Two main circuit topologies suitable for the railway power quality controller（RUPQC） based on the modular structure are proposed. They both have a cascaded structure on the alternating current（AC） side with low level harmonics. To solve the capacitor voltage balance problem in the cascaded converter， a mathematic model of direct current（DC） capacitor voltage in the proposed topologies are deduced. The advantages， disadvantages and application of existing balancing methods for back-to-back converters are analyzed， which have bidirectional real power flow. The results show that the DC component of capacitor voltage has positive correlation and negative correlation with modulation ratios of absorbed and emitted active power respectively. In the end， the control scheme of RUPQC devices is given， the effectiveness of the topologies and control algorithm proposed is verified by simulation examples.