Advanced Control And Wind Power Generation Application Of Cascaded H-Bridge Converters

With the rapid development of modern society and economy,energy and environmental issues have become one urgent issue that human being has to face.Because of its flexible and rich functions,power electronic converters play an important role in adjusting energy consumption structure and increasing the effective utilization of energy and have become the core components and key technologies in modern energy systems.Taking the cascaded H-bridge multilevel converter as the research object,this dissertation presents the following two aspects: optimization of model predictive control for cascaded H-Bridge static synchronous compensator(STATCOM)and the critical issues in cascaded H-Bridge wind energy conversion system(WECS).One issue in MPC for cascaded H-Bridge is its computation increasing exponentially with the cascaded stages.To solve this problem,a modeling method with partially stratified optimization method is presented to rebuild the optimal problem.According to the priorities of the control objectives,the MPC for cascaded H-Bridge STATCOM has been divided as current MPC and capacitor voltage balancing(CVB)-MPC.Then the computation complexity of current MPC is reduced to a polynomial level and the computation complexity of CVB-MPC is reduced to an relatively small exponential level.Finally,a mixed optimization approach with dynamic programming and 0-1 programming is presented to re-determine the optimization order of CVB-MPC and transform the three-value quadratic integer programming problem into two 0-1 programming problem.With the above-proposed modeling and optimization methods,the computation complexity of MPC for cascaded H-Bridge STATCOM can be reduced to a polynomial level.When solving the current MPC with complete enumeration method,the computation burden is determined by the number of non-redundancy voltage vector,which is polynomial level.To solve this problem,a simplified branch and bound approach is presented in this dissertation.By transforming the two-variable integer programming problem into multiple single-variable integer programming problems on the gh coordinate,the global optimal result can be solved only by comparing 4n+1 voltage vector.The analysis and experimental results have proven the time complexity of current MPC can be reduced from polynomial level to linear level.Based on the above optimization approaches,the algorithm execution time of MPC for cascaded H-Bridge STATCOM has been reduced significantly.However,powerful processors are still required for the online implementation of MPC.This dissertation presents a parallel implementation method with DSP and FPGA.The MPC algorithm is implemented by the DSP and FPGA coordinately.A parallel comparisonbased sorting algorithm with pipeline technology is proposed to accelerate the sorting algorithm.By running the DSP and FPGA in parallel,the time complexity and space complexity of MPC for cascaded H-Bridge STATCOM can be reduced to the linear level.The MPC for cascaded H-Bridge STATCOM with any stages can be implemented with a low-cost DSP and a small-size FPGA.Experimental results have proved the correctness and efficiency of the proposed approach.When experimentally testing the cascaded H-Bridge medium-voltage high-power WECS,two oscillation issues,i.e.an envelope oscillation issue and a medium-frequency issue are observed at the grid-link inverters.First,the reason for the envelope oscillation issue is analyzed and a global carrier synchronization approach with carrier synchronization Phase Locked Loop(PLL)is proposed.To solve the medium-frequency oscillation issue,a decoupling analysis method with a linear transformation is proposed to transform the inverter currents as the summation current and difference current.Then,single-variable control theory,i.e.bode diagram can be directly used to analyze the summation current and difference current respectively.It was found that the summation current and difference current have a same current regulator but have different coupling impedances.When the impedance of summation current is much larger than that of difference current,it is hard to select suitable parameters for current regulator.Using small regulator parameters cannot satisfy the control performance of summation current.And using large regulator parameter may decrease the stability margin of difference current,causing medium-frequency oscillation issue.The proposed decoupling analysis method can provide guidelines to design the current regulators and hardware circuits of the multiple-paralleled inverters.Experimental results of a three-phase five-level WECS have proved the above methods.