Multi-Domain Investigation of Distance Protection in Converter-Dominated Grids: From Static to EMT and HIL

The increasing penetration of power electronic converter-based generation and HVDC systems is changing the fault behaviour of transmission networks. Distance protection schemes traditionally rely on predictable fault currents and stable impedance trajectories provided by synchronous generators. In converter-dominated grids these assumptions no longer hold due to current limitation and control-dependent fault current contributions of grid-following and gridforming converters. As a result, the apparent impedance seen by distance relays can deviate significantly leading to maloperation or reduced dependability and security. This paper presents a structured multi-domain framework for assessing distance protection performance in converter-dominated grids. The framework combines static short-circuit analysis, electromagnetic transient simulations and real-time Hardware-in-the-Loop testing using a hardware commercially available distance protection relay. Identical test cases based on a modified IEEE 9 node benchmark system are applied across all domains. The results show that synchronous generator dominated grids provide the most stable protection behaviour while both grid-following and grid-forming dominated scenarios exhibit increased impedance deviation and zone misclassification. Static short-circuit analysis reveals systematic impedance displacement relative to protection zone boundaries. Electromagnetic transient simulations show that time-dependent impedance behaviour can delay or prevent stable zone entry under weak grid conditions and high fault resistance. Hardware-in-the-Loop testing confirms that additional signal processing and implementation effects further influence protection performance. The study demonstrates that reliable distance protection assessment requires a coordinated multi-domain approach rather than reliance on a single analysis domain.