Research On Grid-Connected Control And Secondary Ripple Suppression Technology Of Two-Stage Photovoltaic System

At present,our country is facing the shortage of fossil fuels and severe environmental problems,solar energy has been widely used as a rich green new energy,and the photovoltaic industry has been vigorously developed.In order to further improve the utilization of solar energy and improve the power quality of grid-connected,taking the two-stage single-phase photovoltaic grid-connected system as the research object for the application scenarios of small and medium power levels,this thesis analyzes and studies the grid-connected control technology and the secondary ripple problem.Firstly,analyzed the composition of the two-stage photovoltaic grid-connected system and studied the control technology of the entire system.Through the analysis of the working principle of the photovoltaic panel,the front-stage active clamp flyback circuit and the rear-stage dual-buck circuit of the grid-connected inverter,and the filter,the control of the maximum power of PV panel output,DC bus voltage control and grid-connected current control are studied,and the phase-locked loop control is used to realize the unit power factor grid-connected.Secondly,in order to improve the performance of deadbeat control strategy and reduce its sensitivity to parameters change,an improved deadbeat control strategy based on uncertainty and disturbance estimation is proposed.By analyzing the influence of lag one beat delay and model parameters perturbation under the deadbeat control on system performance,a two-step prediction method is adopted to compensate for the control delay;the change of model parameters is regarded as a disturbance to the system,the inverter output voltage is corrected to reduce the prediction current error,thereby improving the robustness of the system to model parameters change.Through simulation and comparative analysis,it is verified that the proposed strategy improves the control performance of the system,and reduces the THD of the grid-connected current when the model parameters change.Then,aiming at the problem of secondary ripple in single phase grid-connected inverter,a compound control strategy combining feedforward and feedback is proposed.Analyzed the generation and propagation mechanism of the ripple,according to the established small signal model of the DC-DC stage active clamp flyback circuit,the transfer function of the reverse current was derived,using secondorder general integrator to realize the extraction of the double frequency component of the converter output current.Under the premise of without changing the circuit topology,the forward propagation of the secondary ripple current is suppressed through the optimization of the control structure.Simulation results show that the proposed control strategy effectively reduce the secondary ripple content in the converter input current.Finally,a hardware-in-the-loop simulation experiment environment based on RT-LAB and a photovoltaic grid-connected system experimental platform based on the DSP-TMS320F28035 controller are built,and the control strategy proposed in this thesis is experimentally verified.