Distribution Networks Providing Reactive Power As An Ancillary Service: Hierarchical Integration of Online Feedback Optimisation and Fuzzy Control

This paper proposes a hierarchical control framework for Volt/VAR control in distribution networks via coordinated Distributed Energy Resources (DERs) and On-Load Tap Changers (OLTCs). Monolithic Mixed-Integer Quadratic Program (MIQP) approaches face practical challenges such as high computational load and oscillatory OLTC behavior. To bridge this gap, this paper proposes a hierarchical framework combining a fuzzy logic supervisory layer for slow, discrete OLTC actuation with an Online Feedback Optimisation layer for fast, continuous DER reactive power adjustment. Simulations on a 110-kV distribution network show the proposed controller guarantees operation within network limits, achieves accurate reactive power setpoint tracking at the transmission interface for ancillary service provision, and significantly reduces OLTC usage compared to a reference MIQP controller. The framework represents a robust and practical solution for reactive power control with ancillary service provision in modern power systems.