Review of recent developments in grid codes: Focus on compliance testing and grid-forming inverter-based resources

As inverter-based resources (IBRs) integration and cross-border electricity trade grow, harmonized grid codes become critical. Large-scale deployment of IBRs requires standardized, well-defined compliance verification requirements and testing procedures. Moreover, the effective integration of grid-forming (GFM) IBRs in low-inertia power systems needs clear grid code specifications and robust verification frameworks. Therefore, this paper presents a comprehensive review of grid code requirements regarding GFM IBR integration and grid code compliance testing. It offers an in-depth literature review of current methodologies for grid code compliance verification, with a particular emphasis on those recommended by European regulatory frameworks. The paper examines grid code specifications for key GFM functionalities, including voltage-source behavior, inertia emulation, fast fault-current injection, sink for harmonics, sink for unbalance and oscillation damping. Additionally, it discusses various simulation-based test frameworks to verify compliance with core grid-forming capabilities. The primary focus is on European grid codes; however various national and international grid code requirements are also discussed in detail for comparison. This review serves as a foundation for innovation and policy development, supporting system operators, manufacturers, and researchers in deepening their understanding of current regulations and emerging trends.