Simulation of MMC-based converter for flexible direct current transmission of offshore wind farm

The topological structure and control strategy of converter for the flexible direct current transmission of off-shore wind farm are studied.The structure of MMC(Modular Multilevel Converter) is adopted as its main circuit and its equivalent mathematical model is established.The sub-modules of each bridge arm are sorted according to their capacitor voltages,which,combined with the arm current direction,determines the states of sub-modules to ensure that the instantaneous capacitor voltage of every sub-module is equal within the same arm.Dual close-loop vector control strategy is applied:the converter at wind farm side controls DC voltage while the converter at grid side controls active power.The inner loop adopts the current feed-forward decoupled mode to control the active power and reactive power independently.The simulation system model is set in Matlab and the advantage of adopted topological structure and the correctness of applied control strategy are verified.