Quasi seven-level operation of multilevel converters with selective harmonic elimination

Generally, selective harmonic elimination (SHE) complexity for multilevel inverters increases with the increase in number of levels. In this paper, a novel approach for SHE is introduced. This approach is based on cancelling the 5th and 7th order harmonics irrelevant of the number of levels in three-phase multilevel converters, such that the solution for the angles required for the proposed SHE mimics that of the seven-level inverter (i.e. quasi seven-level operation of an n-level converter). The advantages of the proposed SHE approach are the simplicity in format and independency of number of levels. The proposed approach can be extended for more of low order harmonic cancellation. A method to obtain initial values for the SHE equations according to the reference modulation index M is proposed and investigated thoroughly. Compared with existing methods, the proposed method does not involve complex equation groups and is easier to be utilized in the case of a large number of switching angles, or multiple switching angles per voltage level in multilevel inverters. A 17-level cascaded inverter with different modulation indices working in Quasi Seven-Level Operation is considered as an example to validate the proposed concept. Simulations using MATLAB/SIMULINK platform has been conducted to validate the proposed concept.