Research On Key Techniques Of Cascaded Multilevel Converter Based On H-bridge Topologies

As a preferred solution for medium voltage large capacity energy conversion,multilevel converters are attracting increasing research interests from domestic and overseas scholars.Among numerous multilevel converters,H-bridge cascaded multilevel converter owns the most promising research and application value due to its excellent modular structure,highly redundant design,easy expansion,low generation of output harmonics,and the advantage can be directely connected to medium or high voltage distribution network overcoming the poor voltage withstanding performance of single power electronic device.This thesis focus on some key technologies on cascaded multilevel converter based on H-bridge topologies.Firstly,this thesis introduces modulation strategy for H-bridge and its cascaded multilevel converter.To facilitate the design of the control system,H-bridge cascaded converters of STATic synchronous COMpensator(STATCOM),wind turbine side and grid side inside isolated back to back(BTB)cascaded wind power converter are normalized equivalent to controllable voltage sources,and gets the input and output characteristic model for H-bridge cascaded converter under both ABC static coordinate and DQ rotating coordinate,following the analysis of main circuit topology,operating principle of H-bridge cascaded STATCOM as well as isolated BTB cascaded wind power converter.Current reference detection method of different compensation mode for balanced/unbalanced load have been introduced,with multiple synchronous rotating transform based low frequency typical harmonics selective detection method proposed.Secondly,to improve the harmonic compensation and depression ability of cascaded converter,PI and fast repetitive controller(FREP)based compound current control is introduced for balanced system under DQ rotating coordinate with the aid of state variable decoupling method,as well as proportion resonant(PR)controller based individual phase current control is designed for unbalance system under ABC static coordinate.System control strategy of isolated BTB H-bridge cascaded wind power converter based on individual side current control and DC-link voltage hierarchical control is proposed with the design principle of controller parameters given,where hierarchical control is clearly divided into three levels,namely total DC-link voltage regulation control,DC-link voltage balance control between clusters of each phase and DC-link voltage balance control in a cluster.The DC-link voltage hierarchical control is also suitable for cascaded STATCOM.Simulation or experimental results show the correctness and effectiveness of selective harmonic detection method,current track as well as DC-link voltage balance control.Then,this thesis investigates the operation principle of a T-type H-bridge in detail,and proposes a modulation strategy for T-type H-bridge which takes the five-level waveform achieved by uni-polar double frequency CPS-SPWM as the output object and reverse derivate PWM signals of the T-type H-bridge according to its switching modes according to the principle"changing the minimum number of switch status to achieve an output level" aiming at reducing switching losses,with modulation strategy for T-type H-bridge cascaded multilevel converter furtherly introduced.To suppress the DC-link voltage fluctuation's influences of T-type H-bridge cascaded STATCOM,frequency adaptive moving average filter(MAF)is applied,with the effects of MAF in DC-link voltage feedback loop on DC-link voltage control stability analyzed.Simulation and experimental results show the correctness and feasibility of the proposed modulation strategy,DC-link voltage control method and its stability analysis.Finally,a double-DSP+FPGA+CPLD based universal digital control platform is designed for H-bridge cascaded multilevel converter.The platform owns a robust hardware design,a high portability software design as well as a industrial personal computer based human friendly user interface design.Aimed at undesired pulse competition on CPS-SPWM due to low switching frequency in cascaded multilevel converter,the thesis analysis its generation mechanism and presents a novel method for eliminating the competition pulses for which the PWM is restricted to change only once during both the rising half cycle and the falling half cycle of the carrier.Experimental results on three-phase 10kV/±10MVar H-bridge cascaded STATCOM industrial prototype and single-phase T-type H-bridge cascaded STATCOM prototype show the correctness of proposed pulse competition elimination method,as well as reliability and validity of the universal digital control platform.