| From the domestic and foreign research status,new energy,especially solar energy gradually become the research focus of distributed generation.A large number of the distributed energy merge into the power grid through power electronic equipment,affecting the quality of the grid.The research on how to improve the power quality and conversion efficiency of grid-connected inverter as an important part of energy conversion is a hotspot of technology.It is the main research direction to improve the conversion efficiency and the power quality by developing the topology of PV grid-connected inverter,the suitable modulation method and control strategy.In this paper,the more in-depth theoretical analysis and simulation experiments of the key issues,such as voltage interference suppression,inverter topology and harmonic suppression of the photovoltaic grid-connected inverter are made.The main contents are as follows:(1)The performance of passive filter with different structure is analyzed.The resonance suppression problem using LCL filter and the complexity of the control system structure are studied.The stability of grid-connected inverter and the quality of output power is studied.In order to solve the problem of low-frequency harmonics caused by voltage distortion,a new control strategy is proposed based on the double-closed-loop control strategy with LCL filter.By using the mathematical variable elimination method in control theory,the mathematical model of the proposed control strategy is established.The parameters are arranged by Bode diagram and mathematical analysis.The correctness and optimization of parameter setting and grid disturbance suppression are proved by simulation model and experiment.The method can not only eliminate the interference from the low-frequency harmonics caused by the change of the grid voltage,but also greatly reduce the THD of the grid-connected current,improve the quality of the grid-connected power,and reduce the order of the high-order control system,and optimize the system structure parameters.(2)A novel multilevel inverters topology with simple structure is studied,which is the combination of the interleaved parallel technique and the coupling magnetic circuit structure based on a commom two-level half-bridge inverter.The topology and working principle of the novel multilevel PV inverter are analyzed in detail.In order to prove that the topology can improve the system efficiency and cost,the state,stress and loss of the power switching devices in the new multilevel topology are analyzed.The SPWM modulation strategy is adopted to reduce the influence on the DC ripple,and the open-loop control strategy is used to build the simulation and experiment platform,and the stability and feasibility of the system are analyzed.(3)The mathematical model of the novel multi-level inverter topology is established.Based on the analysis of the circulating current under different conditions,the power current and the circulating current have different impedance characteristics through mathematical analysis and deduction.In order to reduce the harmonic interference of the novel multilevel inverters,the traditional PR regulator is proposed to suppress the harmonic interference.The simulation and experiment results show that the proposed method can reduce the Low-frequency output current,improving the output current quality.However,the traditional control method can not effectively eliminate the cross circulating current caused by the couple inductor,an optimal hybrid control method is proposed,Simulation and experimental results show that the proposed method can suppress the circulating current effectively,reduce the THD of the output current,and improve the output power quality. |
