Research On Topology And Control Of Transformerless PV Grid-Connected Inverter

The shortage of fossil energy and the problem of environmental pollution are important issues for economic and energy development.Under the direction of cleangreen and high-quality development,the use of renewable energy for power generation can solve the environmental problems brought by traditional power generation.Among them,solar energy is one of the most valuable renewable energy sources,and photovoltaic power generation has become the focus development direction in China.As an energy conversion device of photovoltaic power generation system,the efficiency of grid-connected PV inverter is the key to the overall efficiency.Compared with traditional inverters,transformerless PV inverters have the advantages of high efficiency,low cost and small size.However,the high frequency operation of the switch tube will cause the common mode voltage to change on the photovoltaic modul to the ground,thereby generating leakage current,affecting power quality and operational safety.In the research of single-phase inverter topology,it is a common and effective measure to improve the topology that can suppress the leakage current.The improved topology is achieved by adding an auxiliary switch and matching the corresponding modulation method,which can stabilize the common mode voltage and suppress leakage current.There are three commonly used topology improvement methods:DC decoupling,AC decoupling,and midpoint clamping.According to different improvement methods,the existing topologies are classified,and the working principle and power loss of different inverter topologies are analyzed.Aiming at the point voltage suspension problem in DC decoupling and AC decoupling topology,this paper proposes a new T-type active midpoint clamped PV inverter topology TAC-Heric(T-type Active Clamp Heric),and introduces four Working mode under SPWM in detail.The new topology is verified to be more effective in suppressing leakage current through simulation.In order to make the new TAC-Heric topology safe and efficient,the PI+QPR composite control strategy is proposed.This control can achieve no static tracking and control the DC signal well,which makes up for the defects of existing PI control and PR control strategies.Finally,according to the newly proposed TAC-Heric inverter grid-connected system,inverter parameters calculation,component selection and program design are performed.Based on the TAC-Heric topology,A simulation model of inverter was built in Matlab/Simulink,and the control system of single PI control,PR control and PI+QPR composite control strategy wassimulated and analyzed respectively.The comparison simulation results show that the PI+QPR controller can improve the tracking ability of grid-connected current,and the ability of resisting frequency disturbance of power grid,and the ability of restraining low-frequency DC component.The results show that the new T-type active midpoint clamped PV inverter topology TAC-Heric has high efficiency and low leakage.Under the control of PI+QPR composite control strategy,it can track the grid without static difference and grid-connected current.The harmonics are small,achieving safe and reliable grid connection.