With the growing share of inverter-based resources (IBRs), and in particular grid-forming (GFM) units, system behavior during and immediately after fault events is changing. While fault ride-through (FRT) requirements are well established (e.g., VDE-AR-N-4105 to -4130), there is still little guidance on transient behavior within the first few grid cycles after fault inception and clearance. Field observations and simulation studies indicate that effects such as phase-angle jumps, over- and undervoltages, and high rates of change of frequency (RoCoF) can occur in this time frame and are not sufficiently covered by current standards. These phenomena may lead to unintended disconnections or adverse interactions between grid-following and grid-forming units.

This work presents the results of the OptiTransient project, which addresses these challenges by systematically investigating transient phenomena and deriving corresponding performance requirements. A particular focus is placed on the interaction of grid-following and grid-forming converters, including the impact of different control structures and the specific behavior of grid-forming units under fault conditions.

The analysis is based on a combination of literature review, detailed EMT simulations, as well as laboratory and field measurements. Transient effects during realistic fault scenarios are characterized, including phase-angle jumps and transient overvoltages, and their impact on converter stability and tripping behavior is assessed. Based on these investigations, quantitative performance indicators are defined and translated into practical and testable requirements.

In addition, different control strategies for grid-forming converters are evaluated, with a focus on their behavior under current limitation and during severe faults. Selected approaches are further developed and validated using hardware-based testing and real-time simulation environments. Complementary to this, suitable test setups and procedures are identified to enable realistic reproduction and verification of transient conditions.

The results provide a technically grounded basis for defining transient performance requirements for inverter-based resources. They support the extension of existing grid codes and contribute to the development of reproducible testing and validation methods, offering practical guidance for manufacturers, system operators, and grid code development.