Tuning a Cascaded Online Feedback Optimization Controller for Provision of Distributed Flexibility

TL;DR

The paper addresses the challenge of coordinating a large fleet of flexibility-providing units across multiple grid layers for ancillary services, using a cascaded online feedback optimization (OFO) framework that relies on real-time measurements and limited grid models. Each grid layer hosts an OFO controller that integrates an optimization problem with feedback, leveraging a sensitivity-based gradient and a projection mechanism to compute feasible updates to the setpoints. The study demonstrates that the performance of the cascaded system is highly sensitive to the tuning of individual controller gains, with strong interdependencies that can yield faster convergence or, if misconfigured, oscillations that propagate through the cascade. The approach offers robust, scalable vertical coordination for distributed flexibility without full grid models, enabling faster and more reliable responses to flexibility requests across grid layers.

Abstract

Coordinating a high number of flexibility providing units (e.g. to provide ancillary services for the transmission system) across various grid layers requires new control concepts. A flexibility request at a point of common coupling can be met by utilizing a cascaded control structure based on online feedback optimization. In this paper the influence of the parameterization of the individual controllers on the performance of the hierarchical flexibility provision is studied on a three-level test system. The results show a high interdependency between the choice of control parameters of one controller and the behavior of other controllers as well as a significant impact on the accuracy and speed of flexibility provision. With a careful tuning, a cascaded structure based on online feedback optimization can achieve efficient vertical coordination of flexibility providing units.

Figures (6)

• Figure 1: Cascaded structure of individual OFO controllers
• Figure 2: Block diagram of a single OFO controller
• Figure 3: Cascaded structure of individual OFO controllers as directed graph
• Figure 4: Active power flow at the EHV/HV PCC depending on the controller gain of the OFO controllers
• Figure 5: Active power flow to the requested set point from superordinate OFO controller