Redundancy and Derating Strategies for Modular Multilevel Converter for an Electric Drive

Reliability is an important issue in medium-voltage electrical drives. The modular multilevel converter (MMC) is an inherently fault-tolerant topology and an interesting solution when quadratic loads are used. Redundancy strategies can be used to extend system failure capability. The strategies presented in the literature affect the dynamic performance and the power losses of the MMC. In addition, for isolated and inaccessible areas, such as ore slurry pumps, which are widely used in mining industries, the maintenance time and cost are high. This paper compares four redundancy strategies applied in a MMC for an electric drive and presents a derating strategy to maintain the system operating after a greater number of failures. The results showed that hot redundancy schemes have less impact on the dynamic performance of the system than the strategy that operates with the nominal submodule number. Besides, hot redundancy presents up to 14.72% more power losses. For quadratic loads, such as the ore slurry pump, it was concluded that, for 14% of failures, the speed reference should be reduced to at least 28% in order to keep the system operating.