Hybrid modulation strategy for eliminating low‐frequency NP voltage oscillations in NPC using redistribution of NTV duty ratios

Abstract

Neutral point (NP) clamped (NPC) inverters produce low-frequency NP voltage oscillations when modulated with nearest three-vector (NTV) methods for certain loading conditions. Non-NTV modulations can address these oscillations at the expense of increased switching frequency, output voltage distortion and complexity. This study presents a new generalised algorithm to implement hybrid modulation strategies which can also eliminate NP voltage oscillations using a combination of the aforementioned methods. The proposed algorithm avoids abrupt changeover between the two modulations and ensures minimum contribution of non-NTV operation and hence its drawbacks. Moreover, the algorithm is computationally efficient compared with its counterparts, since it only uses redistributed NTV duty ratios to generate NTV and non-NTV modulations instead of using two separate complex algorithms which result in increased computational burden. A non-NTV method called selected three-vector (STV) modulation is chosen to perform the detailed analysis of above proposed algorithm for hybrid NTV–STV modulation. Additionally, a simplified STV (SSTV) is derived to develop a new hybrid NTV–SSTV modulation strategy which further reduces losses and complexity compared with the hybrid NTV–STV modulation, without affecting the performance of NPC. The proposed algorithm and the derived hybrid modulation strategy are validated using simulation and experimental results.