| Photovoltaic DC microgrid is a system converting solar energy to quasi DC by photovoltaic panels, directly transferring the microgrid power energy to users. There are complex nonlinear phenomena in photovoltaic DC microgrid, one of which is chaos.The system cannot normally work or be out of control when it is in the chaos state.There are few references on the study of chaos phenomena and chaotic control in photovoltaic DC microgrid so far, it is of significance practice to study the chaotic theory in photovoltaic DC microgrid, and to promote the application in distributed photovoltaic system.The common low-order DC-DC converter in photovoltaic DC microgrid has been studied in this dissertation. Firstly, the state equations of voltage-mode Buck,current-mode Boost and Buck-Boost converter under the continuous current mode(CCM) are derived respectively, the piecewise switch model and discrete iterative mapping model are also set up. Then the chaotic phenomena in the three converters are analyzed respectively by means of bifurcation diagram, phase diagram, poincare section,inductor current waveform and output voltage waveform. A Method to identify the state of chaotic phenomena in DC-DC converter based on Wigner-Ville distribution (WVD)is proposed. The issues have been solved to judge the DC-DC converter works in chaos or in period-1 or period-2 or period-4 state.As a high-order DC-DC converter, the nonlinear characteristics in fourth-order Superbuck converter have been analyzed. Firstly, the piecewise switch model and discrete iterative mapping model in Superbuck converter under CCM mode have been established by analyzing the state equations of the converter. The evolution from stable to the period-2 bifurcation and chaotic state in Superbuck converter has been gotten by analyzing the inductor current waveform, output voltage waveform, bifurcation diagram,phase diagram, WVD and poincare section.Stability criteria of Boost, Buck-Boost, Superbuck converter under current mode have been derived from reference current by analyzing DC-DC converter. Using parameter resonance perturbation method, the chaotic controlling in Boost, Buck-boost and Superbuck converter under current mode has been realized by changing perturbation parameter, it makes the converter from chaotic state to period-1 state.The output energy of the photovoltaic DC microgrid using maximum power point tracking technology can be increased. To solve chaotic searching method do not be directly applied in the maximum power point tracking of photovoltaic array, a photovoltaic array MPPT method based on variable-scale chaotic optimizing searching has been proposed using Logistic mapping modified by power series generate chaotic variable. Simulations and compared with P&O method showed that the proposed method in this dissertation could track the external environment changing accurately and quickly, and avoid the weakness of falling into local optimum in partial shading conditions. |
PDF Full Text Request