Open Switch Fault- DCM Mode Analysis of Neutral Point Clamped Inverter

Neutral Point Clamped (NPC) Multilevel Inverters (MLIs) find tremendous application in high and medium power and is proposed to be used in Electric Vehicles (EV). Compact size than other MLIs and advantages like low voltage stress on switches, high power quality, high DC bus voltage, reduced DC link current and low size phase conductors than 2-level inverters emphasis the use of NPC for EV applications. Due to the presence of large number of switches, switch faults are unavoidable during the operating lifetime of MLIs. The operation of NPC during open switch faults of inner switches are different and severe than outer switches open circuit faults. The shift of NPC from CCM to DCM mode of operation with inner switch fault causes the presence of abnormality in the pole voltage. In this paper, a voltage equation for analyzing the pole voltage during single inner switch open circuit fault of NPC inverter is proposed. The equation attests the occurrence of abnormal pole voltage during fault. The equation is verified by simulation and experimental results.