On Inherent Redundancy of MMC-Based STATCOMs in the Overmodulation Region

Modular multilevel converters (MMCs) are frequently featured to their modular structure, which results in fault-tolerant operation and easy redundancy realization. Nevertheless, in order to reach a given redundancy factor, more cells must be included in the converter, which directly affects the costs. This paper discusses the inherent redundancy of a modular multilevel converter (MMC) based static synchronous compensator (STATCOM) operating in the overmodulation region. Analytical expressions for the limits of the converter linear region were developed in order to define the minimum required dc-link voltage. Moreover, sensitivity analyses were implemented in order to show the effects of grid voltage variations, different output impedances, power factor and injected currents. The results indicated that the operation in overmodulation region has significant inherent redundancy. For a MMC based STATCOM with 26 cells, the converter can ride through 4 failures per arm without significantly increasing the output THD or reducing the injected current into the grid.