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Comparing single-stage grid-connected photovoltaic (PV) system with power quality improvement, in this paper, a high-capacity three-phase four-wire grid-connected PV system based on Boost converter + dual-level four-leg inverter is proposed, which not only allows a wide range of input voltage, but also compensates unbalance current of the local load comparing three-phase three-wire PV system. This paper explains the control principle of each power stage of the system and the unified control strategy of combination of PV grid-connected generation and active filtering in detail, and then the Bode plots of open loop gain for current loop and voltage loop are shown to indicate stability of the unified control according to the system equivalent model. Simulation results on MATLAB/Simulink are presented to demonstrate practicability of the unified control strategy. Applying proposed topology and control method to a 100 kVA grid-connected PV system based on FPGA+DSP controller, the experimental results are given to verify correctness of the whole system.