In the context of climate protection, the energy system is undergoing a structural transformation. Many synchronous generators, which are installed mainly in the transmission grid, are going to disconnect from the grid as part of the decarbonisation. This results in a reduction in their inherent characteristics, like instantaneous reserve. On the one hand, these structures are being replaced by, e.g., grid-forming (GFM) battery storage systems at the extra-high and high-voltage levels. These GFM structures can use their inertia to limit frequency gradients and provide instantaneous reserve. On the other hand, within the energy transition, there is an expansion of converter-based generation capacity at the distribution grid. In the future, a GFM control structure is also conceivable there.

This paper aims to examine the extent to which the provision of instantaneous reserve capacity at lower grid levels differs from that in the transmission grid. First, an assessment of the status quo regarding the potential of instantaneous power provision at the low-voltage grid is shown. The expected results of the investigations are then derived theoretically. In particular, active power losses are taken into account. Subsequently, a benchmark-grid simulation model is dynamised. For that, battery storage systems with GFM control structures are modelled, simplified and parametrised to investigate the provision of instantaneous reserve capacity. For validating the used GFM generation model, the instantaneous reserve provided is evaluated using reference frequency gradients from relevant regulatory requirements. GFM generation will be installed in two main simulation scenarios: at the low-voltage and extra-high-voltage levels, in order to realise a comparison. For the equivalence analysis, power steps are simulated at the transmission grid level. The resulting imbalance between generation and consumption is instantly compensated by the GFM units using their available instantaneous reserve. It is expected that, due to the differing impedances and grid structures between the point where the power step occurred and the GFM unit at different voltage levels, varying amounts of power loss will occur. To investigate and quantify this effect, the influence of the voltage level on the active power adjustment (as instantaneous reserve) of a GFM unit is evaluated under the same fault conditions. Test scenarios are defined that simulate positive and negative power steps during the battery storage system’s charging and discharging modes. Other effects and causal relationships may be investigated.

The study is simulation-based. The simulation results allow a comparison of the required instantaneous reserve capacity at the low-voltage grid with that at the transmission grid. The results are categorised in terms of necessary measures, recommendations or the prioritisation of the expansion of GFM generation in distribution grids, particularly low-voltage grids. In summary, the aim of this paper is to assess the equivalence of instantaneous reserve provision at the transmission and distribution network levels.