Current Constraints of Grid-Connected Converters Under Grid Asymmetrical Faults

Renewable energy generation is integrated into the power system through grid-connected converters. The converters should possess fault ride-through (FRT) capability in order not to disconnect from the grid and to provide reactive power support during grid faults. After a grid asymmetry fault occurs, the grid-connected converter should generate both positive-sequence reactive currents and negative-sequence currents to support positive-sequence voltage restoration and suppress the rise of the negative-sequence voltage in the power system. With the injection of negative sequence current, the three-phase currents of the gridconnected converter are asymmetrical, which may lead to the damage of semiconductor switching devices in one or more phases of the converter due to overcurrent. In this paper, the expression of three-phase asymmetrical peak currents is calculated based on the requirements of the grid code, and the maximum operation boundary of positive sequence active current is derived under the constraint that no overcurrent occurs in all three-phase currents under grid asymmetrical faults. Furthermore, an asymmetrical FRT strategy taking into account the current constraints is proposed, and the validity of the maximum current boundary is verified by simulation.