Research On Operation Control Of Three-Phase Grid Connected Photovoltaic Power System

Due to the important status of solar power in the global power energy system, solar resource has the potential to be the main alternative energy source to replace traditional ones in the future. Therefore PV grid-connected generating technology becomes an important research field. This thesis focuses on control strategies in three-phase grid- connected PV systems, which includes modeling of three-phase grid-connected PV systems, maximum power point tracking(MPPT) methods for PV arrays, grid-connected control of single-stage and two-stage PV system and filter design method.Firstly, the electrical characteristic and output characteristic of solar cell act a vital role in solar research, especially in MPPT research. This thesis presents theoretical analysis and experimental results based on a DC/DC push-pull topology of constant step-size and variable step-size MPPT methods. Furthermore the serious power loss caused by sampling error in the starting stage is analyzed, and an improved strategy is proposed that constant voltage tracking method is adopted when PV system startup. Avoiding the collapse of the DC bus voltage of the single-stage three-phase system when the irradiation changes greatly, an improved method based on the traditional P&O method is proposed. And the viability of this method is verified by experiments on the one-stage three-phase platform.Then the topology and principle of three-phase PV grid-connected inverter are analyzed and the mathematical model of the system is established. Based on the contemporary research of grid-connected control strategies, this thesis analyzes different methods by categorizing them according to different grid-connected filters. Summarize the design principles of L and LC filter, this thesis gives out the advantages of LCL filter when treating the system as a current source. Comparing several design methods of LCL filter, an optimal method is used to give out the parameters of the filter while ensure the less use of inductance magnetic materials, good performance in filtering and high resonant frequency. And a design sample is also given.By analyzing model of grid-connected inverter with LCL filter under dq axis, it is obvious that the control variables of have strong coupling under dq coordinate. It is very complicated to decoupling, so a two-loop control strategy underαβcoordinate is adopted which chooses the capacitor current as the inner control variable and grid-connected current as the outer loop variable. Moreover, a designing method is given to get the parameters of the two-loop controller. In order to cover the static error of the PI controller under stationary coordinate, PR controller is adopted. The performances of PI and PR controller with the same parameters are compared through experiments and simulation.In order to satisfy the IEEE Std 929-2000 standard and the requirements of the grid-connected system in China, a hybrid controller incorporating repetitive control and state feedback pole-assignment control is proposed. Utilizing the feedback of inductance current and capacitor voltage, the system poles can be assigned as wish to achieve good dynamic performance. Besides, the repetitive controller is adopted to improve the static performance which can modify the quality of the grid-connected current to accord with the grid-connected requirement. The two mentioned control strategies are realized based on the three-phase PV grid-connected system. The experimental results prove the availability of the two strategies.Finally analyzing the operation characteristic of two-stage grid-connected PV system, as well as the co-ordination problem between the two converters, two different control strategies are analyzed. The experimental platform is based on the combination of the DC/DC push-pull converter and the three-phase inverter. The availability of the control strategy is verified by simulation.