Research On Multi-Stage Model Predictive Direct Power Control Of Active Front-End Modular Multilevel Converters

In recent years,modular multilevel converter(MMC),with merits of strong scalability,good harmonic characteristics,fault-tolerant ability and so on,'has received extensive attention in applications of high-voltage direct current system,high-power motor drive,and power quality improvements.In this thesis,active front-end MMC is taken as a research objective,the finite control set model predictive control(FCS-MPC)is employed as an important tool,and the instantaneous power compensation strategy is integrated to study the high-performance control of MMC under balanced and unbalanced conditions.The main contributions of this thesis are as follows:Firstly,a multi-stage model predictive direct power control method is proposed to mitigate large computational burden and eliminate the weighting factor dete'rmination of FCS-MPC approach.Cost functions corresponding to control variables of MMC are designed separately so as to eliminate the weighting factors in the control process.Furthermore,the output voltage level is restricted with an attempt to narrow the finite control set,accordingly,the execution time of MPC algorithm is reduced.Simulation analysis shows that the proposed strategy can achieve comparable performance with traditional FCS-MPC method,while the computation power is reduced and weighting factors are eliminated.Secondly,a cascaded structure based lower switching frequency method is proposed to deal with the high switching frequency and high power loss induced by FCS-MPC algorithm.The capacitor voltage balancing process is divided into two parts:voltage equalization and frequency restriction.Two cost functions are evaluated sequentially,thus the weighting factor in single cost function is eliminated.Simulation results show that the proposed method can achieve a lower switching frequency with good performance of other control objectives.Thirdly,a signal delay based power compensation method is proposed to improve the deteriorated performance under unbalanced grid-side conditions,including power fluctuation and grid-side current distortion.The proposed method is capable of eliminating active power ripples or reactive power ripples and maintaining sinusoidal grid-side current.Compared with other approaches,the proposed one can obtain compensation term by using delayed signal,thus the phase-locked loop and sequence extraction is not needed.Finally,both simulation and experiment results are provided to validate the effectiveness of the proposed method.