Research On Optimization Control Technologies For Modular Multilevel Converters

The modular multilevel converter(MMC)keeps many advantages when compared to the conventional voltage source multilevel converters,such as highly modular structure,flexible voltage and power level adjustments,flexible control approaches,etc.Given its wide potential application prospect,the MMC has drawn lots of interest since 2001.By taking the high-power drive applications as a background,this dissertation introduces some foundational studies relative to the MMC,including modeling analysis,system design,and modulation strategies.On this basis,optimization control researches are furtherly carried out focusing on the vriable frequency operation and model predictive control of the MMC.The topology and operating principle of the MMC are introduced at first,and its ac-side and dc-side characteristics are analyzed by deriving the continuous mathematical model.A downscaled experimental prototype is constructed,for which a sub-module capacitance selection method is designed.In addition,to meet the control requirement of large number of switching devices in the experimental prototype,a digital control platform with a three-level control hardware structure is also designed.Then,the principle of outputting N+1 and 2N+1 voltage levels is derived,and relative implementations are analyzed with sub-module unified PWM and carrier phase shifted PWM,respectively,which are appropriate for medium-voltage applications.The experimental results of these two modulations validates the effectiveness of the experimental prototype and the correctness of the mathematical analysis for the modulations.When served as an ac motor drive,the sub-module capacitor voltages fluctuate severely under low frequency operation.The essential reason of this matter is uncovered by analyzing the instantaneous power of the upper and lower arms in each phase unit of the MMC.Then,a capacitor voltage fluctuation suppressing method(CVFSM)under low frequency operation based on the high frequency common mode voltage and circulating current injection is proposed to maintain the instantaneous power the upper and lower arms balanced dynamically.On this basis,a variable frequency operation control strategy is proposed based on the 2N+1 carrier phase shifted PWM to acquire a better output harmonic characteristic under low switching frequencies.The appropriate frequency range of the CVFSM and switching mode are also studied to prevent the CVFSM from bringing useless high frequency circulating current in to the MMC under the normal frequency operations.To reduce the computational burden of the finite control set model predictive control(FCS-MPC)strategy for the MMC,the discrete mathematical predictive model of the system is derived.Then,a new fast model predictive control(FMPC)strategy based on the voltage sorting algorithm is proposed by optimizing the implementation of the control objectives and simplifying the rolling optimization process.The computational burden can be significantly reduced while the advantages of the conventional FCS-MPC strategy reserved with the proposed FMPC strategy.In addition,the dv/dt of the output voltage of the MMC can be limited effectively.Same as the other FCS-MPC strategies,the switching frequency of the FMPC strategy is also unfixed,which could not ensure the steady-state current control performance under low current control indexes and low sampling frequencies.However,the current control precision increases and the average switching frequency decreases with the control index,which make the FMPC strategy especially suitable for the MMC systems with large number of sub-modules.The MPC strategies are conducive to simplify the control system for MMC-based rectifier.However,the steady-state performance of the FCS-MPC strategies is unsatisfied under unbalanced grid voltage conditions.A modulated model predictive control(M2PC)is proposed in this dissertation to addresses this matter by adding an appropriate modulator into the MPC algorithm.At the same time,a new approach to obtain the reference circulating current control is presented based on the second order generalized integrator with frequency-lock loops(SOGI-FLL)to ensure the circulating current control accuracy under unbalanced grid voltage conditions.An optimal control strategy based on the M2 PC is designed in ?? stationary reference frame for the MMC-based rectifier with the aim of keeping the dc-link voltage stable.Relative high-power simulation and low-power experimental resultas indicates that the M2 PC is superior to the conventional FCS-MPC in the aspect of both ac-side current control and ac circulating current component suppression.The M2PC-based optimal control strategy is helpful to the MMC-based rectifier in its steady-state performance under unbalanced greid voltage conditions.