Research On Control System Of Photovoltaic Grid Connected System

The problem of depletion of energy and the global environmental pollution ismore and more severe, today. The exploitation and utilization of solar energy attractspeople’s attention all over the world. Photovoltaic power generation is an importantform of solar energy utilization, has widely applicable prospect and the researchsignificance. Taking photovoltaic grid-connected generation system as the researchobject, and is carried out with depth research in this dissertation.The main contents and contributions of this dissertation are as follows:1. This dissertation describes the structure, principle, operating characteristicsof photovoltaic cells and analyzes the MPPT principle of the two stagesgrid-connected PV systems. On the basis of analyzing the common maximum powerpoint tracking algorithm, an improved variable step-size incremental conductance isproposed, and validated by simulation.2. According to the three-phase grid-connected converter circuit structure, themathematical model is deduced. The converter vector control method is analyzed. Theinstantaneous power mathematical model of three-phase voltage-source converter isestablished based on instantaneous power theory, analyzed the cross-coupling effectsof instantaneous power on the power change rate. A novel direct power control ofthree-phase grid-connected converter for suppressing power cross-coupling effects isproposed. The two control strategy of three phase grid-connected converter arecompared and analyzed, and verified by the simulation and experiment. Thesimulation and experiment results shows that vector control has a better steady-stateperformance than direct power control presented in the thesis, direct power controlhas a better dynamic performance than the vector control.3. Because the switching state of three-phase converter is limited, the modelpredictive control is applied to it. The active and inactive components of line currentare used as the target variables to construct the value function. Through the onlineprediction of the goal, the optimal switch state is selected and the trigger pulse is directly output. Compared with vector control it has the characteristics of simplecontrol structure and without additional modulation technology. The simulation andexperimental research of model predictive control is carried out. The results show thatthe MPC has better dynamic performance than the vector control and its outputcurrent harmonic content is bigger than the vector control.4. For single-phase grid-connected converters, a virtual quartered current andvoltage are constructed using second-order integrator based on the actual single-phaseinput current and voltage. Based on these electric parameters, a αβ coordinate systemis formed. The vector control principle is applied to single-phase grid-connectedconverter. The instantaneous power model of dq rotating coordinate frame wasderivate and analyzed based on instantaneous reactive power theory and predictionmethod. A predictive power control strategy based on this instantaneous power modelis presented. The two kinds of control strategy are verified by simulation andexperiment. The results show that the two kinds of control methods can achieve unitypower factor and reactive power compensation, the steady state performance of thevector control and the predictive power control is closed, the predictive power controlhas better dynamic performance than the vector control.5. On the analysis of the unbalanced grid voltage, the control scheme forremoving two times power grids frequency fluctuation of active power is derived. Thesimulation research of the control strategy is carried out. The results show that thismethod can effectively restrain fluctuation of the active power, improve steadyperformance of grid-connected system under the condition of unbalanced gridvoltage.6. On the basis of the system hardware and software design according to thesystem requirements, experiment platform is built based on XC2785of the infineoncompany and debugging the system. Three-phase and single-phase grid-connected PVsystems are realized.