Time-Domain Backup Protection for Converter Dominated Hybrid AC–DC Microgrids

This article presents a backup protection scheme for hybrid ac–dc microgrids. A time-domain technique is proposed to isolate the faulted dc and ac subgrids from the point of common coupling using ac and dc currents, respectively, in case the primary protection fails. Symmetrical component decomposition in a bipolar dc microgrid is used to obtain the mode-0 and mode-1 current at the terminal of the bidirectional interlink converter (BIC). The superimposed component of mode-1 current acts as the decisive discriminator between ac and dc subgrid faults. One cycle moving average of the phase currents at the BIC terminal identifies the ground faults in dc subgrids. Further, the pole-to-pole faults in the dc grids can be detected using two consecutive zero-crossing widths in the phase currents at the ac terminal. The proposed protection scheme is validated on a hybrid ac–dc microgrid, simulated using a real-time digital simulator and a digital signal processor in a hardware-in-loop scenario for different faults on either side of the microgrid.