Contribution of negative-sequence controlled distributed generation to power system stability under unbalanced faults: A discussion paper

The transformation of the power system in terms of efficiency and sustainability will further lead to increasing converter-coupled generation and demand. This changes the system´s characteristics and influences its stability. Despite these fundamental changes, the secure operation of the power system must be maintained at all times. DG and any converter-coupled generation of relevant size are requested to stay connected to keep power equilibrium and to support the voltage during faults. The balanced fast voltage control currently applied for all types of faults could be improved by an unbalanced injection of short-circuit current among the three phases depending on the fault. A technical solution is an active control of the negative-sequence during unbalanced faults. However, introducing a general requirement, e.g. in grid codes, for this new feature must be well justified. This paper is intended to start a structured discussion of unbalanced fast voltage control by converter-coupled generation for unbalanced faults from a system perspective.