Cascaded Multilevel Inverter Based Transformerless Traction Drive for Railway Applications

Abstract

Electric Railway Traction Drive has been introduced as a solution to the environmental problem caused by the diesel or steam engines. Generally, an AC electrified railway system is supplied with 25kV, 50 Hz AC supply. It is fed to the traction motor after stepping down to three phase, 400 V, 50Hz with the help of a transformer. This magnetically coupled transformer lead to high weight, several losses and reduced efficiency. The railway electric traction requires high voltage operation. This is achieved with the help of multilevel inverter. Among the various multilevel inverters, the cascaded multilevel inverter is best suited for railway traction application because of its modular structure and use of low rating devices. The three phase induction motors are widely used in the railway traction drive because of its low cost and weight, better torque characteristics, high reliability and less maintenance due to the absence of brushes. This thesis presents the application of the cascaded multilevel inverter in the transformerless railway traction drive. Cascaded inverters up to eleven level have been simulated to find that THD increases with the increase in the voltage level. Various modulation techniques- Phase Shifted Modulation, Level Shifted Modulation and Selective Harmonic Elimination techniques were implemented in the multilevel inverters to find out the best modulation techniques among them. It was found that SHE technique resulted in low THD. Thus, an IGBT based-cascaded eleven level inverter with SHE method has been modelled to lower the supply voltage to a level convenient for the traction induction motors. This eliminates the need of a transformer in the railway traction drives and also results in the reduction in the Total Harmonic Distortion of the voltage to be supplied to the traction motors.