AC Hubs for Offshore Wind Power Plants Connected with HVDC Transmission Systems

Abstract

This work presents a novel control approach for AC Hubs, interconnecting offshore wind farms and different mainland grids via HVDC links. There are several proposals for a European overlay grid by different associations. This is where AC Hubs come into play. In order to implement such a grid in the North Sea, wind farms and long distance HVDC transmission lines need to be interconnected. The two options considered for interconnection are multiterminal HVDC configurations or AC Hubs. This work concerns itself with a novel approach for the control of AC Hubs. A technical model based on common control strategies is established and verified by simulation in MATLAB/Simulink. The main objective is to guarantee flawless operation of the AC Hub in absence of fast and reliable communication. This is achieved by applying a droop control scheme, which is similar to control strategies that are typically applied to microgrids or synchronous generators in the conventional electrical grid. After introducing the control strategy, a system layout is chosen, which is loosely based on existing data of offshore wind farms and literature discussing the future scenario of an HVDC grid in the North Sea. Issues with power sharing of parallel power converters, power transmission between different nodes as well as operation during faults are addressed within this work. The results show that by applying the proposed droop-control scheme, the operation of the AC Hub is possible without fast and reliable communication systems.