The high-voltage direct current (HVDC) power transmission short-circuit fault protection is a big challenge, and it is the main obstacle to promote the HVDC multi-terminal networks. A natural commutation current topology (NCCT) of hybrid HVDC circuit breaker (DCCB) integrated with limiting fault current is proposed in this paper, which shares several obvious merits, such as lower peak fault current, shorter fault isolation time and low turn-off loss. The main contributions in this paper are made as follows. (1). A second-order RLC circuit in novel NCCT circuit breaker is utilized as the current limiting circuit to dramatically reduce the fault current rising slope and its peak value. (2) A asynchronous NCCT circuit breaker has been investigated to broaden the proposed NCCT circuit breaker's optimal working area. (3) By taking China Zhangbei 500 kV four-terminal DC grid as an example, an optimization design technique of the asynchronous NCCT-DCCB is developed to demonstrate the process of the theoretical analysis and parameter design. The correctness of the proposed NCCT-DCCB and the feasibility of the optimization design technique are confirmed by EMT simulation on PSCAD/EMTDC and hardware-in-loop experiment on PLECS-RtBox.

Due to the low impedance of DC (Direct Current) power transmission line, short‐circuit current has a very high increment speed, which causes problems as difficulty in current limitation method. This paper proposes a new method to realize the current‐limiting control of short‐circuit current by using active control of power electronic transformer. The method improves the topology of dual active bridge modules that contribute the DC transformer, and installs an IGBT (Insulated Gate Bipolar Transistor) switch in reverse direction on supporting capacitor branch of each module to control the capacitor on or off, which can guarantee the storage energy within the capacitor will not release in initial stage in case of DC short circuit fault happen. Therefore, there is no large inrush current flow out. Furthermore, combined with the requirements of current tracking control during low‐voltage ride‐through, the transformer output current under short‐circuit condition is actively controlled in order to meet the needs of renewable energy power generation equipment. Finally, the simulation and hardware‐in‐the‐loop experiments verify the correctness of this method.

Due to the characteristics of high efficiency and flexible transformation ratio, full-bridge CLLC resonant DC transformers have been increasingly used for DC grid in recent years. Based on the analysis of the working principle of CLLC resonant DC transformer circuit, this paper analysis the current gain characteristics by using fundamental wave analysis method, the parameter influences of resonance network frequency and the transformer gain is also being concerned. On the basis of using soft switching method, this paper will introduce a method of inverted displacement phase control. The working range will be further refined in the original circuit, and the linear control region will be expanded as well. In this way, the output current will have a precise control under the condition of short circuit, which will satisfy the low voltage ridethrough requirements of DC transformers applied to the grid connection of renewable energy. Finally, the method proposed in this paper is verified by simulation and hardware-in-the-loop experiment.

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