MMC Control Strategy for Converter Lifetime Optimization Based on Thermal Stress Analysis of Lower IGBT Device in Half-Bridge Submodules

GigaWatt-scale offshore wind farms are being connected using the Modular Multilevel Converter (MMC), in which submodules act as the main building block. The extensive number of connections has a significant effect on the system's security of supply. Periodic weather conditions and time-dependent labor capacity complicate maintenance, emphasizing the need for more flexibility when replacements are required. Traditionally, submodules are redundant or over-dimensioned to increase reliability, but do not resolve flexibility issues faced by operators. This article presents a novel operation method introducing flexibility in the submodule stress distribution. Specific submodules can be used more or less frequently depending on maintenance achievability. The technical condition of each submodule is determined, and then a selection window with a configurable length determines the submodule-specific insertion frequency, affecting the remaining useful lifetime. The capacitor voltage balance can be guaranteed with traditional sorting methods and is compatible with the lifetime optimization algorithm using a priority factor. The priority can be divided between lifetime optimization and capacitor voltage balancing depending on the operator's needs. This is superior to the traditional methods due to the ability to control the submodule-specific deterioration pace. Analytical evaluation and simulation studies demonstrate the effectiveness of this control approach.