| This thesis was supported by National Natural Science Foundation of China(51277184).With the exhaustion of global energy resources, renewable energy resources should be explored to take the place of the conventional ones. Distributed generation(DG) system is an effective way for the integration of new energy resources. In this century, among these resources, photovoltaic(PV) has become one of the most important power suppliers because of its cleanness and easy installation. During the last two decades, PV technology has been explored and applied by developed countries in Europe and America. It turns out that the intermittence of integrated large-scale PV systems will impact the grid, and on the other hand, the disturbance in the grid voltage will do harm to the PV system and even reduce the service life of PV panels. This thesis has mainly discussed about the control strategies of PV systems under balanced and unbalanced grid voltage disturbances. The main contents are abstracted as follows:① A generally used environmentally amendatory PV cell model has been built and the output feature with varying temperature and illumination has been explored. The control strategy of PV systems on the DC side has been studied and built in Simulink. The PV model with Maximum Power Point Tracking(MPPT) control has been simulated under different temperature and illumination.② Three control strategies of PV converter of the grid side—Grid-forming control strategy, Grid-feeding control strategy and the Grid-supporting control strategy—have been studied. The grid-supporting strategy has been further discussed and simulated. In allusion to the voltage disturbance caused by load switching and generator switching, a simulation model of a PV-storage station has been built and the V/f droop control is applied. The droop coefficients have been determined based on the capacity of the station. The station is required to offer extra powers to compensate the grid voltage disturbance. In addition, based on the grid-supporting PV-storage station model, a multi-point PV microgrid with combined control strategies has been built in Simulink, and the cooperative operation process of PV systems applying different control strategies has been simulated.③ Control strategies of PV systems under unbalanced grid voltage disturbance have been discussed. The impact of unbalanced voltage on the PV system has been discussed, and the sources of output power fluctuations and DC voltage fluctuation have been deduced. This work has summarized the conventional controllers that are used under unbalanced voltage and has discussed their advantages and weakness among. In allusion to the restrain of power fluctuations, three kinds of commonly used outer loop power control strategies(IARC、PNSC、BPSC) have been presented and simulated.④ Based on the IARC outer loop power control strategy, a flexible current control method for PV systems under unbalanced voltage by bringing in two adjustable coefficients has been explored. The RMS values of current harmonics under different control coefficients have been deduced. Two standards, GB/Z 19964-2005 and IEEE Std.1547, are studied to limit the injected harmonics, and the feasible region for the coefficients has been explored. The relational expressions among the 3rd order harmonics, the active power fluctuation and the reactive power fluctuations have been found. A flexible harmonic-injection based power control strategy of PV systems has been designed. This control strategy can suppress the power fluctuations and the DC voltage fluctuation of the PV system, while the injected current harmonics can be limited within the harmonic standards, This method has been verified in PSCAD/EMTDC. |
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