The Horns Rev 2 and 3 projects, owned by Ørsted and Vattenfall, respectively, are connected by a 33kV cable, designed to provide back-up power in the event of an export cable outage at either site.
Offshore wind turbines require an electrical supply at all times to maintain remote operation of components and sensors, and to avoid damage or build up of moisture.
Traditionally, offshore wind projects have used diesel generators — either on a platform or individually located on each turbine — to power the machines during periods when the export cable is out of action.
Recently, while transmission system operator (TSO) Energinet carried out four days of scheduled maintenance on the Horns Rev 2 link, Horns Rev 3 provided the power to its neighbouring turbines.
The 8km link was installed at the same time as Horns Rev 3's inter-array cables in 2018, saving the deployment of a separate vessel to carry out the work.
The financial details of the transaction was not unveiled by either developer, but outages caused by export cables are often compensated by the TSO.
"Offshore wind turbines must always be equipped with electricity, otherwise there is a risk of considerable damage. At the same time, the cable enables a quick and easy start-up of the offshore wind farm when the export connection is back online," said Niels Møller Jensen, Vattenfall interface manager for Horns Rev 3.
The cable also replaces the use of fuel and operating hours for the diesel generators, and removes the need for service boats to maintain them, Jensen added.
This is the first time two offshore wind farms with different owners have backed up each other, Vattenfall said.
Ørsted's 209MW Horns Rev 2 project was completed in 2009. It comprises 91 Siemens Gamesa 2.3MW turbines.
It's new neighbour, the 406MW Horns Rev 3, uses 49 MHI Vestas V164 8.3MW turbines and is currently undergoing commissioning procedures.