Optimal design of modular multi-level inverter with reliability consideration

The modular multi-level converter is an upcoming topology for drive application in the megawatt power range. This paper presents a comprehensive reliability survey for this kind of converter which can be used for its optimal designing. In this survey, converter total failure rate, is calculated for different number of output levels to find the optimal condition in which converter can provide the best possible reliability. MIL-HDBK-217F(2) is used for elements failure rate estimation and voltage sorting method is used for capacitors voltage balancing. Converters with the same output rating and different levels are studied. Simulation results are used for loss calculation to be used for semiconductors junction temperature calculation. Switches junction temperature is used in their failure rates calculation. Reliability comparison between MMC systems with different number of levels is done. Lastly semiconductors loss and output voltage THD are added to optimization to find optimal number of levels. Optimization verifies 7-level and 9-level converters as optimal converters for medium voltage medium power drive application, for Level Shifted (LS) and Phase Shifted (PS) - PWM respectively.