Research On Control Strategy Of Double-stage Grid-connected Photovoltaic Converter

In order to solve continuous consumption of conventional energy and a series ofproblems caused by its using, the energy structure has to gradually make a strategictransition from “primarily in fossil energy and supplemented by clean energy” to“predominantly in clean energy and accompanied by fossil energy”. As one of the effectivemeasures to deal with energy problems, the grid-connected photovoltaic generation hasbecome a research hotspot in both domestic academics and overseas’, and it also widelyused in many countries worldwide. In this thesis, the MPPT control strategy which enablesthe system to track the maximum power point and the LCL filter-based grid-connectedcontrol strategy are investigated for the double-stage grid-connected PV converter.Firstly, this thesis analyzes a variety of environmental problems caused byconventional energy usage, and summarizes the development status of grid-connected PVsystem at home and abroad, topologies of the system in common use at present andexisting control techniques of PV converter.Secondly, the structure and working principle of a double-stage grid-connected PVsystem are presented, the type selection of PV module and the design of PV array areperformed, and the output characteristics of PV array is analyzed based on the engineeredmathematical model of PV cell; the working principle of Boost circuit and DC/AC as wellas its modulation are illustrated briefly, the characteristics of LCL filter is analyzed, and themain parameters of the circuit are designed. The transmission lines of grid-connected PVsystem are introduced briefly.Thirdly, a MPPT control strategy, which possesses the structure of power-voltagedouble closed-loop and is based on the gradient is put forward for the Boost converter infront stage. The proposed strategy directly starts from the power-voltage characteristics ofPV array and gives the voltage corresponding to the maximum power based on its gradient,thus it achieves MPPT automatically by closed loop voltage control. It has someadvantages of clear function hierarchy, clear physical conception, easy parameteradjustment, and perfect capacity of disturbance elimination.Fourthly, based on the conventional vector control strategy, the capacitor currentfeedback technique of the active damping control is introduced, and a power decoupling control with active damping for the inverter with LCL filter in back stage is given toachieve both the constant DC-side voltage control and unity power factor control. Theproposed strategy has some merits of simple structure, clear physical conception, and easyimplementation, and it also increases the damping without adding any power losses so asto improve the stability of the system.Finally, the simulation model for a double-stage grid-connected photovoltaic powergeneration system of10kW is built up, and the simulation results show that in the cases ofillumination intensity mutation and natural illumination, the system can track illuminationintensity changes quickly, and achieve both maximum power point tracking and unitypower factor control, so that the system gets a good dynamic regulating performance andstatic characteristics.