Requirements and verification procedures for grid-forming units – the German approach to ensure power system stability under very high penetration of inverter-based sources

The ongoing transformation of the electrical power system in various parts of the world raises the question of how and to what extent inverters with grid-forming capabilities can be installed ensuring power system stability at very high share of inverter-based generation. While a systemic need for a certain portion of grid-forming power plants is widely confirmed, the method to determine the exact need and the quantification and testing of the grid-forming capability is under discussion. Germany has chosen a two-stage approach to meet the systemic demand for grid-forming capabilities as quickly as possible. In a first step, Germany plans for the new market-based ancillary service “inertia” by the TSOs from 2025. In a second step, based on the national implementation of the European network code “requirements for generators” (RfG2.0), a certain minimum inertia capability is expected to become a mandatory requirement for newly connected large power plants from 2027/28. The inertia market is considered a vital means and incentive for rapid establishment of grid-forming capabilities in the grid. It shall allow for the fast achievement of essential core properties such as voltage source behaviour or phase jump power from inverter-based grid-forming power plants while procuring power reserves to encounter power deficit and surplus in the grid, based on positive and negative inertia products, respectively. The German approach to translating the indistinct term “grid-forming” into such inertia products is presented, and their quantification and validation are elaborated upon by giving an overview of technical requirements, as well as corresponding verification procedures as prerequisites for participation in the inertia market.