Envelope-based criteria and RMS open-source model for power park modules under future grid-forming capability requirements

Grid-forming (GFM) capability is gaining attraction in European grid connection codes and utility specifications. While a broad consensus is emerging regarding the expected functionalities of GFM behavior and the test procedures used to demonstrate its stabilizing effects, the associated compliance criteria remain an active area of research. This paper proposes an envelope- based approach to assess compliance across representative and standard test scenarios, including phase angle jumps, voltage magnitude steps, short-circuit ratio variations, and rate-of-change-of-frequency (RoCoF) events. These envelopes are derived from analytical equations and incorporate time-domain performance metrics such as rise time, settling time, and tolerance bands. Additional margins are included to account for potential manufacturer-specific implementations, by allowing variations in key model parameters such as virtual inertia, damping, and effective impedance. The proposed approach is validated using both RMS and EMT generic models applied to characteristic test cases, offering a practical and flexible framework for evaluating future GFM requirements without over-specifying converter hardware.