Barriers and Insights to Compute Multi-Period Cost Curves of Active Power Aggregated Flexibility from Distribution Systems for TSO-DSO Coordination

The sources of flexibility hosted in active distribution systems (ADSs) can contribute to transmission grid operation security through coordination mechanisms between the operators of distribution and transmission systems. Piece-wise linear cost curves of active power aggregated flexibility of an ADS at the interface with the transmission system can be calculated for one period. However, the time coupling of energy devices such as energy storage systems and flexible demand challenges drastically conceptually and computationally the calculation of cost curves in a multi-period timeframe. This paper first unveils the barriers in computing such cost curves. Then, the paper develops for the first time a methodology, with two versions, for calculating piece-wise linear cost curves of active power/energy aggregated flexibility of an ADS. The core of the methodology relies on combining tailored multi-period AC optimal power flow calculations under various objectives and sets of constraints. The value and computation efficiency of the methodology are demonstrated using a 34-bus benchmark ADS under 24 periods and a 191-bus ADS under up to 96 periods. Finally, the paper provides new insights and research directions to overcome some of the identified barriers.