Reliability of DC-link Capacitors in Three-Level NPC Inverters under different PWM Methods

Pulse-width modulation (PWM) method plays an important role to ensure high efficiency and reliability of the three-level neutral-point-clamped (NPC) inverter. However, a comparison of the PWM methods in previous research has only considered power loss and reliability of power devices, while other reliability-critical components such as dc-link capacitors are not taken into account in the analysis. In this paper, the impact of PWM methods on the reliability of the dc-link capacitors in three-level NPC inverters is investigated by considering the capacitor power loss and their thermal stress. The evaluation results indicate that the PWM method which aims at minimizing the switching loss may introduce a significant power loss in the dc-link capacitors due to high amplitude of the low-frequency neutral-point (NP) voltage fluctuation. In that case, the thermal stress of the dc-link capacitors can be significantly increased, especially when the discontinuous modulation scheme is applied, affecting the reliability of the dc-link capacitors.