A Novel Submodule Level Fault-Tolerant Approach for MMC With Integrated Scale-Up Architecture

A submodule (SM) level fault-tolerant method for modular multilevel converters (MMCs) with a scalable control design for medium- and high-voltage applications is proposed in this article. A large number of SMs, which is typically required in high-power applications, is divided into sets with a smaller number of SMs. A local controller is assigned to each set for the capacitor voltage balancing (CVB) and switching signal generations, and the SM level protection. A CVB control method is proposed to maintain SM voltages among sets within a prespecified band under SM failures, in which an MMC arm can operate with unequal numbers of SMs among sets. The computational and communication burdens are reduced in each local controller. Most importantly, changing the number of SMs is achieved without significant modifications in the control system, which improves the expandability and flexibility of the MMC system to easily scale-up to higher voltage levels. A ±320 kV/1000 MW MMC system has been studied in a real-time digital simulator (RTDS) and field-programmable gate arrays boards to validate the proposed fault-tolerant control method under SM failures.