Significantly more photovoltaic capacity could be connected to substations between medium- and high-voltage connections if battery storage systems absorbed peak generation, thereby improving the utilization of existing transformers. This is the conclusion of a short study conducted by the Fraunhofer Institute for Energy Economics and Energy System Technology IEE on behalf of the German Solar Association (BSW-Solar), focused on German rural substations.
For a defined group of 1,313 substations heavily reliant on renewable energy sources in rural areas, the Fraunhofer IEE modeled a theoretical expansion potential of approximately 150 gigawatts of additional photovoltaic capacity and 60 gigawatts of battery storage capacity. The starting point was real-world operating data from 20 transformers in 13 substations. The results obtained were then extrapolated to the larger group of substations using similarity analyses.
The study focuses primarily on transformer capacity, which has not been fully utilized to date: Since transformers only reach their power limit temporarily, intelligently controlled battery storage systems could absorb peak generation and release the energy later.
In the reference scenario, the researchers assume that the installed capacity from existing renewable energy plants and additional photovoltaics can reach 200% of the respective transformer capacity. A sufficiently sized battery storage system absorbs power peaks when the transformer would otherwise reach its load limit.
According to the calculations, the resulting curtailment of the additionally installed photovoltaics decreases to an average of about 1% across the investigated transformers. This requires that the substation, battery storage, and photovoltaic systems are considered together and dynamically monitored and limited. The storage systems would have to adjust their operating mode accordingly to the actual power flows at the substation.
The calculated 150 GW, however, are not matched by a correspondingly large, already proven, available grid connection capacity. The study does not include detailed calculations of grid bottlenecks and grid conditions within the upstream and downstream networks. Rather, it examines what additional potential could arise from better utilization of existing transformers throughout a 24/7 cycle. According to the authors, this approach could reduce or postpone grid expansion measures.
This study was presented in advance at a recent conference in Berlin, with charts showing an example of a substation where transformer use does peak at maximum capacities throughout the day. However, by identifying regular windows when the transformer is not fully loaded, a substation could avoid being classified as fully congested.
The German Solar Association links the study results to demands regarding the grid package planned by the German Federal government. The Association opposes the long-term designation of large regions as grid bottleneck areas, where further expansion of renewable energy could be restricted by a planned redispatch reservation. Instead, existing grid capacities should be better utilized first.
The association is calling for, among other things, greater digitalization of medium- and high-voltage grids and the deployment of grid-supporting storage systems at bottleneck points. It refers to tenders under Section 11a of the German Energy Industry Act. Furthermore, the association argues that charging storage systems with solar power downstream of the grid connection point should not be hindered during temporary bottlenecks. According to BSW-Solar, such measures should complement the still necessary grid expansion and reduce its scope.
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