Control Systems for Hybrid Power Plants in Weak Grids

This PhD thesis addresses the critical challenge of integrating large-scale renewable energy sources (RES) into weak power grids, where system strength and voltage stability are particularly vulnerable due to high impedance and a high reliance on inverter-based technologies. The research focuses on Hybrid Power Plants (HPPs) as a viable solution to improve voltage stability and increase the renewable power evacuation capacity of weak grids.

The study explores the limitations of existing control strategies and proposes a hierarchical control framework for HPPs that coordinates active and reactive power across wind, solar, and storage technologies. This control approach is designed to improve power evacuation capacity while preserving voltage stability. A key innovation involves shifting reactive power control to the high-voltage side of the grid connection transformer, thereby reducing impedance and enhancing the voltage profile at the point of connection. This allows for more efficient utilization of available renewable capacity and avoids premature voltage collapse that typically constrains weak grid operations.

The thesis also provides a comparative evaluation of grid-following (GFL) and grid-forming (GFM) inverter control schemes. It demonstrates that while GFL controllers suffer from instability under low short-circuit ratio (SCR) conditions, GFM controllers can maintain synchronism and provide effective voltage support even in severely weakened grid environments. These findings are supported by detailed electromagnetic transient simulations using PSCAD, as well as validation on the Real-Time Digital Simulator (RTDS) platform, confirming the robustness and scalability of the proposed methods.

Further investigations highlight the advantages of voltage control strategies over reactive power control in weak grid scenarios. Using Q–V droop-based voltage regulation, the HPP is shown to maintain stable operation across a broad range of SCR values, including extremely weak grid conditions where traditional methods fail. This emphasizes the necessity of adopting adaptive, voltage-focused control systems in future renewable power plant designs.

Overall, the thesis contributes to a deeper understanding of system behavior under high renewable penetration and redefines the conventional criteria for assessing grid strength: Instead of relying solely on SCR, the study advocates for the use of short-circuit power as a more accurate indicator of integration capacity, particularly in overplanted scenarios where traditional metrics may be misleading. The research also underscores the importance of context-specific solutions that align technical, economic, and regulatory dimensions in diverse geographical settings.

By developing and validating novel control methodologies tailored to weak grids, this work provides a comprehensive and practical framework for facilitating the reliable and secure integration of renewable energy. It offers both theoretical insights and applied engineering solutions that support the broader transition toward sustainable and resilient power systems.