Research On Non-isolated Mid-point Clamp Photovoltaic Grid-connected Inverter

The contradiction between the increasing energy demand and the decreasing traditional fossil energy and the resulting environmental problems has become a powerful driving force for the development of new energy,and photovoltaic power generation has become one of the main development trends of new energy.As the conversion device between photovoltaic power generation system and AC power grid,the efficiency,reliability and safety of photovoltaic inverter will have a great impact on the overall performance of grid-connected power generation system.At present,nonisolated photovoltaic grid-connected inverter has many advantages,such as light weight,small volume,low cost,low loss and so on.It is very suitable for medium or small power load sites.However,the non-isolated photovoltaic grid-connected inverter does not have an electrical isolation device,so it will produce common-mode leakage current,endangering the safety of personal equipment and so on.Based on this,for the non-isolated photovoltaic grid-connected inverter,this paper has carried on the thorough research,the concrete content is as follows:Firstly,the composition principle of isolated photovoltaic grid-connected inverter and non-isolated photovoltaic grid-connected inverter is analyzed,and their advantages and disadvantages are compared.Secondly,the common-mode leakage current problem of non-isolated gridconnected inverter due to its own structure is analyzed,and the common-mode leakage current analysis model of photovoltaic grid-connected inverter system is established and a variety of solutions to suppress the leakage current are given.Thirdly,through the analysis and summary of many types of typical non-isolated photovoltaic inverters,it is found that the power switching capacitor will produce additional common-mode leakage current in the non-midpoint clamp inverter topology.Therefore,a new type of non-isolated midpoint clamp photovoltaic grid-connected inverter is proposed,which satisfies the electrical isolation between the photovoltaic side and the power grid side in the current continuation stage,and the ground voltage at the midpoint of the output arm is clamped to half of the DC bus voltage through the clamping unit.Thus,the voltage suspension problem of common mode voltage at zero level output is solved,the influence of switch junction capacitance is eliminated,the common mode voltage is constant,and the common mode leakage current of the system is suppressed.Fourth,the photovoltaic grid-connected inverter system based on the proposed inverter topology is established.According to the performance index of the system,the parameters of the power switch tube and the output filter are analyzed in detail,and the control strategy of the photovoltaic system is introduced.The voltage and current double closed loop control mode is adopted,that is,the proportional integral control(Proportional Integral,PI)and voltage feedforward control strategy are adopted in the grid-connected current inner loop,and the PI control strategy is used in the DC voltage outer loop control.Finally,based on the above theoretical analysis,the proposed new topology is simulated and verified by experiments,and good simulation results and experimental results are obtained,and the performance indexes are also in accordance with the theoretical analysis results in the previous paper.The correctness of the proposed topology and good common mode leakage current suppression ability are proved.