Simultaneous Selective Harmonic Elimination and THD Minimization for a Single-Phase Multilevel Inverter with Staircase Modulation

Power electronic converters for medium and high power / voltage applications employ low frequency fundamental switching frequencies and therefore minimize switching losses. There are two main control techniques of switching angle estimation - Total Harmonic Distortion (THD) minimization and Selective Harmonic Elimination (SHE). In case of the first control technique, the switching angles are found based on the THD minimization without being specifically focused on eliminating some exact harmonics. For the SHE control technique, all degrees of freedom, due to available number of switching angles, are used for the elimination of certain low order harmonics. The basic idea of combining those two control techniques consists of using some degrees of freedom for eliminating certain harmonics and using the others for minimizing the remaining THD content. This generalized problem formulation includes classic minimal THD and SHE problems as special cases. Theoretical developments are verified by the set of experimental cases for the voltage and current THDs selecting characteristic working points over the modulation index range.