As inverter-based resources (IBRs) are deployed at ever-increasing scale across transmission, distribution, and customer networks, the fundamental nature and capabilities of power systems are undergoing a profound transformation. This shift leads to a loss of inertia and effective short-circuit strength, creating new stability challenges. Stakeholders worldwide are increasingly recognizing the need for additional inertia and enhanced system support and are examining how grid-forming (GFM) technologies can be deployed and integrated as ancillary services. For example, in 2023 the German Federal Network Agency initiated market-based procurement schemes for inertia and other grid-forming capabilities. In this context, the Forum for Network Technology and Network Operation in the VDE (FNN) released the second edition of its guideline, which defines the requirements for units that wish to participate in the German inertia market. The guideline and its technical descriptions are pioneering in this field. The next step is to validate and demonstrate the applicability of the defined tests in practice.

In line with this objective, Fraunhofer IWES, in collaboration with Huawei, is currently conducting a dedicated test campaign on a MW-scale battery energy storage system (BESS). The system is being tested and validated in accordance with the FNN guideline together with a certification buddy, providing practical experience with the specified procedures and empirical evidence on their applicability to grid-forming resources. In this paper, the experiences gained during the execution of this measurement campaign are presented with focus on the practical implementation. In addition to providing an overview of the campaign and FNN guideline, selected tests and their results are presented in more detail and discussed. From the perspective of a test institute, there are strong similarities to conventional grid compliance tests in terms of test procedures and workflows, e.g. for fault ride-through, phase jumps, or RoCoF. These similarities enable the efficient use of existing frameworks and the exploitation of synergies in test planning, execution, and evaluation. Methodologies have also been developed and applied to test the islanding capabilities of the grid-forming unit in Power Hardware-in-the-Loop (PHIL) setups.