Research On Several Optimization Control Methods Of Modular Multilevel Converter

Modular Multilevel Converter(MMC)is widely used in flexible HVDC transmission because of its advantages such as modular design,scalability,high transmission capacity,good harmonic characteristic,capability of decoupled active and reactive power control and so on.MMC has a great development prospect,receiving attention of the academic and industry circles at home and abroad.In this dissertation,several optimization control methods of MMC are proposed following a literature review.In this paper,the research status of modular multilevel converter(MMC)in topology structure,mathematical modeling,parameter design,control technology and so on is deeply studied.It is concluded that the MMC still has some problems such as further optimization design of circulating current control,large fluctuation of capacitor voltage in sub-module,long start-up time of MMC device and complex control system.The decoupled model between ac and dc side of MMC is built,by ignoring switching processes of power electronic devices.The typical three-layer control system of MMC-HVDC:valve group level,converter level and system level control is introduced and analyzed.These work provide the theoretical basis for a series of optimal control methods proposed later.Aiming at the problem that the capacitance voltage fluctuation of MMC sub-module is too large and the required capacitance value of sub-module is too large,a novel optimal capacitance voltage fluctuation of MMC sub-module control method based on 2nd harmonic current is proposed.Firstly,the analytical expressions of the main electrical parameters,such as the capacitance fluctuation voltage of MMC sub-module and the bridge arm circulation,are derived by using the analytical method considering the circulation control.Then the calculation formulas of second harmonic current amplitude and initial phase reference value are given when the capacitor voltage fluctuation of sub-module is limited to a certain range,and the corresponding control strategy is designed accordingly.Finally,through modeling and simulation,the results show that compared with the traditional control method which completely suppress the circulating current,the proposed method can further reduce the voltage fluctuations of the sub-module capacitance,thereby reducing the required value of the sub-module's capacitance.Aiming at a Full-bridge MMC based mobile DC ice-melting device,a comprehensive optimization control method is studied.Firstly,the DC voltage output range of full-bridge and half-bridge sub-module is compared to provide reference for the selection of sub-module topology of dc de-icer based on MMC.Then constant power charging strategy and constant modulation index charging strategy are proposed,and the conclusion that constant power charging is faster than constant modulation charging is obtained.Four main control strategies are designed:reactive power,ice melting current of each phase,average voltage of all sub-modules and carrier phase-shifting pulse width modulation.In order to simplify test conditions and reduce test loss,a novel rated-current zero-power test scheme is proposed.Finally,the start-up test,rated-current zero-power test,the STATCOM operation test and the transient test are carried out respectively.The test results show that the charging speed of constant power charging is fast in the controllable charging stage.The designed control system has good transient characteristics and high stability.