High power factor control system in multilevel converters for AC heavy traction drives

A quasi-zero reactive power demand which is mandatory in single-phase AC (25 kV/50 Hz) traction systems is considered. Multilevel converter structures and high-performance control techniques have been developed to fulfil this goal in AC heavy traction drives. The tasks of the three level rectifier are described in detail. The current control logic is particularly suitable for applications to multilevel bridges. Advantages deriving from the use of a medium-voltage converter with three different DC potentials are discussed. Digital simulation performed using the general-purpose electromagnetic transients program made it possible to verify the effectiveness of the regulating techniques on the whole system. By controlling converters suitably, the reactive power flowing was reduced to quasi-zero values having a constant voltage on the DC-link and minimizing the harmonic content of the line current in every load condition. Another regulation technique was studied to balance the voltage on condensers which, acting exclusively on the rectifier and not using dissipating techniques or raising the device switching frequency, manage to keep the unbalance DV within a prefixed band.<>