Reliability Improved Discontinuous Pulse Width Modulation for NPC Inverters Operated at High Modulation Index

In a three-level neutral point clamped (NPC) inverter, the power devices and dc-link capacitors are considered the most reliability-critical components, and their wear-out failure is primarily caused by thermal loading resulting from power losses. It has been found in a previous study that a discontinuous pulse width modulation (DPWM) reduces the power loss and improves the reliability of power devices. However, it reduces the reliability of the dc-link capacitors, and thus the reliability of the NPC inverter. This article proposes a DPWM method called reliability improved DPWM (RI-DPWM) to improve the reliability of the NPC inverter by reducing the thermal loadings of both the power devices and dc-link capacitors while keeping the advantage of the conventional DPWM, reducing the power loss without additional deterioration in output current total harmonic distortion compared with conventional DPWM. The proposed RI-DPWM method is a good alternative to the conventional DPWM to improve the efficiency and reliability of the NPC inverter operated at high modulation index which is greater than or equal to 0.898 with a power factor of 1 or close to 1. The effectiveness of the proposed method is verified through simulations and experiments.