Multi-mode Hybrid Modulation Strategy for Three-level Converters based on Half-wave Symmetric SHEPWM

Selective harmonic elimination pulse width modulation (SHEPWM) is a mainstream choice for low switching frequency applications. To realize the entire speed range operation of driving systems, switching between different modes is inevitable. Three-level half-wave symmetric SHEPWM has more control degrees of freedom to deal with various problems, but the phase distribution and switching angle are quite different from quarter-wave symmetric SHEPWM. Thus, some conventional smooth switching methods can not be directly applied to three-level half-wave symmetric SHEPWM to avoid the current impact problem. In this paper, by analyzing the harmonic characteristics of half-wave symmetric SHEPWM and smooth switching conditions, the proper simultaneous switching point is derived, and then a multi-mode hybrid modulation strategy based on half-wave symmetric SHEPWM suitable for three-level converters is obtained. It can avoid the current shock at the switching time under various power factor and modulation ratio. Some numerical analysis and simulation are carried out to prove the correctness and feasibility of the proposed strategy.