| With the increasingly serious environmental problems and the continuous reduction of the price of photovoltaic panels,solar power generation has gradually become an ideal choice to solve environmental and energy problems.Among them,photovoltaic gridconnected inverter,as the core equipment of grid-connected photovoltaic power station,has become the focus of research by scholars at home and abroad.Cascaded H-bridge(CHB)inverter has the advantages of simple layout,high reliability,modularization and the least number of devices required to synthesize the same voltage level.Connecting the photovoltaic string directly to the DC bus not only meets the requirements of CHB topology requiring a large number of independent DC power supplies,but also eliminates DC/DC converters.Therefore,CHB photovoltaic grid-connected system has broad application prospects in the field of photovoltaic power generation.In order to solve the problems of intermodule and interphase power imbalance,and effectively suppress the voltage fluctuation of DC bus,the main research contents of this paper are as follows:Affected by partial shielding,self damage and other factors,the transmission power between H-bridge modules of single-phase CHB photovoltaic grid-connected system is unbalanced,resulting in over-modulation of H-bridge modules with high transmission power,which affects the quality of grid-connected current.Therefore,based on the traditional voltage and current double closed-loop control,three interbridge power balance control strategies are studied in this paper,and realizes the stable operation of the system under the unit power factor by input power distribution,the third harmonic injection and the hybrid pulse width modulation.The principle and overall control structure of the three strategies are analyzed,and their stability margin is mathematically deduced.At the same time,the advantages and disadvantages of various methods are compared and analyzed.In the three-phase CHB photovoltaic grid-connected system,in addition to the power imbalance between the above modules,there is also the problem of interphase power imbalance,resulting in the imbalance or even distortion of three-phase grid-connected current.Therefore,this paper studies three interphase power balance control strategies.Among them,the interphase power balance control adopts the fundamental frequency zerosequence voltage injection method,double minimum-maximum zero-sequence voltage injection method and weighted minimum-maximum zero-sequence voltage injection method,and the three methods are combined with the interbridge power balance control method based on input power distribution to realize the stable operation of the three-phase CHB photovoltaic grid-connected system.The power balance range of three interphase and interbridge power balance control strategies is compared and analyzed.The simulation models of single-phase and three-phase CHB photovoltaic gridconnected system are built in Matlab/Simulink to verify the effectiveness of the above control strategy.At the same time,a 10 k W three-phase CHB seven-level photovoltaic gridconnected system simulation experimental platform is built.DSP is used to realize the control algorithm and issue control instructions,and the switch pulsation signal is generated by FPGA.The effectiveness of interphase and interbridge power balance control strategy and DC bus voltage fluctuation suppression strategy are verified by experiments. |
