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This paper analyzed three control strategies (direct power control, per-phase harmonic reference generation method, conventional Ip-Iq method) for active power filters in three-phase four-wire systems. They were applied to a 4-leg inverter to compensate input currents of an office building. Differences in harmonic detecting and switching sequence generation of these three control strategies were analyzed. Per-phase harmonic reference generation control is used to reduce capacity of dc bus capacitor. Direct power control is designed for better harmonic compensation as well as smaller power capacity. The simulation results are presented to substantiate the theoretical analyses.