Inertia Response and Frequency Stability in Renewable Energy-Dominated Power Systems: Review of Virtual Inertia Techniques

As global power systems transition toward increasing penetration of renewable energy sources (RESs), such as solar and wind, maintaining frequency stability in converter-dominated low-inertia grids has become a critical challenge. This review examines the role of inertia in power system dynamics, emphasising the consequences of reduced mechanical inertia, the resulting increase in the rate of change of frequency (RoCoF), and the associated stability risks in grids with high inverter-based penetration. Inertial, primary, and secondary frequency response mechanisms are discussed alongside potential cascading failures, protection system triggering, and pathways toward fully renewable grids are assessed. Virtual inertia techniques, including synchronverters, swing-equation-based methods, virtual synchronous generators (VSGs), droop control, Virtual Oscillator Control (VOC), and matching control, are evaluated in terms of benefits, limitations, implementation complexity, and Technology Readiness Levels (TRLs). A key contribution is a multi-criteria evaluation framework that classifies these methods by control adaptability, scalability, and communication requirements, providing system operators with a structured basis for strategy selection. A comparative assessment of Phase-Locked Loop (PLL) synchronisation methods, including SRF-PLL, DDSRF-PLL, FLL-PLL, and Kalman filter-based approaches, is presented under weak-grid, unbalanced, and harmonic-distorted conditions. The integration of virtual inertia with energy storage technologies, such as batteries, supercapacitors, and flywheels, is also discussed, along with its role as an ancillary service within evolving electricity markets and grid codes. Collectively, this study provides a unified reference to advance intelligent, scalable, and deployment-ready frequency control in low-inertia renewable power systems, offering both theoretical insights and practical guidance for future high-RES grid architectures.