A fault-tolerant grid-forming converter applied to AC microgrids

•Bidirectional grid-forming converter with a fault-tolerant functionality.•Delta connection of three single-phase voltage source inverters (VSIs).•The converter can operate in three different modes of connection.•A control strategy is developed to allow having a stable operation.•Experimental results are presented to validate the proposed topology. A crucial converter for the islanded operation of AC microgrids is the grid-forming converter. This central converter supplies electrical loads, and assures the distributed generators operation, since the malfunctioning of grid-forming converter can jeopardize the whole microgrid. This paper proposes a bidirectional grid-forming converter with a fault-tolerant functionality applied to islanded AC microgrids using a centralized control architecture. The converter is based on a delta connection of three single-phase voltage source inverters (VSIs) using an H-bridge topology for three-phase three- or four-wire systems. For this latter case, a zigzag transformer is used to provide the neutral conductor for the system. In addition to establishing voltage and frequency references for the microgrid, the converter is capable of changing its configuration from delta to open-delta, and vice-versa, in a smooth mode and without interrupting the microgrid operation. Thus, in case of failure of one of the grid-forming VSIs, the system can keep supplying the microgrid. A control strategy is developed to allow a stable operation in all operating modes. Experimental results are presented with a converter prototype to validate the proposed topology, control and mode of connection.