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Power quality was always a major concern in designing an electric supply system for railways. Since electric railcars are usually single-phase loads, they draw high amounts of negative sequence component of currents, in addition to harmonic contents and transient currents. Therefore, many compensation methods were examined to improve the power quality indices. The active power quality conditioner (APQC) can be considered as an ideal compensator for high-speed railway, which contains a three-phase converter connected to the traction substation through a step-down transformer. However, with the growth of railway loads, the nominal rating of the solid-state high-frequency switches of APQC increases seriously, which in turn, results in an exponential growth of the cost of power-electronic switches. Therefore, for a very high-capacity railway system, it is not economic to apply an APQC. As a solution, a combination of APQC with the static VAr compensator is proposed in this study, which reduces the rating of APQC, and improves the power quality of the system. Simulation results validate the pre-defined hypothesis. Moreover, the performance of APQC depends on the DC-link operation, for which genetic-algorithm optimisation has been applied to obtain an optimum design of a stable DC-link voltage.

Railway power conditioners are widely adopted to improve Power Quality of electric railways. The present study focuses on the main architectures and the simplest, but effective, control method (PI controller): DC-link voltage and output current sequence are the controlled quantities and optimization is reached iteratively, showing the consequences of each choice. The final objective is to identify criteria that may be applied equally well to different compensator topologies.