Research On Power Balance Control Strategy Of Single-phase CHB Grid Connected Photovoltaic Inverter

In recent years,with the increasing depth of global research on new energy power generation technology,the proportion of photovoltaic power generation has been greatly improved.This has greatly promoted the research of photovoltaic grid-connected inverters,not only many new photovoltaic grid-connected inverter topologies and grid-connected structures,but also novel control strategies have emerged,and related research has effectively improved the power generation efficiency and system operation stability of photovoltaic systems.Among them,Cascaded H-Bridge(CHB)multilevel inverter is particularly outstanding in photovoltaic power generation,with independent MPPT control,easy expansion and no transformer advantages.However,due to the uncertainty of light and ambient temperature,the study of the power imbalance of CHB photovoltaic grid-connected inverters is becoming more and more important.This paper takes single-phase CHB photovoltaic grid-connected inverter as the research object,focuses on inverter loop control and power balance control,and conducts indepth and comprehensive research,the main research content is as follows.Aiming at the problem of photovoltaic cells affected by light intensity and ambient temperature,the equations of the basic parameters of photovoltaic cells and these two influencing factors are obtained by establishing their mathematical models,and the calculation of important parameters in the equations is analyzed.For the quantitative analysis of photovoltaic cells affected by these two factors,a photovoltaic cell simulation model is established in MATLAB/Simulink,and the output characteristic curve of photovoltaic cells when the light intensity or ambient temperature changes is obtained by using the control variable method.By establishing a mathematical model of the single-phase CHB photovoltaic grid-connected inverter system,the key problems of inverter control are analyzed.Aiming at the loop control problem of single-phase CHB photovoltaic grid-connected inverter,this paper adopts independent MPPT control to improve the power generation efficiency of photovoltaic panels;The voltage control loop adopts the traditional PI controller to control the DC side voltage as a whole,which not only maintains the stability of the DC side voltage,but also simplifies the control structure and reduces the amount of calculation.For the current control loop,the control effect of the two controllers was analyzed,and the PQR controller was selected and the influence of its control parameters and parameter calculation method were analyzed.For the shortcomings of traditional voltage and current dual closed-loop control,a sample-and-hold holder is added to increase the voltage loop bandwidth,a better SOGI-PLL phase-locked loop is used to improve the phase angle detection accuracy and speed,and according to the grid-connected current transfer function,the grid voltage is fed forward to eliminate its influence on the grid-connected current.Aiming at the power imbalance problem of single-phase CHB photovoltaic grid-connected inverter,the mechanism of system power imbalance and the modulation problem that may occur in system operation under power imbalance conditions are first analyzed,and three power balance control strategies are proposed on this basis.Among them,the power balance control strategy based on duty cycle correction corrects the duty cycle signal of each H-bridge unit according to the output voltage of the DC side photovoltaic panel of each H-bridge unit,in the case of mismatch between the output power of the DC side and the AC side of each unit,the inverter can still operate normally,but this control method is only applicable to the case that the adjustment system of each H-bridge unit is less than 1,and the power imbalance exceeds a certain limit and cannot play a control effect;The power balance control strategy based on third harmonic compensation can cope with the power imbalance problem of overmodulation of the H-bridge unit,compensate for a quantified third harmonic on the modulation unit modulation wave to reduce the modulation amplitude,but this control strategy can only expand the modulation range of the H-bridge to 1.155;the power balance control strategy based on the hybrid modulation technology combines the advantages of square wave and PWM wave to expand the H-bridge modulation range to 4/π,which can cope with the more severe power imbalance problem.Finally,the corresponding hardware platform is designed and built,and the feasibility of the proposed method is verified by experiments.