Research On Improved Space Vector Modulation Method For Single-phase Cascaded PV Inverter

Environmental problems and the exhaustion of traditional fossil energy have promoted the development of new energy.Solar energy has received more and more attention as a non-polluting and resource-rich new energy source.Compared with several other multilevel inverters,cascaded multilevel inverters have the advantages of high efficiency,less harmonics,and modularization.Based on these advantages,the paper adopts the grid-connected system of cascaded PV inverters as the research object,and researches on the modulation method.The paper first briefly describes the current energy situation and photovoltaic technology,the control strategy of grid-connected inverters and the working principle of cascaded inverter modulation methods,and summarizes the advantages and disadvantages of various control strategies.Then the working principle of the H-bridge cascaded inverter and the maximum power point tracking(MPPT)technology is described in detail.There is power imbalance between the cells of the single-phase cascaded inverter.The general modulation method needs to add power balance control to restrain the imbalance,control the DC voltage,and make the control system complicated.An improved space vector modulation(SVM)is proposed to solve the power imbalance among the cells of cascaded PV inverters,and the quality of output waveform of traditional carrier phase shift modulation method caused by imbalanced DC voltage of each cell.The modulation method combines the DC voltage control of each cell with the modulation method,and considers the actual situation of DC voltage imbalance among each cell in the vector selection,and does not need to additionally increase the power balance control algorithm,thereby simplifying the control system.The modulation method is improved on the basis of space vector modulation.The level vector ranking method is proposed to narrow the space range of the optimal vector selection,the suboptimal vector can be found by properly expanding the vector set without the optimal vector,so that the optimal vector combination can be obtained.The improved space vector modulation can not only make the DC voltage rapidly track the DC voltage instruction of each cell through vector selection,but also ensure the quality of output voltage waveform when the DC voltage of each cell is imbalanced.The single-phase cascaded PV inverter adopts a dual closed-loop control strategy.The current loop adopts repeated control,which realizes non-error control of the photovoltaic system,improves the stability of the system.The voltage outer loop takes the sum of the difference between the output voltage instruction and the DC voltage sampling value given by each cell MPPT module as the input,and adopts PI control.The power balance control is no longer needed,which reduces the operation of the control system and improves the stability of the PV cascaded inverter.Finally,a single-phase three-cell cascaded PV inverter model is built in this paper.The various simulation comparisons verify that the improved space vector modulation method can make the DC voltage converge rapidly and ensure the quality of the output voltage waveform when the DC voltage of each cell is imbalanced.