The Research On Precharge Control Strategy For Modular Multilevel Converter And Prototype Development

Modular Multilevel Converter(MMC)has the advantages of modular structure,high output waveform level,and common DC bus.Compared with other multi-level converters,these advantages make it foreign scholars interested in research,and had made a wide range of engineering applications in high-voltage large-capacity VSC-HVDC transmission and flexible AC transmission and other fields.On the basis of reading the existing literature,this paper analyzes some key technologies of MMC by means of simulation and experimental verification,and puts forward an effective solution to these key technologies.MMC topology is characterized by each arm which is the use of power module cascade way,and through the bridge arm in series reactor to reduce the the bridge arm circulation caused by voltage imbalance between the phase.Firstly,based on the power module of the half-bridge structure,the basic working principle of MMC is studied.Based on the mathematical model of MMC,the design of prototype stage controller with a novel direct current control strategy is analyzed.Secondly,it is necessary to precharge the capacitor of the power module to its rated value before the normal operation of the MMC-HVDC transmission system.The control method of the precharge of the power module is studied from the AC grid side start-up and the DC bus side start-up respectively.The charging process is divided into two stages:controllable precharging and uncontrollable precharge.A precharge control strategy using a feedforward control and a constant current in the controllable precharge stage is proposed.NLC modulation and carrier-based PWM modulation for MMC has its own characteristics.Firstly,the relationship of the N+1 level modulation and the 2N+1 level modulation of the MMC in different modulation strategies with the upper and lower arms correspond to the carrier phase angle differenceis is analyzed.Secondly,the MMC output characteristics and the internal dynamic characteristics of different modulation strategies are compared and analyzed respectively from the normal operation of the system and the actual operation of the existence of over-modulation and carrier phase angle error conditions in two cases.Experiment is the most effective way to verify the theoretical analysis,I participated in the design and build a three-phase MMC passive inverter experimental device.As the hardware circuit needs to measure a large number of sub-module voltage in the MMC bridge arm,so a set of DPS+CPLD+FPGA-based master-slave control system to achieve multi-channel rapid sampling,voltage equalization control algorithm,and PWM Pulse transfer.By means of experiment and analyse,the CPS-PWM strategy used in MMC is verified.The measured experimental waveform verify the feasibility of the control system architecture.