Symmetric droop control for improved hybrid AC/DC microgrid transient performance

A droop-controlled, hybrid ac/dc microgrid represents a robust architecture that can coordinate the operation of multiple distributed sources while minimizing the power conversion stages. While the reduced-order nonlinear dynamics of grid-forming, droop-controlled inverter-based microgrids have been rigorously shown to be stable for a wide range of operating parameters, the problem of analyzing the transient stability characteristics of droop-controlled hybrid ac/dc architectures has not been adequately addressed. This work reviews the link between the Virtual Synchronous Machine control concept and grid-forming droop control, and introduces a new droop control strategy for ac/dc hybrid microgrids, termed ‘symmetric droop control’ (SDC). SDC better addresses the dynamic interactions between the ac and dc sub-grids of the hybrid microgrid, by ensuring that the interfacing inverter appropriately represents the dc sub-grid dynamics when interacting with other inverters. SDC can help to ensure that the reduced-order, nonlinear dynamics in both the ac and dc networks remain predictable and well-behaved during large disturbances in the ac network.