Hierarchical Provision of Distribution Grid Flexibility with Online Feedback Optimization

TL;DR

The paper tackles dispatching distribution-grid flexibility for ancillary services by introducing a hierarchical Online Feedback Optimization ($OFO$) framework. It decomposes the global problem into layer-specific optimization problems controlled by primary and secondary OFO controllers, coordinating at the PCC without relying on explicit full-grid power-flow models. The approach leverages online measurements and a sensitivity-based gradient projection to achieve accurate set-point tracking with high reliability while preserving privacy and reducing computation. Across single-layer, experimental Volt/Var interaction, and multi-layer case studies, the method demonstrates robust constraint satisfaction, fast convergence, and scalable performance, making it well suited for congestion management in grids with incomplete models, especially at low voltage levels.

Abstract

Utilizing distribution grid flexibility for ancillary services requires the coordination and dispatch of requested active and reactive power to a large number of distributed energy resources in underlying grid layers. This paper presents an approach to hierarchically dispatch flexibility requests based on Online Feedback Optimization (OFO). We implement a framework of individual controllers coordinating actors, contributing to flexibility provision, to track a requested operating point at the interface between grid layers. The framework is evaluated in terms of performance during coordination and possible interaction between individual controllers, both central and distributed. Results show high reliability and robustness of the OFO controllers as well as an efficient dispatch of active and reactive power. Its computational efficiency and capabilities in set point tracking during online grid operation are making OFO a promising approach to the flexibility dispatch problem.

Figures (11)

• Figure 1: Time-scales of different ancillary services on transmission system level.
• Figure 2: Hierarchy of individual OFO controllers
• Figure 3: OFO controller for a single grid layer as implemented in this paper.
• Figure 4: A single iteration of the OFO controller in grid operation.
• Figure 5: Converging behavior of controller for set point of $P_{\text{PCC}}=-14.5\unit{kW}$.