Multilevel inverter based electric traction drives

Electric Railway Traction Drive has been introduced as a replacement to the existing steam and diesel run locomotives. Generally, an electric railway system requires a transformer to step down the high voltage (25 kV, 50 Hz) to the low voltage level (400 V, 50 Hz) which is fed to the traction motors. This transformer adds extra weight to the system along with the several losses and hence reduced efficiency. The railway electric traction requires high voltage operation. This can be achieved with the help of multilevel inverter (MLI) which eliminates the need of transformer in the railway traction system and also results in the reduction of the Total Harmonic Distortion (THD) of the voltage to be supplied to the traction motors. In this paper, various modulation techniques of cascaded H-bridge inverter are analyzed, which can reduce THD for three-level inverter to multilevel topologies. Three methodologies adopting phase shifted carrier pulse width modulation, the level shifted carrier pulse width modulation and selective harmonic elimination (SHE) modulation concepts have been employed in this paper. The simulation of different modulation techniques has been done using MATLAB/SIMULINK. It was found that SHE modulation technique results in a lower value of THD. An eleven-level inverter with SHE modulation technique is used for electric traction drive; plugging and a combination of capacitive braking and DC injection braking method are employed for braking purpose. Different control methods like V/f Control and Direct Torque Control (DTC) are employed for speed control of traction motor. In DTC torque and flux was controlled directly by using required voltage vectors