High-voltage dc/dc converters play an important role in HVDC grids. Isolated modular dc/dc converters (IMDCCs) based on modular multilevel converter (MMC) technology provide a good solution to high-voltage applications. In order to reduce the size of the system, the IMDCC is required to be operated with a high ac link frequency, but this will lead to increased switching loss and thus degraded efficiency. This paper proposes a soft-switching operation scheme for such an IMDCC. In this scheme, a quasi-square-wave (QSW) modulation method is employed, where the chain-links generate quasi-square terminal voltages with reduced dv/dt. With such chain-link terminal voltages, the arm currents which provide good condition for the soft-switching operation of the QSW-IMDCC can be obtained. Since soft-switching can be achieved for the power switches, the proposed scheme will suffer less switching loss, thus improving the efficiency of the converter. Moreover, a capacitor voltage balancing control strategy is proposed. This strategy does not need any arm current sensors, thus reducing the cost. The proposed soft-switching operation scheme and capacitor voltage balancing control strategy are verified by the simulation results.

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The DC line fault current clearance for series MTDC is discussed in this paper. A new method is proposed to increase the availability of the series MTDC system during DC line faults and decrease the dependency on telecommunication. In the proposed method, the DC lines in the series MTDC system are divided into several different protection zones. The DC faults at any low voltage DC lines can be cleared with the coordinated control of the converter stations adjacent the fault line while the DC power transmission at other DC lines are remained to some extent; for DC faults at high voltage DC line, although the power transmitted at the fault pole during the DC fault reaches zero, the emergency power support between inverters is possible. The DC fault clearance speed is fast because the telecommunication among the converters in the fault pole isn't needed. The validity of proposed method is verified by simulation results.