Leakage Current Suppression Strategy Of Non-Isolated Three-Level Grid-Connected Photovoltaic Inverter

In order to achieve the goal of “peak carbon dioxide emission and carbon neutrality”,the proportion of renewable energy generation connected to the grid is increasing year by year,which dominates the photovoltaic power generation.Non-isolated photovoltaic grid-connected systems are widely used due to their small size and low cost.However,lack of electrical isolation leads to the formation of conduction interference loops,generating common mode currents,that is,system leakage currents,reducing system stability,increasing system losses,and even threatening personal safety.To this end,the following works are carried out.First,a mathematical model of the three-level inverter photovoltaic grid-connected system is established to analyze the causes and factors that influence the leakage current.The algorithm cost function is improved,and the model prediction direct power control strategy is constructed to accommodate leakage current suppression.The common-mode voltage amplitude control term and the rate of change control term are incorporated into the cost function to compare with the traditional model prediction direct power control strategy on the suppression of leakage current and to verify and analyze the effect of common-mode voltage on leakage current.Second,the three-level four-leg inverter structure is used to further improve the performance of the grid-connected system.To avoid the problem of difficult selection of weight coefficients due to too many objective terms in the cost function,a fourth leg is added to constant common-mode voltage fluctuations for the purpose of leakage current suppression.To address the problems of high number of the switching states of the three-level four-leg inverter,the difficulty of algorithm design and the large computational effort of traversal search,an improved model prediction direct power control strategy is proposed to achieve the control objectives of neutral potential control,leakage current suppression,and low complexity of the algorithm,and simulations are performed to verify and analyze its effectiveness.Third,three-level four-leg inverter topologies are analyzed and the low-cost optimization improvement strategies are proposed.Mathematical models are established for different optimized topologies,and equivalent circuit models are constructed.With the total harmonic distortion rate of grid-connected current and leakage current amplitude as the key control indexes,the advantages and disadvantages of each topology are compared and analyzed,and the optimal topology of leakage current suppression is proposed.Fourth,simulation experiments are carried out,using the same model predictive direct power control strategy with leakage current suppression for different three-level four-leg inverter topologies.The key component parameter is obtained by fitting functions and nonlinear programming.The performance advantages and disadvantages of each topology are analyzed,and the optimal leakage current suppression strategy for the non-isolated three-level photovoltaic grid-connected inverter is obtained by comparison.Also,the effect of grid-connected output and the effect of suppression of leakage current of the proposed strategy in the case of parasitic capacitance variation are analyzed to verify the effectiveness and practicality of the proposed topology and control strategy.