The coordination of resistive-type superconducting fault current limiters (R-SFCLs) and DC circuit breakers (DCCBs) have been investigated. However, the use of R-SFCLs with mechanical DCCBs (M-DCCBs) in hybrid transmission systems has not been studied. The objective of this paper is to investigate the parameter requirements of R-SFCLs and M-DCCBs for a detailed system protection strategy in a hybrid AC/DC system considering the influence of AC faults and three types of DC faults. When the AC fault is far from all MMCs, its impact on the DC system is weak. If the AC fault is close to one MMC, it will produce a strong transient response in the DC part. Therefore, a bypass switch in parallel with an R-SFCL is used to remove the R-SFCL from the system if it quenches during the AC fault. Then, the parameters of the protection devices are designed based on pole-to-pole DC fault. System restoration can be achieved within 200 ms after the fault. For pole-to-neutral and pole-to-ground faults, except for the R-SFCLs at the faulty pole, the R-SFCLs at the healthy pole will also quench, but the resistances are very low. The system may be restored within 250 ms after the DC fault.

In order to understand the influence of the average opening velocity on the high-current vacuum arc anode phenomena, high-speed photography was used to observe the anode phenomena of the vacuum arc discharge in vacuum interrupters. The contact diameters used in the vacuum interrupters were 12 and 25 mm, respectively. The contact materials included Cu, CuCr25, and CuCr50. The arc current frequency was 50 Hz, and the arcing time was controlled at about 9 ms. A permanent magnet mechanism with a contact spring was used to adjust the average opening velocity from 1.3 to 1.8 m/s. The experimental results showed that, with the arc current increasing, there was a threshold current I_1st (peak value) at which a high-current anode mode first appeared. Moreover, the first high-current anode mode was a footpoint at the velocity of 1.8 m/s, while, at the velocity of 1.3 m/s, it was most probably an anode spot and sometimes it was a footpoint. The result showed that, at the velocity of 1.8 m/s, the threshold current I_1st was lower than that at 1.3 m/s. Moreover, the threshold current I_1st followed the order of Cu >; CuCr25 >; CuCr50 at both the velocities of 1.3 m/s and 1.8 m/s. Meanwhile, at the higher average opening velocity of 1.8 m/s, the arc energy and arc voltage were higher than or close to those at 1.3 m/s.

Researchers have investigated the interruption properties of CO2 gas, including CO2 and O2 mixed gas. However, there has been no research on the passive resonance DC interrupting characteristics of CO2/O2 mixed gas. The objective of this paper is to learn the passive resonance DC interrupting properties of CO2/O2 mixed gas with superconducting fault current-limiting technology. A proposed passive resonance DCCB that includes a superconducting current-limiting module (SCLM) and a DC interrupting module (DCIM) connected in series was tested for DC voltages of 1.3, 3 and 10 kV and DC currents from 50 A to 10 kA. The experimental results show that the DCCB could successfully interrupt a 10 kA current in a 10 kV DC system. The response time of the DCCB was only 0.3 ms. After interruption, there was no overvoltage. The arcing time and the arc voltage of the SF6 gas and the CO2/O2 mixed gas were also compared experimentally. Experimental results show that CO2/O2 mixed gas may be a good substitute for the SF6 gas in passive resonance interruption. The CO2/O2 mixed gas passive resonance DCCB combined with the SCLM is an effective way to interrupt high short-circuit fault currents in medium-voltage and high-voltage DC systems.

Resistive type fault current limiters (SFCLs) have attracted lots of attention from research interests and engineering applications, due to their self-triggering, fast and effective fault current limitation ability. Fast recovery of SFCL after quench is highly attractive, yet still remains challenge when recover with load compared to recovering without load. We investigated the dependence of recovery time of a helical SFCL coil on the ratio of prospective current over critical current I _pros /I _c . A multilayer simulation model was built in MATLAB/Simulink to investigate the electro-thermal behavior of SFCL coil during a fault and recovery period, and the model was validated by experimental results. Quench simulations were carried out under different I _pros /I _c , ranging from 15 to 119 and in different recovery conditions including recovery with load condition and recovery without load condition, while keeping the fault duration of 100 ms. We observed that the SFCL coil reached 85 K and 195 K at the time when the fault was cleared, in the quench tests with I _pros /I _c = 15 and 119, respectively. SFCL coil recovered without load in 0.5 s to 10 s, depending on the ratio of I _pros /I _c .

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.

The objective of this paper is to determine a breakdown mechanism transition with increasing vacuum gaps. A rod-plane vacuum gap was designed in a vacuum interrupter. A current and voltage conditioning process was performed in order. Two shots of ac power frequency voltage were applied at different vacuum gaps until breakdown occurred, and the vacuum gaps were 1.0 mm, 0.8 mm and 0.5 mm, respectively. The field emission current and breakdown voltage were simultaneously measured. Whether the field emission current and voltage fitted the Fowler-Nordheim theory was used to distinguish the field emission induced breakdown and particles induced breakdown. The results showed that there was no mechanism transition between the two shots at 0.5 mm gap spacing, and there were mechanism transitions between two consecutive breakdowns at 0.8 mm and 1.0 mm gap spacings. The probability of a mechanism transition occurring also increased with the increase in gap spacing from 0.5 mm to 1.0 mm.