With the growing deployment of inverter-based resources (IBRs), particularly grid-forming (GFM) assets, power-system behaviour during the first cycles after fault inception and clearance is becoming increasingly relevant. Existing fault ride-through (FRT) requirements predominantly specify RMS-based behaviour and provide only limited guidance for fast electromagnetic transients. Recorded fault events and simulation studies indicate that phase-angle jumps, transient overvoltages, high rates of change of frequency (RoCoF), and control-related interactions can occur in this time frame and may lead to unintended disconnections of converter-based assets.

This paper presents the framework and key findings of the OptiTransient project. The analysis combines recorded real-world high-voltage fault events, EMT simulations, and laboratory measurements. Relevant transient phenomena are characterised and their dependence on grid-following (GFL) and GFM control structures is assessed. Particular attention is given to converter current-limitation strategies, which strongly influence transient voltage behaviour, available instantaneous power reserve, and post-fault recovery.

EMT studies of future grid configurations show that increasing GFM penetration can improve the damping of transient oscillations and reduce voltage stress during voltage dips and phase-angle jumps. The results also identify converter current headroom and current-limitation design as key constraints for the effective provision of GFM-based instantaneous reserve, including from large battery energy storage systems.

Furthermore, measurements on different FRT test benches demonstrate that the realised transient voltage and current waveforms depend substantially on the test-bench topology and switching principle. An improved phase-wise, magnitude-based current-limitation approach for unbalanced faults is evaluated by simulation and software-in-the-loop testing.

The results provide a basis for measurable transient-performance indicators, realistic EMT studies, and reproducible test approaches for the further development of grid-code and test-bench requirements for converter-based generation and storage assets.