I. Introduction
Since about 1970, power electronics has played an important role in the domain of electric traction because it allows supplying the energy of catenaries to traction motors in a controlled way. Different topologies of power converters have been used according to available semiconductor technology, supply voltage, and frequency characteristics. Currently, for 15-kV 16.7-Hz applications the “state of art” topology of the power circuits for traction with three-phase motors is described in Fig. 1. The voltage of the catenaries is adapted by the main transformer to an appropriate level for the four-quadrant converter. This converter works like a rectifier and provides a continuous voltage to the dc link. A smoothing filter tuned at twice the supply frequency is mandatory. A three-phase inverter supplies a three-phase motor.
Typical ac traction chain.