New current limiting control for grid-forming converter under unbalanced faults

In the future, most power systems will rely on converter based generation. In order to guarantee stable operation so-called gridforming converters were developed. In contrast to synchronous generators, the behaviour of converters is mainly dependent on its control algorithm and not by its physics. This offers a wide range of opportunities for shaping the control in the desired manner. The disadvantage of low overcurrent capability can be overcome by oversized power electronics or by appropriate controls. In this paper we present a control scheme based on the virtual synchronous machine with an approach for overcurrent protection in positive- and negative-sequence. The proposed approach can limit fault currents successfully and keeps its gridforming functionality during the fault. Furthermore prioritization of active and reactive power in positive- and negative-sequence during fault is possible and good transient stability can be observed. The effectiveness is shown in MATLAB/Simulink and laboratory tests.