Research And Implementation Of Control Strategy In Grid-connected Photovoltaic Systems

With the development of photovoltaic (PV) technologies, applications of photovoltaics in grid-connected situations have grown rapidly, indicating that photovoltaics are attractive to produce environmentally benign electricity for diversified purposes. This thesis focuses on the control strategies in grid-connected PV systems, which includes modeling of grid-connected PV systems, maximum power point tracking (MPPT) methods for PV arrays, integration of reactive power compensation function and islanding detection methods in PV systems.The modeling of PV arrays is the base of research on grid-connected PV systems. This thesis presents two models of PV arrays respectively based on the physical and the output characteristics. Both of them can be directly applied in circuit simulation and system analysis.Single-stage grid-connected PV system has advantages such as simple topology, high efficiency, etc. However, since all the control objectives, such as MPPT, synchronization with the utility voltage and harmonics reduction for output current need to be considered simultaneously, the complexity of control scheme is much increased. This thesis proposed a modified incremental conductance MPPT method applied in a single-stage grid-connected PV system, which can remarkably improve stability of the MPPT method. The reactive power compensation for local load is also realized, so as to alleviate grid burden. A DSP is employed to implement the proposed MPPT controller and reactive power compensation unit. Deadbeat algorithm is also employed to reduce harmonics in the output current of the inverter. Simulation and experimental results will show the high stability and high efficiency of this grid-connected PV system. In applications of dispatchable grid-connected PV systems, a Perturb & Observe method combined with an intelligent charging controller for lead-acid batteries is alsoproposed in this thesis.To improve the detection effectiveness of active islanding detection methods when there are several PV systems feeding the same island, an active islanding detection method based on positive power disturbance is proposed in this thesis. The method can be easily implemented in PV systems and will not increase the system cost.Based on theoretical analyses, two prototypes of grid-connected PV systems are implemented. Experimental results will verify the proposed control strategy.