European Transmission System Operators (TSO) are currently facing a tremendous number of connection requests from batteries individually rated for dozens to hundreds of MW each. Such large-scale installations have a significant impact on power flows, which may result in grid congestions. But unlike other assets which behavior easily be predicted (e.g. based on weather forecasts for wind or solar generation, or existing consumption patterns for loads), batteries cannot be realistically modelled in long-term connection studies since they may decide to apply various business strategy over time (arbitrage, Frequency Containment Reserve, automatic Frequency Restoration Reserve, capacity markets…).

To guide future battery connection requests toward portions of the grid with expectedly lower congestion risks, more and more TSOs provide grid hosting capacities for substations. And due to the uncertainty regarding their behavior in a few years, worst-case power injections are usually considered for batteries. Besides, hosting capacities may be associated with a maximal number of curtailments per year in some countries to enable larger storage units connections for specific substations.

For a given substation, the hosting capacity is computed in a straightforward manner by considering solely the impact of a battery connected to this specific substation on power flows over a wide variety of conditions (various load and generation snapshots, under nominal and fault conditions). Thermal limits of transmission assets are used to assess the maximal admissible battering power rating accounting for the worst-case scenario.

However, this approach results in extremely optimistic hosting capacities since the simultaneous impact of batteries connected to nearby substations is not considered on power flows. Consequently, the operation of a storage unit not only curbs the hosting capacity of the substation it is connected to, but also of any other substation of the grid. Consequently, any connection should trigger the computation of updated capacities for the complete grid, resulting in a computational burden for TSOs and fragile hosting assumptions for project developers.

This paper presents an innovative approach to directly compute a new type of hosting capacity for batteries, referred to as “joint capacity”. Contrary to previous ones, those capacities guarantee that any battery connecting to a substation up to the corresponding power rating will not restrict hosting capacities for other substations. Thanks to this major property, joint capacities provide a faithful representation of what the network is genuinely able to host. To illustrate, an application of this work is provided for a sizable portion of the French transmission network, which also takes into account a guaranteed maximum number of curtailments per year.