FlexPlan.jl - An open-source Julia tool for holistic transmission and distribution grid planning

In the ever complex world of the power systems, robust decision making in the context of investment planning becomes a difficult problem. In the decision making process, the costs and benefits of implementing complementary or com-peting technologies (AC versus DC or storage versus classical transmission expansion) need to be assessed. Additionally, the electrification of industrial demand and the integration of smart loads, such as heat pumps or electric vehicles, pave the way for demand flexibility provision on large scale. Considering that the majority of the demand flexibility is located in the distribution grid, there is a need for planning models and tools able to perform combined transmission and distribution grid planning. This paper introduces FlexPlan.jl, a Julia/JuMP-based open-source tool for holistic planning of transmission and distribution grids which includes a complete set of planning candidates for transmission and distribution networks and fully internalizes demand flexibility and storage usage. We use stochastic op-timisation, in order to find robust decisions with respect to different climate conditions and operating hours within given climate years. To keep the optimisation problem tractable, we introduce a novel decomposition between the transmission and distribution grid planning problems. We demonstrate that using the proposed approach a speed improvement of up to 100 times can be achieved for cases with a large number of distribution grids, with negligible increase of the objective function value and a good solution quality in terms of power flow.