Grid-forming (GFM) converters are considered a key technology to maintain voltage and frequency stability in future power systems with high shares of converter-based generation. However, differences in the implementation of grid-forming controls across manufacturers require systematic testing and comparative assessment. Within the GFM-Benchmarking-Project, initiated by Fraunhofer ISE in cooperation with the four German transmission system operators 50Hertz, Amprion, TenneT and TransnetBW, a comprehensive testing procedure has been developed to evaluate the dynamic properties of GFM converters in a black-box approach. This paper, as the second part of the project results, focuses on two fundamental aspects: the voltage source properties and the inertial response of the converters. For the voltage source properties, tests in islanded operation were performed to evaluate the dynamic stabilization capability and to determine the settling times. The results revealed a wide spread, with some devices stabilizing within only a few milliseconds, while others required up to one grid period. The investigation of the inertial response was conducted using both RoCoF-based (Rate of Change of Frequency) and load-based approaches, enabling a detailed comparison of methods and parameterizations. The converters generally provided inertia in line with the expectations, though differences were observed in damping, symmetry between positive and negative RoCoFs, and response under constant frequency deviations. These findings highlight the need for harmonized performance requirements and standardized testing procedures, ensuring that GFM converters can be reliably integrated into future power systems.
Ernst et al. (2026) studied this question.