Analyze and improve lifetime in 3L-NPC inverter from power cycle and thermal balance

Three-level Neutral-point-clamped (3L-NPC) topology is becoming a realistic alternative to the conventional one in high-voltage and high-power application. Studies show that the power cycling mean time to failure (MTTF) of the semiconductor bond wire in 3L-NPC inverter system may be very short under some common conditions. Firstly, this paper shows the impact of some key parameters on power electronic system lifetime according the analysis of semiconductor failure mechanism. Secondly, a switching frequency reduction method based on the position relationship between the flowing current and load voltage is applied to reduce power cycle and switching losses. And then, three-level active neutral-point-clamped topology is taken into account to wake the most thermo stressed device. In order to validate the improve lifetime method in this paper, a 2 MW 3L-NPC converter used in wind energy has been simulated. Finally, the described method has been tested in a 7.5 kVA scalable prototype.