A virtual-filter-based stability enhancement control for grid-connected converter in DC microgrids under unbalanced grid conditions

DC microgrids, considered building blocks of smart grid technologies, are subjected to small-signal instability due to the extensive introduction of power electronics devices. Therefore, in this paper, a sequence Virtual Filter (VF) controller, which considers not only the longitudinal virtual parameters but also the lateral capacitance and resistance, is first developed to increase the flexibility of system stability adjustment compared to the Virtual Impedance (VI) controller. Then, as distribution feeders are normally unbalanced and the application of the proposed sequence VF controller may amplify the distortion of imbalances on DC voltage, a novel Reference Current Generation (RCG) strategy considering the VF controller is proposed for Grid- Connected Converter (GCC) to improve DC microgrid power quality. The double-frequency fluctuation of the DC-link voltage is eliminated by regulating the oscillation of the active power flowing into the converter instead of the Point of Common Coupling (PCC) to 0. The PSCAD simulation results illustrate that the sequence VF controller can enhance the stability adjustment since, in some cases, the system can only be stabilized by adjusting lateral parameters. On the other hand, the proposed RCG strategy can significantly reduce DC voltage fluctuations compared to the traditional approach. Furthermore, incorporating the proposed strategy with the sequence VF controller offers greater flexibility in reducing the negative-sequence current while maintaining a power transfer capacity for GCCs in a master-slave-based DC microgrid, comparable to that of the traditional strategy.