Advanced negative sequence droop control for fault-ride-through operation and system support in weak grids

A control concept is presented to include grid supporting schemes into the droop control of grid-supporting and grid-forming power electronic converters. This control scheme adopts advanced power reference strategies used for grid-feeding converters to voltage controlling converters during unbalanced faults. The strategies are evaluated first with respect to the maximum power injection in case of a fault, which is necessary for frequency and voltage support in the grid. Second, the impact on critical power converter quantities like power oscillations or current and voltage limits are discussed. Relevant fault types are identified and the related three-phase voltage waveforms are extracted. Finally, the optimum injection strategy for two different fault types depending on the sag depth is derived.