Voltage regulation on 25 kV AC railway systems by using thyristor switched capacitor

The function of an AC traction system is to deliver power to the locomotives as efficiently and economically as possible. Two main problems generally affect a normal railway system: the voltage drop and the high levels of harmonic distortion. The maintenance of a minimum voltage level, so that the trains can operate normally, involves a limit in the maximum length of the feeder (typically 25 km), and consequently to the distance between feeding substations. In this paper a shunt compensation approach has been studied, based on the application of thyristor switched capacitors (TSCs) tuned into a single arm filter. They are cheaper devices that allow achieving suitable results in the reactive power compensation for electrified railway systems and limiting the additional harmonic distortion introduced. The capacitors in the bank have been arranged and controlled according to a binary code, so that 2ⁿ levels of compensation are available with n single arm filters. Particular attention has been focused on designing a suitable control law for a TSC bank of this type and in its implementation in a real circuit. The tests carried out both with digital and analogue simulations have confirmed the effectiveness of the proposed compensation strategy