TSO-DSO Flexibility Regions are Non-Convex

Contrary to common assumptions about the convexity of the TSO-DSO flexibility region, this paper shows that, in general, it is not convex. To prove it, a counterexample is provided. This example debunks the concept of convexity within the TSO-DSO feasible region. Minimum voltage limits has important relevance in the non-convexity. The findings presented here invite for a call for a critical reevaluation of the methodologies proposed in the literature, as these primarily rely on the assumption of convexity.