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Due to the use of diverse electric loads, the power quality problem in the power distribution systems can be very complex, especially in three-phase three-wire or four-wire distribution systems. This paper proposes a unified power quality conditioner (UPQC) with one cycle control (OCC) that has the functions of harmonic suppression, reactive power compensation, unbalanced load correction, and source neutral/zero-sequence current limitation. The advantages of this UPQC include no need for reference calculation, vector operation, and flexible structure for three-wire or four-wire system. Experimental and simulation results from a prototype confirmed the feasibility of the proposed approach.