Improved selective harmonic elimination pulse-width modulation strategy in multilevel inverters

In the standard selective harmonic elimination (SHE)-pulse-width modulation (PWM) technique for multilevel converters, in order to eliminate N 2 1 non-triplen odd harmonics of the ac-side voltage and to regulate the output voltage amplitude, N switching angles need to be determined. Therefore a set of N non-linear transcendental equations must be solved to find the desired switching angles for any value of the modulation index. However, solutions to the non-linear equations are not feasible for the entire range of modulation region. This study proposes a modified SHE-PWM technique that extends the standard modulation region and generates a stepped voltage waveform within a wide range of modulation indices. As compared with the standard SHE-PWM technique, the constraint associated with elimination of the harmonic with the highest order is relaxed and the corresponding equation is disregarded. Consequently, a reduced number of transcendental equations are employed to find the N desired switching angles. Since the switching frequency remains constant, converter´s efficiency does not change. However, in order to produce a pure sinusoidal waveform, a higher order filter is required. A genetic algorithm is used to solve the equations and to determine the switching angles. The proposed technique can generate stepped voltage waveforms with a wide range of modulation indices. Performance of the proposed SHE-PWM technique for a seven-level cascaded H-bridge converter, based on simulation studies, is evaluated and experimentally verified.