Modulation Strategy for Three-level Neutral-Point-Clamped Converter Achieving Clamping Diodes Loss Control

For the medium voltage (MV) three-level (3L) neutral-point-clamped (NPC) converter, minimizing the clamping diodes volume are desired to reduce the cost and footprint of the converter, not to mention the simplification of the laminated bus-structure. In the traditional 3L modulation methods, the conduction losses of clamping diodes change significantly under different operation conditions, e.g. power rating and power factors. Therefore, the selection of diodes needs to cover the worst scenario. In this paper, to help reduce clamping diodes cost and volume, a new modulation strategy is proposed to control the losses of the clamping diodes by operating the 3L NPC converter in the combination of 3L modulation and quasi-2-level (Q2L) modulation. The detailed modulation strategy is introduced in this paper. The total device loss comparison of the proposed modulation strategy, Q2L modulation, and 3L modulation is then presented. To verify the proposed modulation strategy, 3L NPC phase leg with 10 kV SiC MOSFET is tested, and the proposed modulation strategy is verified.