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This paper proposes a novel and simple control method for an active power quality compensator (APQC) used in electrified railways. The proposed method consists of only constant do capacitor voltage control, which is generally used in APQCs. No detection method for harmonic, reactive and negative-sequence components is necessary. Thus we offer the simplest way to compensate the harmonic, reactive and negative-sequence currents on the primary side of the Scott transformer in the traction substation. The basic principle of the proposed control method is discussed in detail, and then confirmed by digital computer simulation using PSIM software. A prototype experimental model is constructed and tested. Experimental results demonstrate that balanced source currents with unity power factor are obtained on the primary side with the proposed control method, which uses only constant do capacitor voltage control.