap alone. This is explained by analyzing the three stages of transients induced by a PB. A model was established to simulate transients induced by PB of the vacuum gaps in series. The simulated results could match the experimental results well. An evolution process from a PB to the four breakdown types was deduced. The influence of PB on insulation coordination and breakdown voltage of the vacuum gaps in series was analyzed according to the abovementioned analysis.

The performance of vacuum circuit breakers (VCBs) in the interruption and protection of an electric power system is highly dependent on the contact conditions; thus, the assessment of the contact conditions is essential. The field emission current has been recognized as an effective parameter with which to investigate the contact conditions both qualitatively and quantitatively. The traditional measurement method that uses the field emission current to determine the contact conditions is performed under ac voltage and requires the VCB to be dismantled, which limits the application of this method. The objective of this study is to develop a new measurement method for the contact conditions in a VCB with the field emission current during the closing operation under dc voltage. The new measurement method does not require the VCB to be dismantled and can simplify the processing of experimental data. Then, the new measurement method is applied to investigate the contact conditions during capacitive switching. The obtained result shows that the field enhancement factor $\beta $ of the contact surface oscillates up and down during the capacitive experiment, which can verify the conditioning and deconditioning effects of the inrush current on contact surfaces.

A quick elimination of short-circuit faults in a short time can significantly promote the transient stability of an AC power grid. This paper proposes controlled fast vacuum breaking (CFVB) of a short-circuit current within a short-arcing time. The CFVB technology performs the following: determining a lower-limit opening speed of 3.5 m/s for a 40.5 kV fast vacuum circuit breaker (FVCB), quickly predicting the fault current-zero within 4.0 ± 0.4 ms to command the FVCB to open at a selected current phase-angle, and optimizing an electromagnetic repulsion actuator with an opening time of 2.5 ± 0.05 ms to achieve an accurate opening operation. The CFVB technology was deployed in a prototype of single-phase multiple-break 363 kV FVCB to validate its feasibility. The validation involved test duties of T100s(b) and T100a following standards IEC 62271-100 2017 and GB/T 1984-2014. Test results demonstrated that by adopting the CFVB technology, the 363 kV FVCB can successfully interrupt a fault current of 80 kA rms within the first half-wave fault current with a short-arcing time of 2.5 ms. Further application of the CFVB technology could help promote the transient stability of power distribution and transmission systems.

In part I of this series article, it was found that a negative charge process after a partial breakdown (PB) could reduce the breakdown voltage of vacuum gaps in series. This part proposes the use of a bridging resistor to mitigate this negative charge process and improve the breakdown voltage of the entire arrangement. Two commercial vacuum interrupters (VIs) of the same model were connected in series, with each VI having a grading capacitor connected in parallel. A bridging resistor was then bridged between two floating potential middle points: at the middle point between the VIs and at the middle point between the grading capacitors. Experiments were performed using various gap distance arrangements and resistance values. A noncontact measurement method using an electric field sensor was proposed to measure the floating potentials of the two VIs. Experimental results showed that the negative charge process could be mitigated using the bridging resistor $R_{g}$ by blocking the discharge current from the grading capacitor into the broken-down VI. The voltage distribution between the two VIs became uneven because of the bridging resistor, but the breakdown voltage of the series-connected gaps increased significantly, regardless of the resistance value. The increase rate was influenced by the gap distance arrangement. With a symmetrical gap distance arrangement for the two VIs, a maximum increase rate of 37.4% was reached when $R_{g} =40\,\,\text{k}\Omega $ . With asymmetrical arrangements, when smaller gaps shared higher voltages, the increase rate reached a maximum of 52.8% with $R_{g} =70\,\,\text{k}\Omega $ , but it only reached a maximum of 38.9% when larger gaps shared higher voltages with $R_{g} =10\,\,\text{k}\Omega $ .