Sub-Synchronous Control Interaction in Grid-Forming VSCs with Droop Control

This paper analyzes the sub-synchronous control interaction of droop-controlled grid-forming voltage-source converters (VSCs). The complex-valued impedance model of the droop-controlled VSC in the stationary frame is proposed first, based on which the grid-VSC interaction is characterized. It is revealed that the sub-synchronous resonances can be induced in stiff grids, owing to the interactions of inner voltage control loop and the outer power control loop. Experimental tests corroborate the theoretical analysis.