The global transition toward a sustainable energy future is fundamentally transforming the structure of power generation. Large, centralized conventional power plants are increasingly being replaced by a growing number of smaller, decentralized energy resources. While this development offers new opportunities, it also introduces significant challenges for maintaining power system stability, particularly due to the rising share of converter-based generation. In this context, grid-forming inverters have emerged as a key technology for enabling stable operation of power systems with high shares of renewable energy. Unlike conventional grid-following converters, these inverters operate as controlled voltage sources and are capable of providing synthetic inertia, thereby contributing to enhanced grid stability and system strength.

Recent regulatory developments and testing guidelines in Germany define the practical integration of grid-forming inverters (see VDE FNN Guideline –Technical requirements for grid-forming capabilities including provision of inertia). The latest revision of this document provides a more detailed specification of the properties, requirements, and operational behavior of grid-forming systems, for instance defining requirements for the provision of instantaneous reserve.

In addition to defining technical requirements, the FNN guideline also specifies concrete test sequences for verifying grid-forming capabilities and formulates quality criteria’s for proper control design. The proposed testing procedure comprises 16 individual test sections designed to assess different aspects of grid-forming capability. These tests range from the verification of voltage-source behavior and the response to phase angle jumps, to the evaluation of converter behavior under current limitation and the capability to provide instantaneous reserve.

The paper examines selected requirements from the FNN guidance in detail and evaluates selected sequences through laboratory experiments using a free programmable state of the art grid-forming inverter. The implemented grid-forming control should fulfill the defined quality criteria and is experimentally tested with respect to the required system behavior specified in the FNN guideline.