Research On Control And Protection Of Modular Multilevel Conterver

With the increasingly serious energy shortage and environmental pollutionproblems, the state will vigorously develop and utilize renewable clean energy,optimize the industrial structure. However, wind，solar and other renewable energybecause of their inherent dispersion of small and far away from the load centercharacteristics makes the AC transmission technology uneconomical. Flexible DCtransmission technology solves this problem. Recent years, the HVDC basing onModular Multi-level Converter (MMC) has potential growth. This paper does someresearches on MMC.Firstly, a mathematical model of MMC is built. A systematic controller of theMMC-HVDC is designed according to the mathematical model. Then some in-depthresearch of the Direct Current Control and the Indirect Current Control is done. Thecomputing methods of the arm inductance and sub module capacitance parameters ofMMC are given. A system of the MMC-HVDC of61levels is set up usingMATLAB/SIMULINK. Depending on the simulation result we can see that DirectCurrent Control has good current response characteristic and restricts the currentwell. The Direct Current Control has been extensively used in the high-power projectof VSC-HVDC.Secondly, the valve control of MMC is studied particularly. The voltage level ofthe output voltage of MMC is increased twice by the improvement of the DPWMmethod and the carrier phase shifting PWM method. This paper proposes a newvoltage-sharing method which is fit for carrier phase shifting PWM method. Thevoltage-sharing method has low switching frequency and works well on balancingvoltages of the arm capacitance, so it is good for engineering application.Finally, a new-style controller which is good for restricting circulating current of MMC is designed. The controller has simple structure and is adept in restricting DCcurrent which is produced by DC fault. The launch strategy of MMC-HVDC whoseinverter side connected with different load is researched well. This paper also buildsa negative sequence mathematical model of MMC whose AC side is fault andproposes two different control methods which could restrict negative current. Thesimulation result indicates that these two methods restrict negative current well andmake the MMC operate continuously when the AC fault happens.