Assessment of Aging and Performance Degradation of Supercapacitors Integrated into a Modular Multilevel Converter

The interest for modular multilevel converter (MMC) with energy storage systems (ESSs) has increased quickly over the last decades. Since an MMC has several hundreds of sub-modules (SMs) and ESS performances usually deteriorate quickly, the ability of the converter to provide the expected service during its lifetime must be investigated. In this work, supercapacitors are used as ESS and the reduction of their capacity is first calculated through lifetime simulation of the ESS using aging models. Then, simulations are used to analyze the available energy of the proposed converter considering dispersions for the initial parameters of ESSs and for the aging rate. Finally, the results are compared with different levels of redundancy and maintenance intervals to discuss the viability of the solution. For the depicted case study, results show that oversizing the ESSs without carrying any maintenance operations during the lifetime of the converter minimizes the total number of cells required.