  The shift towards clean energy brings about notable transformations to the
energy system. In order to optimally plan a future energy system, it is
necessary to consider the influence of several sectors as well as the
interaction of the transmission grid and distribution grid. The concept of
Feasible Operation Region (FOR) is a detailed approach to representing the
operational dependencies between the transmission and distribution grid.
However, in previous planning procedures, only a simplified expansion of the
distribution grids can be taken into account. With the method presented in this
paper, a Feasible Planning Region (FPR) is developed, which represents the
operational boundaries of the distribution grids for several expansion stages
and thus represents an admissible solution space for the planning of
distribution grids in systemic planning approaches. It hence enables a more
detailed representation of the necessary distribution grid expansion for the
integration of distributed technologies in an optimized energy system of the
future. In this paper, we present the method by which the FPR is formed and its
integration into an energy system planning formulation. In the results, the FPR
is presented for different voltage levels, and its use in power system planning
is demonstrated.
