  In power systems, large-scale optimisation problems are extensively used to
plan for capacity expansion at the supra-national level. However, their
cost-optimal solutions are often not exploitable by decision-makers who are
preferably looking for features of solutions that can accommodate their
different requirements. This paper proposes a generic framework for addressing
this problem. It is based on the concept of the epsilon-optimal feasible space
of a given optimisation problem and the identification of necessary conditions
over this space. This framework has been developed in a generic case, and an
approach for solving this problem is subsequently described for a specific case
where conditions are constrained sums of variables. The approach is tested on a
case study about capacity expansion planning of the European electricity
network to determine necessary conditions on the minimal investments in
transmission, storage and generation capacity.
