Research On Input Power Unbalance Control Of CHBI

In recent years,the world environmental protection problem is more and more serious,and people’s demand for energy is also more and more,the development of new energy and use has been urgent.As a core part of the photovoltaic power generation system,the performance of the inverter will directly affect the quality of grid-connected current,so it is very necessary to study the inverter.Compared with the traditional two-level inverter,the cascaded multilevel inverter has the advantages of less harmonic content of output current,small size,simple modular expansion,direct access to the grid without transformer,and the DC voltage side of the cascaded multilevel inverter is independently completed by the photovoltaic panel MPPT operation,and the inverter operation efficiency is high.Therefore,cascaded multilevel inverters have a very broad application prospect in the photovoltaic field.Due to the influence of lighting,shading,photovoltaic panel damage and other problems,cascaded multilevel grid-connected photovoltaic inverters will have a large power difference between the DC side units,resulting in power imbalance,which makes the larger power units overmodulate,and then cause grid-connected current distortion,and even lead to system instability.In view of the above power imbalance problem,this paper takes cascaded multilevel photovoltaic inverter as the research object.The main work content and innovation points of this paper are as follows:1)Analyze and compare the advantages and disadvantages of various multilevel inverters;The mathematical model of photovoltaic cells was established to analyze the influence of light intensity and temperature on the output characteristics of photovoltaic cells.The mathematical model of cascaded multilevel photovoltaic inverter is established.2)A control method of cascaded photovoltaic grid-connected inverter based on harmonic compensation method is proposed.On the basis of the third harmonic compensation method,the fifth harmonic is further introduced,the third and fifth harmonics are introduced into the overmodulation unit,and the reverse third and fifth harmonics are allocated to the non-overmodulation unit according to the ability to withstand,so that the whole inverter does not introduce harmonics.Finally,the correctness and feasibility of the proposed control method are verified by simulation and experiment.3)A control method of cascaded photovoltaic grid-connected inverter with improved harmonic compensation method is proposed.Further thinking on the basis of harmonic compensation method,the introduction of harmonic instead of fundamental wave,fundamental wave into the overmodulation unit,and the reverse fundamental wave is allocated to the overmodulation unit according to the affordability,so that the whole inverter does not introduce fundamental wave.Finally,the correctness and feasibility of the proposed control method are verified by simulation and experiment.