Abstract

Inverter-based low-voltage microgrids face the issue of imbalanced reactive power sharing among parallel inverter units due to predominantly resistive feeders. This issue can be addressed using large droop coefficients in primary power control loops. However, the large droop coefficients can lead to increased power oscillations and lightly damped modes in the system. To address this issue, this paper proposes an adaptive droop control strategy. This approach adapts the reactive droop coefficient in real-time, ensuring the best possible reactive power sharing while maintaining sufficiently high damping of the inverter oscillatory modes. The paper first derives a mathematical relationship between the damping-ratio of the associated inverter mode and droop coefficients, showing their monotonically inverse dependence. Based on the relationship, an adaptive strategy is formulated that continuously updates the reactive droop coefficient based on the estimated damping-ratio of the critical system modes. The damping-ratio is derived by perturbing the reactive power droop coefficient and processing the inverter's active power response using the Matrix-Pencil mode estimation algorithm. The proposed adaptive droop control offers three key advantages: (i) enhanced reactive power sharing, (ii) improved microgrid stability through adaptive droop coefficient adjustment, enabling inverters to act as stability reservoirs, and (iii) applicability to grids with unknown R/X ratios.