The Key Technologies Of High Voltage Direct Current Transmission System Based On MMC

Due to the modular design feature,the modular multilevel converter(MMC)has the advantages of high output waveform quality,strong fault handling capability,low switching frequency and easy to be manufactured.Therefore,the MMCs are widely used in high voltage direct current(HVDC)transmission system.In this thesis,the simulation model and experimental platform are built for MMC to verify the effectiveness of the theoretical analysis and control strategy.Afterwards,the working principle,mathematical model and control strategy of the MMC-HVDC system with two ports are analyzed and validated by the simulation.The main contents of this thesis are as follows:(1)The topology and applications of the MMC are summarized,and the internal and external models are established to discuss its operating characteristics.In order to reduce the AC side current harmonic,negative sequence current and the internal circulating current,limit the rising rate of the fault current,the selection method of arm inductance are designed.Furthermore,the designing criteria of the sub-module capacitor is also presented according to its allowable voltage fluctuation range.(2)By comparing the methods of carrier-disposition(CD)PWM,carrier phase-shifted(CPS)PWM,improved carrier phase-shifted PWM,nearest level modulation(NLM)PWM,the advantages and disadvantages of these different modulation strategies are concluded.Combined with the first three modulation strategies,the hierarchical controlling strategy for voltage balance is discussed.In addition,the capacitor voltage ranking algorithm suitable for the NLM-PWM strategy is studied.In terms of the circulating current,the generating mechanism and components are analyzed,then the control model is obtained in the rotating coordinate system.At the same time,the average capacitor voltage control model is improved to eliminate the circulating current of double line frequency.The simulation and experiment results indicate that the improved control method has the merit of capacitor voltage balance and good performance in suppressing the circulating harmonic current.(3)A three-phase MMC experimental platform is built.The system architecture and the realization method of the main unit of hardware and software are described in detail.In addition,the design method of the controller parameters is given in the discrete domain.Finally the validity of the theoretical analysis is verified on the experimental platform.(4)Based on the research of MMC,the operating principles of MMCHVDC with two ports are analyzed and the mathematical model is established.By adopting the three-level control strategy in the MMC-HVDC system,the simulation model for the active power supply with two ports is built.The simulation results validate the correctness of the theoretical analysis and the effectiveness of control scheme.