In a consortium with cable company Prysmian, Siemens Energy is to build a submarine HVDC interconnector in the Irish Sea to increase power transmission capacity between England and Scotland. The order is worth more than 1.1 billion euros for the consortium. Customers are the British grid operator National Grid Electricity Transmission (NGET) and its Scottish counterpart Scottish Power Transmission (SPT), which have created a joint venture, NGET/SPT Upgrades Ltd, for this specific grid upgrade, to be known as the Western HVDC Link.

The new connector will have a rating of 2200 MW and will be the first submarine interconnector ever using a DC voltage level of 600 kV, the highest voltage level to date being 500 kV. The Western HVDC Link project will provide the necessary additional capacity on the country’s transmission system as the UK heads toward a low carbon economy. It will help to balance supply and demand within the grid sections in the light of the continued growth of remote and fluctuating renewables. The link is scheduled to be operational by late 2015.

“This order to connect National Grid’s and Scottish Power’s transmission networks underscores our technological leadership in the HVDC field by using the as yet unrivalled voltage level of 600 kV for a submarine interconnector with an also unmatched transmission capacity of 2200 MW. Raising the voltage level in the cable by 20 percent also enhances its transmission capacity by one-fifth. The transmission losses in the cable are simultaneously reduced by one-third,” says Udo Niehage, CEO of the Power Transmission division of Siemens Energy. Higher voltage levels reduce the cost of material used. They also mean lower transmission losses. The total power loss in the link, caused by the two conversions – from AC to DC and back to AC – as well as by the ohmic resistance of the cables will be less than 3 percent.

The consortium comprising Siemens and Prysmian is responsible for turnkey supply of the link. This includes co-ordination and logistics efforts for cable installation along the 420 km-long route and HVDC converters in Hunterston, Scotland and the Wirral area in North West England.

The electric power will be transmitted between these substations at a voltage of ±600 kV via a submarine high-voltage cable supplied by Prysmian and reconverted to the grrid’s 400 kV AC for further transmission and distribution. The link will be able to transmit energy in both directions.