Research On Modulation Technology Of Transformerless Three-Phase Photovoltaic Inverter Based On Leakage Current Suppression

Transformerless photovoltaic systems have been widely used due to small size and high efficiency.However,because of the lack of an isolation transformer,the system will generate leakage current,which poses a threat to the surrounding electrical equipment and personal safety.This thesis mainly studies transformerless photovoltaic inverters,focusing on solving the leakage current and unbalanced neutral point potential problems of two-level three-phase four-leg inverters and three-level three-phase four-leg inverters to ensure the safe and reliable operation in transformerless photovoltaic power generation systems.First,for the two-level three-phase four-leg inverter,the common-mode loop model is established,and the common-mode voltage characteristics under different switching states are deduced.The the existing modulation strategy of leakage current suppression based on space vector modulation and carrier modulation is studied,and its working principle and performance characteristics are analyzed.Based on this,an improved carrier modulation strategy is proposed by selecting different carrier forms reasonably.Compared with the existing modulation strategy of leakage current suppression,it has better leakage current suppression effect,does not increase additional switching losses,and has higher utilization rate of DC voltage and the characteristics of clear principle and simple implementation.Finally,the correctness of the theoretical analysis is verified by simulation.Secondly,for the three-level three-phase four-leg inverter,its working principle and common-mode voltage characteristics are analyzed.For the leakage current problem of the system,the working principle and performance characteristics of the traditional modulation strategy and the existing leakage current suppression modulation strategy are analyzed.A sawtooth wave combination modulation method for suppressing leakage current is proposed.The carrier adopts upward and downward sawtooth waves respectively.Compared with the existing modulation strategies,it has better leakage current suppression effect,lower switching loss and higher DC voltage utilization.Thirdly,in view of the unbalanced neutral point potential of the three-level three-phase four-bridge inverter,the reasons for the fluctuation of the neutral point potential,the existing hazards and the influence on the leakage current are analyzed.On the basis of the sawtooth wave combination modulation strategy,the neutral point potential balance control is proposed,and the neutral point potential balance is realized by injecting an appropriate zero-sequence component,which further improves the suppression effect of the leakage current.The correctness of the theoretical analysis is verified by simulation.Finally,a two-level three-phase four-leg inverter is built,and comparative experiments are carried out to verify the performance advantages of the improved carrier modulation strategy;an experimental platform for a three-level three-phase four-leg inverter is built to verify the advantages of sawtooth wave combined modulation,the effect of the neutral point potential balance control strategy and the influence of the neutral point potential fluctuation on leakage current.