A Novel Fault-Tolerant Operation Approach for the Modular Multilevel Converter-Based STATCOM With the Enhanced Operation Capability

The operation capability of the modular multilevel converter (MMC)-based static synchronous compensator (STATCOM) is limited under submodule (SM) failures. When running in the STATCOM mode, the floating dc-link voltage of the MMC is adjustable. Therefore, this article proposes a novel fault-tolerant operation approach to enhance the operation capability of the MMC under severe SM failures. First, the dc-link voltage is dynamically regulated under SM failures, where the capacitor voltages of all SMs are slightly increased. The modulation ranges of the faulty and healthy phases can be enlarged, and the even voltage stress distribution can be realized. Then, a low-magnitude fundamental frequency zero-sequence voltage (FF-ZSV) is injected to balance the line-to-line voltages. Finally, the virtual energy idea is proposed, where the capacitor voltages of faulty SMs are considered to be clamped at the reference value. As a result, the arm energy reference will be the same in all six arms, and the arm energy balancing control is simplified. Detailed comparisons show that the proposed approach has an enhanced operation capability with lower capacitor voltage increment. Simulation and experimental results verify that even under severe SM failures, the proposed approach can still guarantee the fault-tolerant operation within the specified security region.