Research And Implementation Of Two-Stage Gird-Connected Single Photovoltaic Inverter

As an interface equipment of renewable energy and grid,the grid-connected inverter will gradually occupy a dominant position in the future electric system with the gradual improvement of the permeability of renewable energy in electric system,which directly affects the utilization efficiency of renewable energy.Compared with traditional L filter,the LCL filter has better attenuation capability of high-frequency harmonic.Under the same filtering standard,the LCL filter requires lower inductance value,which can reduce the volume and weight of grid-connected device and reducing power loss and cost.The improved control strategy and hardware implementation of a two-stage grid-connected single photovoltaic(PV)inverter with LCL filter were mainly researched in this thesis.The topology of a two-stage grid-connected single PV inverter,including DC/DC conversion stage,DC/AC conversion stage and the mathematical models of them were analyzed at first.The control strategy of the DC/DC conversion stage and the DC/AC conversion stage of grid-connected single PV inverter was designed and improved,which solves the misjudgment of maximum power point tracking(MPPT)algorithm,the distortion of PCC point current,the slowly dynamic response of grid current and so on.The specific research contents are as follows:(1)The improved variable step size perturbation and observation method was used in the DC/DC conversion stage.Based on the traditional perturbation and observation method,the steady-state oscillation criterion,variable step size disturbance factor and dynamic voltage threshold were introduced.In order to reduce the oscillation of PV system near the maximum power point(MPP),the steady-state oscillation criterion and variable step size disturbance factor were added to detect the duty point of PV real-time and adjust the step size of the disturbance.In order to prevent misjudgment of the algorithm in the case of sudden light intensity mutation,dynamic voltage threshold was added to control the operating range of PV to ensure that the system can quickly adjust back to the new maximum power point when misjudgment occurs.(2)The double-closed loop control of current based on capacitive current feedback was usually used in the DC/AC conversion stage.The capacitive current feedback can eliminate the resonance peak of LCL filter effectively.However,the distortion of the grid-connected current was especially obvious when considering the background harmonics of actual grid voltage.Therefore,based on the double-closed loop control of current based on capacitive current feedback,a grid voltage feedforward control method was designed in this thesis,which can reduce the influence of grid side voltage on the grid current,and improve the power quality of the grid current and the robustness to the grid voltage.(3)The changing information of the output power of PV cannot be feedback to current inner loop directly under traditional single voltage outer loop.In order to improve the tracking speed of grid-connected current in the case of changed output power of PV and reduce the response time,the power feedforward control was proposed based on the traditional voltage outer loop to reduce the steady-state error of the DC bus voltage and improve the response speed of the system.Finally,an experimental platform of 5kW two-stage grid-connected single PV inverter was built.The software and hardware of the system were then designed separately.Simulation and experimental results verified the effectiveness of the controller designed in this thesis.