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A major order for low-loss HVDC technology

In a major project set to be completed by 2016, Siemens is set to increase the power transmission capacity between England and Scotland.

In a major project set to be completed by 2016, Siemens is set to increase the power transmission capacity between England and Scotland

A consortium between Prysmian and Siemens Energy is building a submarine DC interconnector in the Irish Sea.

The customers of this project are the British grid operator National Grid Electricity Transmission (NGET) and its Scottish counterpart Scottish Power Transmission (SPT), which founded the special-purpose joint venture NGET/SPT Upgrades Ltd. for this project. The grid connection between Scotland and England, designed as a low-loss high-voltage direct-current (HVDC) transmission system, will have a rating of 2200 megawatts (MW). It will be the first ever submarine interconnector using a DC voltage level of 600 kilovolts (kV), as the highest voltage level used to date was 500kV. In addition, it will also be the longest 2,200MW capacity HVDC cable in the world

Higher voltage levels in the cables allow higher power transmission capacity with the same diameter of copper in the cable and thus 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%.

The consortium, comprising of Siemens and Prysmian, is responsible for the turnkey supply of the link. This includes co-ordination and logistics efforts for cable installation along the 260 mile route and HVDC converters in Hunterston, Scotland and Connah’s Quay in north Wales.

The 400kV alternating current used in the transmission grids will be converted into direct current in the Siemens HVDC converter stations. One will be installed in Hunterston on the west coast of Scotland and the other will be set-up in Connah’s Quay in north Wales. The electric power will be transmitted between these points at a voltage of ±600kV via a submarine high-voltage cable supplied by Prysmian and reconverted to AC for further transmission and distribution. The link can transmit energy in both directions.

High-voltage direct current transmission technology significantly reduces transmission losses. HVDC is the only viable solution for economical, low-loss transmission of power via cable over distances in excess of 50 miles, because in an alternating current cable of that length a large part of the electrical energy transmitted would be lost in the form of reactive power.

It is expected that the €1.1 billion project will provide much needed additional power transmission capacity as the UK heads towards a low carbon economy. The power link will help to balance supply and demand within the grid sections in light of the continued growth of remote and fluctuating renewables. The sub sea link will bring renewable energy from Scotland to the south and help to meet 2020 renewable targets.

"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 kilovolts for a submarine interconnector with an also unmatched transmission capacity of 2200 megawatts. Raising the voltage level in the cable by 20% also enhances its transmission capacity by one-fifth. The transmission losses in the cable are simultaneously reduced by one-third," said Udo Niehage, CEO of the Power Transmission Division of Siemens Energy.

Nick Winser, Executive Director at National Grid added: “This link will have a vital role to play in meeting both the country’s energy needs and helping to address the problem of climate change. We are investing in an innovative solution using the most advanced technology. As a result, the benefits for consumers and electricity generators in being able to transport power in the most efficient way will be felt for years to come.”

Ignacio Galán, Chairman of ScottishPower, commented: “The HVDC link is a major project in the context of a vital upgrading of the UK electricity grid over the coming years, with the electricity grid between Scotland and England already running close to maximum capacity. The additional capacity from this link will provide a significant boost to renewable energy projects that we are developing in Scotland in order to bring clean energy to consumers throughout the country and at the same time help meet carbon reduction targets.

“We are fully committed to playing our part in the government’s planned infrastructure development programme, and expect to invest a total of £2.6 billion between 2013-2021 on upgrading our transmission network in the UK. Overall, we are projecting investments totalling £12 billion in the UK over the course of this decade, including major offshore wind projects around the country.”

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