Ancillary Services in Modern Power Systems: A Practical Survey

The restructuring of electricity markets has transformed the provision of ancillary services, which are essential to ensuring grid security, quality, and reliability. Traditionally supplied by large, centralized power plants, these services—encompassing frequency control, voltage regulation, and system restoration—are now increasingly delivered by distributed energy resources such as battery storage, electric vehicles, and microgrids. This comprehensive survey reviews the evolving definitions and classifications of ancillary services in modern power systems, emphasizing the impact of renewable energy integration and digitalization on grid stability. Specifically, it highlights the emergence of new ancillary services—such as fast frequency response and synthetic inertia—critical for managing the challenges posed by low-inertia grids in systems dominated by renewables. International case studies from regions including Ireland, Great Britain, and Japan are examined to illustrate how innovative operational strategies and regulatory reforms bolster system flexibility and resilience. The insights presented herein aim to guide policymakers and system operators in designing robust, sustainable power systems for the future.