Comprehensive review of the requirements of AC transmission grid codes and the role of FACTS devices in integrating renewable power plants

• Integrated three-tier framework for GC compliance analysis • Global convergence trends in AC transmission grid codes • Distinct compliance strategies for PV, wind, hybrid plants • FACTS as dynamic enablers of FRT and voltage support • Bridging regulations with practical engineering solutions The increasing penetration of renewable power plants (RPPs), particularly photovoltaic (PV) and wind systems interfaced through power electronics, has fundamentally reshaped power system operation and driven the evolution of alternating current (AC) transmission grid codes (GCs) from basic connection rules toward performance-oriented regulatory frameworks. Modern grid codes impose stringent requirements on fault ride-through (FRT) capability, voltage and frequency support, reactive power control, active power recovery, and power quality. This paper presents a structured review of AC transmission grid code requirements and examines the role of Flexible AC Transmission Systems (FACTS) in enabling practical compliance by renewable power plants. Grid codes from different regions are comparatively analyzed, revealing global convergence in core technical requirements despite regional regulatory differences. The impact of renewable technology type on compliance pathways is investigated, showing that photovoltaic PV, wind, and hybrid plants require fundamentally different control and technical strategies due to inherent converter and system constraints. A comprehensive literature review demonstrates that FACTS devices have evolved from local compensators into active elements of dynamic grid control, enhancing fault ride-through FRT performance, voltage support, reactive power regulation, and power quality. The main conclusion is that sustainable grid code compliance cannot rely solely on converter-based control, but requires coordinated integration of renewable technologies with appropriately selected and controlled FACTS devices within a system-level framework. A three-tier analytical structure is proposed to link regulatory requirements, technology constraints, and network support tools.