Assessing Active Distribution Network Flexibility: On the Effects of Nonlinearities and Nonconvexities

TL;DR

This paper examines how nonlinearities and nonconvexities in active distribution networks affect the accuracy of flexibility estimation, revealing that ignoring these factors can lead to overestimations and impractical solutions.

Contribution

It highlights the importance of considering nonlinearities and nonconvexities for accurate flexibility assessment in active distribution networks.

Findings

Full P-Q flexibility requires perfect coordination and high ramping rates.

Ignoring nonlinearities leads to overestimation of flexibility.

Nonconvex P-Q areas are common without ideal conditions.

Abstract

A widespread approach to characterise the aggregated flexibility of active distribution networks (ADNs) is to estimate the boundary of the feasible network operating areas using convex polygons in the P-Q space. However, such approximations can be inaccurate under realistic conditions where, for example, the nonlinear nature of the network is captured, and the behaviour of flexible units is constrained. This letter demonstrates, using a small ADN example with four flexible units and considering only nonlinearities from the network, that reaching the full P-Q flexibility areas would require perfect coordination of units and high ramping rates. Without these requirements, the P-Q areas become nonconvex. Thus, if the effects of nonlinearities and nonconvexities are ignored, existing approaches in the literature can result in overestimation of ADN flexibility and give rise to impractical solutions, hampering coordination between transmission and distribution system operators.