Improved hybrid SHM-SHE modulation technique for four-leg three-level NPC inverters

In this paper, Selective Harmonic Mitigation (SHM) and Selective Harmonic Elimination (SHE) modulation techniques are combined to generate the appropriated firing pulses for a 4-leg 3-Level Neutral Point Clamped (NPC) inverter. SHM modulation technique is applied to the phase legs in order to get a desired modulation index (ma) and to mitigate the lower order odd non-triplen harmonics (5^th, 7^th, ...) while the fourth leg firing pulses are generated using SHE modulation technique to eliminate the major low order odd triplen harmonics (3^rd, 9^th, ...) completely. It is demonstrated that this hybrid modulation technique can reduce significantly a vast range of harmonic orders in the NPC inverter output voltage when compared with the results achieved when using only SHE modulation technique in the four legs of the inverter. To avoid the generation of odd non-triplen harmonics in the SHM phase legs that can not be eliminated with the fourth SHE leg, the Simulated Annealing (SA) heuristic algorithm used to find the SHM switching angles is tuned in order to provided suitable oddnon-triplen harmonics distortions which can be eliminated with SHE technique. Moreover, since in dynamical applications the modulation index has to be changing continuously, the SA algorithm has also been designed in order to provide a smooth switching angles set to avoid undesired transients harmonic contents.