Optimal PWM in Three-Level Inverter for THD Minimization with Quarter Wave Symmetry Relaxed

Background: Previous works on total harmonic distortion (THD) minimization for multilevel inverters (MLI’s) have made a symmetrical approach that imposes Quarter Wave Symmetry (QWS) and Half Wave Symmetry (HWS) with the aim to restrict the computational difficulties to solve the finalized non-linear expressions. Objective: However, the QWS constraint can be abolished, which leads to more generalized formulations and multiple solutions corresponding to minimum THD for the complete modulation index range. Methods: A systematic search approach using MATLAB in-build function is adopted to investigate ideal angles of switching related to a three-level, three-phase inverter restricted to only HWS as a constraint for two and three (i.e., N = 2 and 3) switching transitions per quarter cycle. Results: On account of the effective minimization of the dominant low-order odd line voltage harmonics, the proposed pulse-width-modulation (PWM) scheme proved its superiority based on minimum THD% performance compared to the previously reported PWM techniques with both HWS and QWS being retained. Conclusion: The simulation and experimental findings show that the proposed non-symmetrical PWM scheme outperforms the classic PWM method limited to QWS and HWS symmetries across the whole modulation-index range, with the added benefit of a general formulation and many solutions.