Research On Single-Phase Two-Stage Non-Isolated Photovoltaic Grid Connected Inverter

Solar energy with its advantages of cleanliness,pollution-free and sufficient reserves,has been researched and developed by various countries among many new energy sources.Photovoltaic power generation uses sunlight to form direct current(DC)signals on solar panels,and outputs appropriate alternating current(AC)signals through the inverter circuit to be incorporated into the power grid to generate electricity.As the core part of the inverter circuit,its topology and control method determine the efficiency,cost and quality of photovoltaic grid connected power generation,which will be applied on a large scale in the future.At present,non-isolated photovoltaic grid connected inverter has the advantages of small space and high efficiency,and is becoming the main research direction.However,the lack of isolation leads to the common mode leakage current,which affects the photovoltaic grid-connected system.To solve this problem,this paper proposes an improved mid-point clamped topology,which includes the following contents:Firstly,the mechanism of common mode leakage current is summarized,and the theoretical conditions for restraining common mode leakage current are obtained.Without considering other parasitic capacitances,the influence of the two most important parasitic parameters of photovoltaic parasitic capacitance to ground and filter inductance on the common mode leakage current is analyzed by frequency domain analysis method.Taking the traditional single-phase full bridge topology as an example,the magnitude of common mode leakage current under two polarity sinusoidal pulse width modulation(SPWM)strategies is compared and analyzed through simulation,and the theoretical conditions for restraining common mode leakage current are verified,which provides a basis for the modulation strategy used in the improved topology proposed below.Secondly,the working principles of three traditional topologies are analyzed,and the influence of switch parasitic capacitance on common mode leakage current in traditional topology is deduced through mathematical formula.In view of this influence,an improved neutral point clamped topology is proposed,which can maintain the same low input voltage as the full bridge inverter,solve the problem of output voltage oscillation of common mode voltage in freewheeling mode,ensure that the voltage in freewheeling path is clamped to half of the input voltage during freewheeling,and effectively suppress common mode leakage current.Then,for the proposed improved topology,six working modes and voltage clamping analysis are carried out under the modulation strategy determined in Chapter 2.The simulation model is built to verify the influence of the parasitic capacitance of the traditional topology switch on the common mode leakage current and the advantages of the improved topology compared with the traditional topology.Finally,based on the improved topology,through the calculation of boost circuit parameters,filter circuit parameters and the analysis and selection of system control principle,the control framework and overall simulation model of single-phase two-stage non isolated photovoltaic grid connected system are established.When the ambient temperature remains unchanged,the illumination intensity is normal and the illumination intensity changes suddenly,the correctness of the control principle and the stable operation of the system simulation model are verified.