Cascaded multilevel inverter for underground traction drives

The elementary concept of a multilevel converter to achieve medium and high voltage by using a series of power semiconductor switches with several lower voltage DC sources. A suitable modulation technique is required to perform the power conversion by synthesizing a staircase voltage waveform to reduce the Total Harmonic Distortion (THD) of the multi-level output voltage waveform. To further reduce the THD for an underground traction application, the level of the cascaded multilevel inverter is increased to the higher value using unequal DC voltage across each H-bridge. In the proposed work, a 9-level cascaded multilevel inverter is simulated as well as experimentally tested in a laboratory prototype for the application of traction system using single DC supply. The concept can be extended up to any level. The proposed topology uses an AC Link with an inherent regulation of the voltage supplies among the H-bridges.