Research On SVM Modulation And DC Voltage Control Method For Cascaded Photovoltaic Inverter

The shortage of environmental issues and orthodox fossil energy have promoted the development of new energy sources,making solar energy receive widespread attention,and photovoltaic power generation is an important way of its utilization.Compared with other inverters,the cascaded H-bridge multi-level inverter topology has the advantages of high system efficiency,low output waveform harmonic content,and easy modular expansion.However,due to the power imbalance between the units of the cascaded photovoltaic inverter,the general modulation method needs to increase the power balance control to suppress this imbalance,so that each unit can work stably at its maximum power point.This leads to an increase in the complexity of the controller algorithm.The unbalanced power or unbalanced DC voltage of each unit in the cascade will degrade the quality of the output waveform.Therefore,this thesis takes the modulation and DC voltage control strategy of the H-bridge cascaded photovoltaic inverter as the research goal.This thesis first briefly describes the research background and significance of photovoltaic inverters,describes the basic principles of the maximum power point tracking technology,control strategy and modulation strategy of the cascaded inverter,and summarizes the advantages and disadvantages of various methods and the latest Technological development.Then take the single-phase three-unit cascaded H-bridge photovoltaic inverter as the research object,analyze its topological structure and working principle,adopt the variable step-length conductance increment method as the maximum power tracking control method,and the voltage and current double closed loop System control structure.Aiming at the power imbalance between the units of the cascaded photovoltaic inverter and its adverse effects on the output waveform,based on the single-phase SVM modulation method,this thesis proposes a space vector modulation method based on capacitor voltage prediction and least squares method.According to the structure of the inverter’s DC side circuit,the principle of capacitance voltage change is analyzed,and the capacitance voltage predictive control model is established.The least square method is used to get the vector selection function,and then the level numerical order method is used to get the optimal vector,thereby combining the waveform modulation and the DC voltage control to form a SVM method that can realize the DC capacitor voltage control.This method not only has low output harmonics,but also has a fast response to DC voltage control,and the adjustment of the unit DC voltage is independent of the voltage and current double-loop control,which simplifies the control system structure.Finally,this thesis uses Matlab/Simulink simulation to prove the control effect and feasibility of the method.An experimental platform for a single-phase three-unit H-bridge cascaded photovoltaic inverter was built on a HIL-based real-time simulation platform.Experiments have proved that this method can quickly converge the DC voltage of each unit,and at the same time have high-quality output waveform.