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This paper proposes a constant DC voltage control based strategy for an active power quality compensator (APQC) used in electrified railways. The proposed strategy consists of only an I-PD based constant DC capacitor voltage control with an added moving average type low-pass filter (LPF). The added LPF improves the response time of the constant DC capacitor voltage control for the APQC. Thus we offer the simplest control method for the APQC used in electrified railways with the improved response time. The basic principle of the proposed control strategy 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 the balanced source currents with the unity power factor are obtained on the primary side the Scott transformer in the traction substation systems, improving the response time by one-fourth as compared to that of the previously proposed control method.