A Flexible Space Vector Modulation Strategy With Hybrid Medium Voltage Vectors for Full-Range Operation of 3L-NPC Inverters

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This article proposes a novel space vector modulation strategy with hybrid medium voltage vectors (MVVs) for three-level neutral-point-clamped (3L-NPC) inverters, which achieves high algorithm flexibility, low switching losses, and low computational burden while ensuring neutral-point voltage (NPV) balance over the full range of modulation indices and power factors. The proposed strategy introduces six additional virtual MVVs at the locations of the six actual MVVs, which allows the space vector distribution to be maintained without modifying the synthesis of the reference voltage vector. A simple judgment scheme is presented to determine whether a virtual or actual MVV should be used, which can be flexibly tuned to greatly reduce switching losses while ensuring always balanced NPV. Besides, a novel and simple method for determining the optimal switching sequence is proposed. Furthermore, carrier-based modulation with adjustable switching patterns is achieved to further simplify implementation and improve flexibility. Simulation and experimental results validate the proposed strategy.