Residual Power Transfer Capability Analysis of an MMC-SST under Submodule IGBT Open-Circuit Fault

Modular multilevel converter-based solid-state transformers (MMC-SSTs) have flexible bidirectional power management capability and are proposed to integrate different types of distribution networks. Due to the large number of insulated gate bipolar transistors (IGBTs) adopted, reliability is one of the major challenges of these grid devices. In this paper, the submodule (SM) IGBT open-circuit fault characteristics of an MMC-SST are analyzed in detail. Unlike conventional MMC applications, the MMC-SST exhibits different fault behaviours due to its unique topology and complicated operation modes. Under certain operation modes, the capacitor voltage of the faulty SM may not be affected by the faults and the faulty unit is able to maintain its power transfer capability. Therefore, the existing fault diagnosis schemes in the literature cannot be directly applied to the MMC-SST to detect and locate all fault conditions. Simulated study in MATLAB/Simulink shows good agreement with the presented analysis. The analysis provides the ground for developing advanced fault diagnosis and fault-tolerant methods for the MMC-SST under SM IGBT open-circuit faults.