Research On Direct Power Control Of Photovoltaic Grid-connected Inverter

Direct power control(DPC)is widely used in photovoltaic grid-connected inverter,but its control effect is limited by the hysteresis band,its variable switching frequency affects the filtering effect of the filter,and it is sensitive to the existence of harmonic or change of frequency in grid.Model Predictive Control(MPC)has been paid more and more attention due to its excellent ability of dealing with constraint conditions.Therefore,with the support of "2011" Hubei Collaborative Innovation project,based on the direct power control of photovoltaic grid-connected inverter and combined with the Model Predictive Control technology,the Model Predictive Direct Power Control(MP-DPC),the Space Vector Modulation Model Predictive Direct Power Control(SVM-MP-DPC),and the Virtual Flux-based Model Predictive Direct Power Control(VF-MP-DPC)of photovoltaic grid-connected inverter are researched in this thesis,and effectiveness of these method is verified by simulation.In addition,the prototype is constructed based on the experimental platform of the wind/solar hybrid generation system,control effect and effectiveness of MP-DPC method is validated experimentally.The main work accomplished and the results in this study are as follows:(1)The structure of photovoltaic grid-connected inverter is studied and its mathematical model is constructed.Firstly,the structure of the two-stage power-frequency isolated photovoltaic grid-connected inverter is simplified.Then,the mathematical model is established according to its topology structure,which lays the foundation for the research and improvement of DPC method of the photovoltaic grid-connected inverter;(2)The MP-DPC method is studied.Firstly,the prediction model of MP-DPC for photovoltaic grid-connected inverter is established.Then,one-step and two-step prediction of MP-DPC algorithm are studied,and an improved two-step prediction algorithm is developed to simplify the complex computation due to the predicted depth.Finally,MP-DPC method is verified in simulations and experiments,and the simulation results indicate that the switching frequency of two-step prediction MP-DPC is lower than DPC and is half of the one-step predicted MP-DPC.(3)The SVM-MP-DPC method is studied.Firstly,the prediction models of SVM-MP-DPC of photovoltaic grid-connected inverter in ?? and dq coordinate system are established and compared.Then,the SVM-MP-DPC method is simulated and analyzed.The fixed switching frequency control is realized.(4)The VF-MP-DPC method is studied.Firstly,the Virtual Flux estimation is improved and verified by simulation.Then,the prediction model of VF-MP-DPC method is established.Finally,the simulation and analysis of VF-MP-DPC method are carried out.The simulation results verify the effectiveness of the VF-MP-DPC method.In summary,average switching frequency is reduced in proposed MP-DPC method of the photovoltaic grid-connected inverter;SVM-MP-DPC method operates with fixed switching frequency;the suppression of gird frequency changes and harmonic are achieved as well as fixed switching frequency in VF-MP-DPC.At the same time,the unit power factor inverter is obained,the grid requirements of THD is met.