An optimal selective harmonic mitigation for cascaded H-bridge converters

In recent years, due to more intransigent regulations in the area of power quality and appearance of new grid codes which limit the maximum allowable harmonic levels, paying attention to the quality of power converters' output signal has become more important. Moreover, in order to achieve high efficiency in medium voltage and high power converters, the switching frequency has to be kept as low as possible. So in this paper, a special implementation of the selective harmonic mitigation pulse width modulation (SHMPWM) method with a very low switching frequency is introduced for multilevel cascaded H-bridge converters to mitigate selected harmonics below the standard levels and to satisfy the grid codes from the THD point of view. In the proposed method, using a 150Hz switching frequency for power switches, the harmonic components are mitigated up to the 41st harmonic. In addition, the advantages of the SHMPWM are shown compared to the SHEPWM for the same switching frequency. To verify the effectiveness of the proposed method, simulations are carried out in PSCAD/EMTDC environment on a 7-level cascaded H-bridge converter.