Study of THD in Different CHB MLIs Controlled With Unipolar and Bipolar Carrier Based Modulation Techniques

Multi-level inverters (MLIs) are gaining popularity due to their inherent advantages like better harmonic distortion, ability of producing higher magnitudes of AC voltages. As these converters uses many cascaded connection of many small units the switches used in devices are subjected to less voltage stresses. H-bridge based multilevel inverter can increase the number of output voltage levels by adding switch components and DC input voltage sources. If it employs seven switches and three DC sources, the number of output voltage levels becomes seven. Although its THD characteristics are improved, it needs output filter to meet general output voltage THD requirement, i.e., 5 % below. By adding PWM switching schemes to the operation of the prior H-bridge switches, it can synthesize more sinusoidal waveform. By this simple alteration in the switching scheme, it can improve the output voltage THD requirement. To verify the high performance of the proposed switching scheme, Unipolar, Bipolar, Third harmonic based Pulse width modulation schemes are considered for analysis and computer-aided simulations are conducted using MATLAB.