Grid Agnostic Droop Control Strategy for Damping Restoration and Optimal Reactive Power-Sharing

In low-voltage microgrids, parallel grid-forming inverters face the challenge of disproportional reactive power sharing due to (i) the local nature of voltages and (ii) the cross-coupling between the frequency and voltage droop control loops resulting from predominantly resistive feeders. A large magnitude reactive droop coefficient (RDC) can decouple the frequency and voltage droop loops. However, the large droop coefficients can lead to increased power oscillations and insufficiently damped modes in the system. To balance these conflicting objectives, this paper proposes an adaptive droop control strategy that adapts the RDCs in real-time, ensuring the best possible reactive power sharing while maintaining sufficiently high damping of the inverter's oscillatory modes. The damping-ratio is derived by perturbing the RDC and processing the inverter's active power response using the Matrix-Pencil mode estimation algorithm. The proposed control system is validated through Controller Hardware-in-the-Loop (C-HIL) experiments. Typhoon HIL-402 real-time (RT) emulator is used to emulate the power stage. A dedicated Windows PC serves as the secondary controller, updating the reactive droop coefficient. Communication between the hardware emulator and the controller takes place over the OPC UA protocol via a LAN. The results demonstrate that the proposed control offers three key advantages: (i) improved reactive power sharing compared to the fixed droop control method, (ii) enhanced microgrid stability under various disturbances, and (iii) applicability to grids with unknown grid $R/X$ ratios.