Railway hybrid power quality conditioner (HPQC) is one of the newly proposed devices for power quality compensation in co-phase high-speed traction power supply for its benefit in reduction of operation voltage over conventional railway power quality conditioner (RPC). However, initial railway HPQC design is developed based on minimum operation voltage at predefined fixed rated load. This may not be applicable in practical condition when load varies. In order to solve this problem, a railway HPQC design with increased operation voltage has been investigated to enhance the compensation capability. Nevertheless, the compensation capability is even larger than that required during load variations such that the operation voltage is higher than it actually requires. A lower operation voltage can actually be used to provide required compensation capability when the coupled impedance is different from the designed rated one. In this paper, the effect of coupled impedance on the compensation capability in co-phase railway HPQCis being analyzed. An impedance-mapping design technique based on required compensation capability is then proposed based on the analysis. The proposed method is advantageous for lower operation voltage and smaller capacitance value. The effectiveness of impedance-mapping technique is verified via simulation and experimental results.