Significantly more PV capacity could be connected at substations linking medium- and high-voltage grids if battery storage systems absorb generation peaks, thereby improving utilization of existing transformers over time. That is the conclusion of a study conducted by the Fraunhofer Institute for Energy Economics and Energy System Technology (Fraunhofer IEE) on behalf of the German solar association BSW-Solar.
For a group of 1,313 rural substations with a high share of renewable energy, Fraunhofer IEE calculated theoretical expansion potential of around 150 GW of additional PV capacity and 60 GW of battery storage. The analysis was based on actual operating data from 20 transformers at 13 substations. The researchers then extrapolated the results to the larger group of substations using similarity analyses.
The study focuses primarily on currently underutilized transformer capacity. Because transformers reach their capacity limits only intermittently, intelligently controlled battery storage systems could absorb generation peaks and discharge the electricity later.
In the reference scenario, the researchers assume that the combined installed capacity of existing renewable energy systems and additional PV installations can reach 200% of a transformer’s rated capacity. Appropriately sized battery storage systems absorb generation peaks whenever the transformer would otherwise reach its load limit.
According to the calculations, curtailment of the additional PV capacity would average around 1% across the transformers studied. This would require substations, battery storage and PV systems to be considered as an integrated system, with power flows dynamically monitored and limited. Storage systems would need to adjust their operation according to actual power flows at the substation.
However, the calculated 150 GW should not be interpreted as an equivalent amount of verified, immediately available grid connection capacity. The study does not include detailed calculations of grid bottlenecks or assess the condition of upstream and downstream grid sections. Instead, it examines the additional potential that could be unlocked through better utilization of existing transformer capacity over time.
The authors estimate that the approach could reduce the need for some grid expansion measures or allow them to be postponed.
BSW-Solar is using the study’s findings to support proposals related to the grid package currently being prepared by Germany’s federal government. The association opposes the long-term designation of large areas as “grid bottleneck areas,” where further renewable energy deployment could be restricted under a planned redispatch provision. Instead, it argues that existing grid capacity should first be used more efficiently.
Among other measures, BSW-Solar is calling for greater digitalization of medium- and high-voltage grids and the deployment of grid-supportive battery storage at bottleneck locations. It points to tender procedures established under Section 11a of Germany’s Energy Industry Act (EnWG).
The association also argues that, during temporary grid bottlenecks, storage systems located behind the grid connection point should be allowed to charge with solar electricity without additional restrictions. BSW-Solar says these measures could complement necessary grid expansion while reducing the overall amount of new grid infrastructure required.
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