Opportunities to support the restoration of electrical grids with little numbers of large power plants through converter-connected generation and storages

The nuclear phase-out in Germany and the decarbonization of the energy supply are accompanied by a profound upheaval of the generation fleet. The decommissioning of large power plants and their replacement by decentralized, renewable generation plants (DG) as well as storage facilities leads to a shift in generation output from power plants connected to the transmission grid to a large number of renewable and decentralized plants connected to the distribution grids. This significantly impacts the approach to restore the interconnected system after a large-scale blackout. In order to be able to plan and instruct the dispatch of generating power from the network operator's control centres when restoring the grid after a large-scale blackout, two aggregation and forecasting methods differentiated by voltage level are presented. For larger farms connected to the high-voltage grid, a structure-based node approach is chosen; for the multitude of small and microunits connected to the medium- and low-voltage grids, an area-based approach is used. These methods do provide the network operator with active power/frequency management in the medium and long-term range. The provision of fast frequency response (FFR) by inverter-coupled units or storage systems stabilizes the frequency in the short-term range. In this way, the frequency nadir following a load connection can be mitigated during network restoration, which accelerates the process. This is particularly relevant against the background of the nuclear and coal phase-out, as it can compensate for the reduction in inertia and controllable feed-in power caused by power plant shutdowns. In a case study, it is shown that in a grid island typical for the early phase of network restoration, the inertia of a 500 MW coal-fired power plant can almost be replaced by a battery storage system of 50 MW which provides FFR.