Research On Maximum Power Point Tracking And Grid-connected Strategies Of Photovoltaic Power System

In the grid-connected photovoltaic system,the two-stage grid-connected structure is widely used,and its pre-stage Boost circuit is mainly used for maximum power point tracking control,and the latter stage plays an indirect role in grid connection.However,the traditional Boost circuit has disadvantages of low boost gain and large output current ripple.The more commonly used maximum power point tracking(MPPT)techniques such as perturbation observation method cannot avoid the occurrence of oscillation and misjudgment.And the effect of grid connection at the later stage depends on the performance of the controller.Based on these problems,the DC-DC circuit topology,maximum power point tracking(MPPT)algorithm and grid-connected control strategy of two-stage photovoltaic power generation grid-connected system are studied to improve the performance of each part.Specific content includes:1.The two-stage structure is determined as a photovoltaic power generation system structure,and a Switched-Inductance-Parallel-Staggered Boost circuit with high gain and low output ripple is provided.Based on the classification of the photovoltaic power generation system structure and analysis of the grid-connected photovoltaic power generation system,the advantages of the two-stage structure as the main topology are discussed.On the premise of theoretically deriving the Boost circuit's boost and output ripple performance,simulations are used to verify the advantages of the Switched-Inductance-Parallel-Staggered Boost circuit's boost boost and output ripple.2.The mathematical model of photovoltaic cells is deduced and a simulation model is established.The output characteristic curve of the constructed photovoltaic cell is given.Based on the analysis of the principle of photovoltaic cell output power regulation,the advantages and disadvantages of several commonly used maximum power tracking techniques are compared.A small signal model of the Boost circuit is established,and the perturbation interval time and the perturbation step size are selected for the perturbation observation method.The minimum perturbation interval and the minimum perturbation step length for the stable operation of the conservation-preserving disturbance observation method are given,and the simulation results are used to verify the conclusions.To avoid turbulence at the maximum power position by disturbance observations,at the same time eliminating misjudgments in the tracking process.An Adaptive-Step-Hysteresis-Perturbation-Observation method which combined the perturbation observation method with the hysteresis comparison method is given,and the advantages of the method are verified by simulation.3.The grid-connected structure of the full-bridge inverter circuit combined with the LCL filter is established,and the LCL filter is modeled.Based on this model,the voltage outer loop and PI,PR current inner loop are designed respectively.In order to eliminate the resonance jump of the LCL filter itself,based on the advantages of the two current loops of PI and PR,a PI-PR current loop design method based on capacitive current active damping is presented.The simulation shows that this method has better harmonic suppression ability.4.Combining the design goals of the photovoltaic grid-connected system,the parameters of the pre-stage Boost circuit and the parameters of the post-stage LCL filter are designed,and the switching devices of the circuit involved are selected,and the experimental prototype is built for the grid-connected experiment.The experiment verifies that the PI-PR current loop based on capacitive current active damping has good grid-connected current control performance.