Hybrid modulation strategy for neutral point clamped three-level inverter based on modulation wave decomposition

For neutral point clamped three-level inverter (NPC TLI), traditional carrier-based pulse width modulation (CBPWM) can cause triple frequency fluctuations in neutral point voltage (NPV) under high modulation index and low power factor. Although virtual space vector pulse width modulation (VSVPWM) can achieve unconditional balance of NPV, it has the problem of high switching loss. To address the above issues, a hybrid modulation strategy suitable for NPC TLI was proposed based on the coordination of multi-constraint objectives through modulation wave decomposition. It is realized by comparing double modulation waves with a single carrier. Two types of constraints for switching times and unconditional balanced neutral point voltage were analyzed. In order to reduce the switching loss under the condition of ensuring NPV balance, the proposed hybrid modulation strategy with the above two constraints was obtained. On this basis, active NPV control methods under these two constraints were discussed. In addition, the performance indicators of the proposed strategy and other strategies were compared. The hybrid modulation strategy can achieve NPV balance under full modulation index and full power factor, and has reduced switching losses compared to VSVPWM. Experimental results verify the feasibility and superiority of the proposed strategy. © 2023 Editorial Department of Electric Machines and Control. All rights reserved.