Railway hybrid power quality conditioner (HPQC) is advantageous over conventional railway power quality conditioner (RPC) for its reduction in operation voltage and device ratings in co-phase traction power compensation. In order to further reduce the power loss during operation, flexible dc control has been proposed. However, the operation voltage range for railway HPQC has not yet been discussed. This is important since in contrast to traditional power quality compensation devices, active power compensation is involved in co-phase traction power. Compensation capability in co- phase traction power thus does refer not only to reactive power compensation but also to active power compensation ability. Satisfactory compensation performance can be provided only when railway HPQC can provide enough active and reactive compensation power output at the same time. In this paper, the effects of operation range in flexible dc voltage control on railway HPQC compensation capability is briefly discussed and analyzed. The analysis is verified via PSCAD simulations. A laboratory-scaled hardware prototype is also constructed to obtain experimental results for further verifications.