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.

The growing integration of renewable energy sources poses significant challenges to the existing power system infrastructure. These challenges include reducing power system inertia, resulting in faster dynamics and deterioration of frequency stability margins. Additionally, the limited capacity of power electronics-based inverter units reduces the sources of reactive power generation, jeopardizing voltage stability. To address these issues, system operators are increasingly required to procure ancillary services to ensure reliable power system operation. This paper presents a comprehensive literature review of the current state of the ancillary services market in Germany, examining the various procurement schemes currently employed and the economics of services. It also highlights the insufficiency of existing ancillary services and identifies potential services that may be necessary to stabilize Germany's future power systems, particularly as renewable energy sources continue to gain prominence. Furthermore, the paper explores the current status of battery storage technology in Germany and highlights its potential to provide ancillary services across different time resolutions. This review aims to benefit academics, researchers, practitioners, and policymakers by enabling them to make informed decisions and effectively navigate the changing energy landscape in Germany.