Grid-forming units are expected to provide voltage-source behavior and frequency-related active power support in converter-dominated power systems. In Germany, the VDE FNN guideline on grid-forming capabilities defines measurement-based verification procedures in fictitious island operation, in particular for voltage-source behavior and grid-security-based primary control (PRNB). While these tests are highly relevant for the qualification of grid-forming units, their conventional realization requires a comparatively complex laboratory setup with dedicated hardware and limited flexibility.

This paper presents a Power-Hardware-in-the-Loop (P-HIL)-based alternative for executing fictitious-island tests and compares it with the corresponding conventional laboratory procedure. The work is based on laboratory investigations and original [DG1] [RB2] methodology development within an ongoing research project on validation methods for grid-forming units. The contribution comprises three elements: (i) development of a generalized real-time-capable model of the fictitious island grid, (ii) implementation of a suitable P-HIL interface for the interaction of two voltage sources, and (iii) benchmarking of the emulated setup against the physical setup using identical FNN-oriented test sequences for voltage-source behavior and PRNB.

The paper focuses on the extent to which the P-HIL setup can reproduce the key observables of the conventional test, including dynamic voltage and frequency response, active-power reaction, and overall test reproducibility. Based on this comparative proof-of-concept, the work derives boundary conditions for stable interfacing, discusses the benefits and limitations of both approaches, and identifies where P-HIL offers clear advantages regarding flexibility, automation, safety, repeatability, and scalable parameter variation.

Beyond the specific test campaign, the paper addresses the broader standardization question of whether P-HIL can provide evidence of sufficient quality for future compliance testing and certification of grid-forming units. It thus contributes to both a practical methodology and comparative validation evidence for extending measurement-based grid-code verification by P-HIL-based procedures.