Inherent Redundancy of SDBC-MMCC based STATCOM in the Overmodulation Region

The single delta bridge cell modular multilevel cascaded converter (SDBC-MMCC) has emerged as an interesting topology for Static Synchronous Compensator (STATCOM) applications. Since this converter is composed of tens/hundreds of components, reliability issues arise and redundant cells must be employed to fulfill the reliability requirements, increasing the initial cost of the STATCOM. In fact, there is a potential for fault tolerant operation when the overmodulation region is considered. This work aims to explore the inherent redundancy of SDBC-MMCC in the overmodulation region. Initially, the boundary conditions between linear and overmodulation region are analytically derived. Then, the inherent fault tolerance is evaluated based on 17 MVA/13.8 kV SDBC-MMCC based STATCOM case study with 24 cells per cluster. The results indicate that the converter is able to exchange rated power with the grid after 1 failure, with harmonic distortion within the limits suggested by the standards.