Selection of the Number of Levels of a Modular Multilevel Converter for an Electric Drive

The modular multilevel converter (MMC) is an inherently fault-tolerant topology and an interesting solution for medium-voltage (MV) electrical drives, especially when quadratic loads are employed. In order to select the best cost-effective solution, this paper presents a design procedure and comparison of MMC based MV electrical drives. The designs with the best IGBTs utilization factor for blocking voltages of 3.3 kV, 4.5 kV and 6.5 kV were chosen for comparison. The comparison is based on cost, performance, complexity and energy losses metrics. Manufacturer data were used in the power losses estimation procedure. These metrics are normalized to an index, introduced in order to select the best cost-effective MMC design. The methodology and comparison are exemplified through a 13.8 kV 16 MW three-phase induction motor which drives an exhaust fan based on real industrial mission profile. The simulation results show that the 29-level MMC has the best performance and lowest drive energy losses. Despite of its higher complexity at a moderate cost, it was evaluated as the best cost-effective solution based on the application mission profile.