Fault-current handling capability of the modular multilevel converters (MMCs) under dc-cable short-circuit conditions is a major concern for the MMC applications on the high-voltage direct-current (HVDC) transmission systems, where the MMCs based on half-bridge submodules (SMs) cannot block the fault currents to protect the converter devices. In this paper, a comprehensive review for the fault-ride-through capability of the HVDC transmission systems based on the MMCs adopting different SM schemes is presented, where the MMCs can block the fault currents and compensate the reactive currents to the electric grid during the dc faults. An analysis of the dc short-circuit faults in the MMC is introduced and then the operation principle of different SM circuits building the MMC for blocking the fault currents is highlighted. The fault-tolerant operation of these MMC schemes as static synchronous compensator to enhance the ac grid stability during the dc faults is also investigated. A comparison in terms of investment cost, loss, volume, and controllability for various MMC topologies is performed. Comprehensive simulation results for the most promising topologies of the MMC with a capability of fault-ride through under dc-fault conditions are finally presented.

This paper proposes an LVRT (low-voltage ride-through) of multi-pole permanent-magnet synchronous generator (PMSG) for wind turbine systems under the grid fault condition and varying wind speed using the energy storage system (ESS) and braking chopper (BC). The proposed topology can reduce the ESS power rating and improve the output performance. A detailed design procedure and control strategy for the ESS and BC are presented. The PSIM simulation results prove that the proposed system can give the ride-through capability and power smoothening for the PMSG wind turbine system. The validity of the proposed system is also verified by experiment results.

This paper proposes a fast harmonic rejecting control design to enable active support of electric vehicle (EV) charging stations to micro-grids under distortion. The proposed harmonic rejecting strategy is applied to control bidirectional converters and storage elements in charging stations to improve performance of charging stations under various distorted conditions. In this approach, power quality of both ac- and dc-grids is improved by actively rejecting unwanted harmonic components in voltages, currents, and powers. A fast dynamic control design can actively reject harmonic components where harmonic filters are used to eliminate unwanted components in signals. The paper shows that the proposed harmonic rejecting method can result in a simple control design in stationary frames without any phase-lock-loop ( PLL ) blocks. A two-stage optimization method is introduced under the predictive control fashion, i.e., optimizing vector angle for control performance and then modulating vector magnitude by coordinating one active- and one zero-switching state. The comparative results prove that not only control performance is resilient under distorted conditions but also the power quality on both ac- and dc-grids is actively improved. Both MATLAB/Simulink and real-time experimental platforms are used to verify the efficacy of the proposed approach in terms of compensating the reactive power, rejecting the imbalance harmonic distortions, and smoothing the dc-voltage.

This study proposes an advanced control scheme for the interlinking inverters of the hybrid AC/DC microgrids, which facilitates a seamless transition between grid-connected and stand-alone/islanding modes for the microgrid. Due to a nonlinear relationship between the terminal voltages of the voltage-source inverter (VSI) interfacing through inductor–capacitor (LC) filters with the grid voltages and currents, a feedback linearization technique (FLT) is employed to control the interlinking VSI under both grid-connected and islanding operations. The FLT-based current controllers are applied in the grid-connected mode, in which they adjust the power exchange between the DC and AC subgrids and mitigate the distortion of the grid currents produced by nonlinear loads. Under the stand-alone operation, the AC bus voltages are directly regulated by the FLT-voltage controllers of the interlinking VSI. In order to reduce the inrush currents and voltage overshot at the instant of mode switching, the FLT-based controllers are performed all the time regardless of the operating modes, where the voltage references for the VSI are not changed abruptly. The control performance of the VSI is highly satisfactory with low-transient overshoot values of the voltages and currents and low total harmonic distortion (THD) values of the grid currents and AC bus voltages are about 3.5% and 2.7%, respectively, under the nonlinear load condition. The validity of the new control strategy is verified by the simulation work, which investigates the operation of a hybrid AC/DC microgrid.