Utilising a Lagrangian approach to compute maximum fault current in hybrid AC-DC distribution grids with MMC interface

Hybrid AC-DC networks are transforming high-voltage transmission and medium-voltage distribution grids by embracing the advantages of both AC and DC systems, which facilitates the inclusion of renewable energy sources and distributed generation. As modular multilevel converters (MMCs) are vastly employed in such hybrid networks, determining their maximal fault current in worst-case scenario is a critical design factor for planning and implementation of a reliable protection scheme. This study develops a novel mathematical framework that applies a Lagrangian energy method to calculate the maximal fault magnitude. This method allows to account for converter's internal energy and compute its impact on the amplitude of the fault current. It is shown when the converter is interfacing weak AC sources with high internal impedance such as wind farms or solar farms, dumping the internal energy of the converter into the fault is the salient contributing factor of the fault magnitude. Furthermore, to distinguish and classify the output overcurrent as either ignorable transients or destructive faults, a perceptron with sigmoid threshold is employed. The model is verified using a simulated medium-voltage hybrid AC-DC distribution network.