Gradient-Based Search Algorithm for Selective Harmonic Elimination in Cascaded H-Bridge Multilevel Converters

This article develops a gradient-based search algorithm for selective harmonic elimination (SHE) to address the problems of high-computational cost, low convergence speed, modulation index error, and slow dynamic response often associated with SHE algorithms. First, the dimension of the search space is reduced by deriving intuitive equations to increase the search speed. Second, the desired modulation index is achieved by applying the proposed constraints. Third, gradient equations are modified to move and hold points in the reduced search space. Extensive comparative simulation studies show that, compared to conventional SHE methods, the proposed algorithm is highly efficient in producing smooth switching angle curves with minimal fluctuations over the entire modulation index range. The proposed method minimizes execution time and modulation index error, even with unbalanced dc input voltages. These superior characteristics are also experimentally verified on cascaded H-bridge inverters with different number of stages up to 25.