A New Design of Z-Source Capacitors to Ensure SCR's Turn-Off for the Practical Applications of ZCBs in Realistic DC Network Protection

The installed capacity of renewable energy resources in dc format is increasing in modern power systems, and thus, the protection of high-power dc power delivery from fault is critical in HVdc transmission and MVdc distribution networks. Based on thyristors, Z-source circuit breaker (ZCB) is a promising device for high-power protection, with the capabilities of bidirectional power flow and automatic/controllable turn-off function. In this article, a new specific method for calculating Z-source capacitances is developed based on the reverse-recovery characteristics of SCR, which was not considered in the prior-art solution properly. The proposed method can ensure the turn-off action of SCR in ZCB's practice for realistic dc network protection. At first, it is revealed that the problem in the prior-art solution of ZCB parameter identification, which does not handle the SCR's reverse recovery well, and thus, could result in a failure of tripping action. To solve the problem, the specific method is proposed to adjust the Z-source capacitances, in order to accumulate enough electrons for the postfault depletion region buildup in SCR, and thus, guarantee the SCR's turn-off for a successful dc circuit protection. In the specific derivation, the required tripping time of ZCB is also considered and can remain unaltered after Z-source capacitance adjustment. Finally, experimental tests have verified the problem solving by comparing the proposed to the prior-art solution and proven its accuracy. The proposed method can be added as a supplement to the prior-art solution for the practical design of ZCB. This research article helps to increase the technology readiness level of ZCB solution and practice in realistic dc network protection.