| Currently, many countries pay much attention to the research and application of the renewable energy resources. This paper focuses on three-phase single-stage grid-connected system. The topology, control strategy and anti-islanding protection of the PV system are analyzed and studied.First, the T shape LCL filter is designed. The design method and process of parameter calculating are given. The three-phase grid-connected system is analyzed mathematically. It's proved that the three-phase inverter can be decompounded to two single-phase inverters through abc ?αβcoordinate transform and controlled in the equivalent single-phase inverter. The controller of grid-connected system is predigested.Second, several methods of the Maximum Power Point Tracking (MPPT) and grid-connected controlling are analyzed. Their advantages and weakness are compared. This paper uses current optimal gradient variable step MPPT and designs a hybrid controller incorporating repetitive control and state feedback pole-assignment control for grid-connected current controlling. The paper poses the system control diagram. The stability of system is analyzed.Next, the system model is established by using MATLAB. The result of simulation proved that the control method is correct. Under the direction of the simulation, the grid-connected work of the three-phase single-stage PV grid-connected system is realized on the experiment platform. The experiment proves that the control strategy can track the PV array's maximum power point correctly, efficiently reduce the current distortion and achieve the unit power factor grid-connecting.At last, anti-islanding is safety critical issue in grid-connected system. In this paper, the basic theory of AI (Anti-Islanding) is analyzed. Several islanding detection methods are analyzed. Active Power Disturb and Active Frequency Drift are designed in this paper. The two methods are compared to each other. Their properties are pointed out.
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