Low-Frequency Harmonic Elimination Technique in Three Phase Cascaded H-Bridges Multilevel Inverters for Renewable Energy Applications

This paper proposes a fundamental frequency selective harmonic elimination (SHE) technique for medium and high power three phase cascaded H-bridges (CHB) l-level inverters, where low switching frequency assumes an important role. Respect to classical SHE methods, it is capable to delete more harmonics from the output voltage waveform and to provide lower total harmonic distortion (THD) values. A mathematical proof is given to demostrate that the undeleted harmonics are only those of order n = 2k (l + r) ± 1, k = 1, 2, . . . where r is an integer parameter that assumes the values −2, −1, 0 defining three different sets of switching angles, that are provided in the whole range of modulation index. The method assumes different dc voltage sources and pulses patterns, defined for the three sets of switching angles. Simulated results are shown to confirm the validity of the proposed procedure. For l = 5 and l = 7, the results obtained are compared with those carried out by applying the classical SHE technique at the same conditions. Very good performances, in terms of number of deleted harmonics and THD, are observed when r = 0. In comparison to classical SHE, the proposed method reduces computational complexity because it must not solve non linear equations.