Research On Model Predictive Control Strategy Of Modular Multilevel Converter

Modular multilevel converter is highly modular,has good output characteristics,easy to expand and so on,and quickly becomes a research hotspot compared with the conventional multilevel converter.However,the traditional closed-loop control strategy mostly uses proportional integral controller or proportional resonant controller,which has complex control structure and poor dynamic performance,while model predictive control is easy to deal with multivariable nonlinear system and has fast response speed.The output waveform harmonics of traditional model predictive control strategy are widely distributed,which makes the design of output filter difficult.To solve this problem,the Model Predictive Control with Optimal Switching Sequence(OSS-MPC)is designed to achieve constant switching frequency of MMC devices.Thus,the practical value of model predictive control in high voltage and high power system is improved.This thesis studies the mathematical model and working principle of MMC,analyzes the carrier modulation strategy of MMC,including the carrier phase-shift pulse width modulation strategy and the carrier cascade pulse width modulation strategy,and combines the voltage closed-loop voltage equalization algorithm and voltage sequencing voltage equalization algorithm,analyzes the relative merits among this modulation strategies,and builds a five-level MMC simulation model.The corresponding modulation strategy and voltage equalization algorithm are verified.In order to achieve the control goal of fixed switching frequency,an OSS-MPC strategy for three-phase MMC is designed based on space vector,the voltage prediction model of MMC is analyzed,and the selection principle of seven-stage switching sequence and the action time of each voltage vector is given.At the same time,an optimal switching model predictive control strategy based on two dimensional control region is designed to realize independent control of each phase.The circulation suppression strategy of eliminating the weight factor is adopted,which is compatible with two OSS-MPC algorithms.Finally,through simulation analysis,the Voltage Sorting Combined MPC(VSB-MPC)strategy and the traditional one are compared and verified,and the results demonstrate that the algorithm is actual and working.In order to simplify the control system structure of MMC converter in MMC-HVDC system,this thesis combined with OSS-MPC strategy based on space vector,the optimization control of MMC inverter was designed in detail.The traditional control strategy of MMC double closed-loop vector system and its corresponding circulation suppression strategy are analyzed.The mathematical model of MMC inverter station is studied.The OSS-MPC strategy is applied to the inner loop current control.Through simulation,the traditional double closed-loop control strategy,the control strategy using OSS-MPC in the current inner loop and the control strategy using VSB-MPC in the current inner loop are compared and analyzed,and the superiority of OSS-MPC in the control of MMC system is verified.According to the control strategy described in this thesis and the corresponding MMC structure,hardware experiments are designed based on the MMC experiment platform,and the control performance of OSS-MPC strategy and VSB-MPC strategy is compared and analyzed.Experimental results show that OSS-MPC strategy has superior steady-state performance.