The Modular Multilevel Converter (MMC) is considered as the most promising converter technology for HVDC applications. The topology of the MMC is comprised of thousands of distributed sub-module (SM) capacitors which behave as energy storage devices. This distinct feature of the MMC brings an additionnal degree of freedom – the internally stored energy – which does not exist in conventional Voltage Source Converters (VSCs). In order to ensure the proper function of the MMC, this energy must be maintained within a certain range. This range varies in accordance with the operating power and the associated grid conditions. Knowing the upper and lower limits of the energy is important for maintaining adequate margins from the present level of the energy. However, due to the complexity of the internal dynamics of the MMC, it has never been clarified.
Patent FR3053854 (B1) WO2018007741 (A1)