Modulation Methods for Three-Level Neutral-Point-Clamped Inverter Achieving Stress Redistribution Under Moderate Modulation Index

This letter investigates the loss and thermal behaviors of a three-level neutral-point-clamped (3L-NPC) inverter undergoing moderate modulation index, which is typically presented during minor voltage sags of the power grid or speed changes of the electric machines. A series of new space vector modulation methods is then proposed to relocate the thermal loading among the power devices. It is concluded that with some of the proposed modulation methods, the device loading in the 3L-NPC inverter can be effectively modified and optimized, and the junction temperature of the most stressed devices can be also relieved. The control ability of dc bus midpoint potential, which is one of the crucial considerations for the 3L-NPC topology, is also investigated when applying the proposed modulation methods.