Power cycling lifetime improvement of three-level NPC inverters with an improved DPWM method

This paper investigates the power cycling lifetime of a three-phase three-level neutral-point-clamped (NPC) inverter used in a 50-kVA adjustable speed drive (ASD). Considering the short lifetime of NPC inverters under low output frequency conditions, an improved discontinuous pulse width modulation (DPWM) method is introduced to extend the inverter lifetime. This improved DPWM method is achieved by injecting a zero-sequence signal with relative high frequency into the voltage reference signals, which is in order to obtain lower swing value of the junction temperatures in the Insulated Gate Bipolar Transistors (IGBTs). Both simulation and experimental results are presented to verify the effectiveness of this solution.