The DC side fault of the high voltage DC transmission system based on modular multilevel converters high voltage DC (MMC-HVDC) is easily affected by the double influence of AC transient electrical quantity infeed and overvoltage and overcurrent, seriously threatening the safe operation of the power grid. Aiming at the problem above, a fault suppression strategy in dual mode of isolation and current limiting is proposed. Based on the half-bridge-full-bridge hybrid MMC, the fault transient mathematical model considering AC feed and sub-module switching feed is analyzed. AC isolation module with double conduction self-resistance structure and current-limiting control module is constructed, which can be adaptively adjusted to the system’s direct current parameters. The ±500 kV double-ended hybrid MMC system model is established to verify the scheme’s effectiveness. In addition, the influence of high AC modulation parameters on the suppression effect is investigated. PSCAD/EMTDC simulation results show that AC active and reactive power feed-in peaks are reduced by 388.69 MW and 119.82 Mvar, respectively, under dual-mode control. The peak value of DC overvoltage and overcurrent is reduced by 1211.53 kV and 22.5 kA, respectively, the fault self-clearing time is shortened by 43 ms, and the fault suppression effect is remarkable. In the high AC modulation ratio operating state, the modulation parameters are positively correlated with the AC power feed and negatively correlated with the overvoltage and overcurrent of the DC system. The research results provide support for the fault protection scheme of the hybrid MMC flexible and direct power grid.

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In the isolated island operation of microgrids, affected by the different equivalent circuit impedance between distributed generators, the traditional droop control cannot evenly divide the reactive power borne by distributed generators according to the different line impedance. Therefore, firstly, the reasons affecting power sharing are analyzed, and then an improved droop control method based on traditional droop control is proposed; that is, the time change rate of voltage is used as the droop variable, and a voltage recovery mechanism is added to the reactive droop controller, in order to adjust the reciprocal of voltage to zero in the steady state, so as to stabilize the voltage. Aiming at the problem that the droop controller cannot always realize the equal distribution of reactive power, a voltage compensation link suitable for improved droop control is designed and added to eliminate the line impedance difference between distributed generators and improve the accuracy of reactive power distribution. Finally, the effectiveness and feasibility of the proposed control strategy are verified by building a “wind light storage load” microgrid simulation model on the MATLAB / Simulink platform.