Interaction analysis of vector- and droop-controlled VSCs in parallel

Voltage source converters (VSCs) can be either vector-controlled to follow the variation of the primary energy sources, or droop-controlled to support the grid frequency/voltage. To meet both the local and system-level demand, hybrid operation of VSCs with the two control schemes is foreseen, while the dynamic interactions within such a system is less focused. This paper investigates the stability issues of a system with both types of VSCs in parallel connection to the main grid, particular attention is given to low-frequency outer power control dynamics. Net-damping criteria is used for qualitative stability evaluation. Comparative study with only vector-controlled case indicates that the hybrid controlled VSCs system can have a better stability for the smaller negative damping region presented. In addition, a potential low-frequency instability phenomenon, due to the interaction between the AC voltage control mode of vectored-controlled inverter and the droop control mode of droop-inverter, is identified by participation factor analysis. Detailed time-domain simulations are carried out to verify the above analysis.