COBRAcable: Interconnector to the Netherlands

 

Project background

In September 2019 the first interconnector between Denmark and the Netherlands was commissioned. Theinterconnector has a capacity corresponding to the annual electricity consumption by approx. 700,000 households. The purpose of the COBRAcable is to improve the cohesion of the European transmission grids by increasing the exchange of excess wind power to neighbouring countries and strengthening the electrical infrastructure, the security of supply and the European electricity market.

 

The interconnector yields a Danish socio-economic surplus of approx. DKK 1bn. This surplus is i.a. based on the assumption that Danish power producers can export electricity to an expanded European market.

 

The connection contributes largely to the introduction of renewable power production in that both countries are able to manage much more sustainable production, i.e. buy and sell wind power and solar power across borders whenever there is a surplus situation in either one of the countries. This increases the value of sustainable energy. At the same time, the connection ensures a high level of security of electricity supply, as more and more wind energy flows into the systems “as the wind blows”.

 

The connection is designed to meet future requirements by presenting the opportunity for future offshore wind farms in the North Sea to be connected to the COBRAcable. Furthermore, the cable can be part of a future interconnected offshore electricity grid between the countries bordering the North Sea.

 

Technical description

The COBRAcable consists of two parallel, approx. 300km long, DC submarine cables linking western Denmark and the Netherlands together. The project also comprises approx. 20 km of onshore cable. The cables are connected to converter stations built in Endrup, east of Esbjerg, Denmark and Eemshaven in the Netherlands, respectively.

 

The cables are supplied by Italian Prysmian who also manages the installation of the cables.

 

The two converter stations are supplied by German Siemens AG.

 

The new Voltage Source Converter technology used for the project, provides the option for connecting offshore wind farms to the cable. Hence, the COBRAcable can be the first stepping stone towards the establishment of an actual offshore grid in the North Sea, capable of unpinning the expansion of wind power and strengthening the European electricity transmission grids.

 

The installations

 

The converter stations

The converter stations in Endrup and Eemshaven connect the 400-kV (kiloVolt) AC grid to the DC COBRAcable.

 

The converter stations transform the AC of the national grids to DC and vice versa.

 

The COBRAcable is built with the latest converter technology, the VSC – Voltage Source Converter – and not the well-known LCC technology – Line Commutated Converter – used in most of Energinet’s other DC connections.

 

The VSC technology is based on IGBTs – insulated gate bipolar transistors – whereas the LCC technology is based on the use of thyristors. The VSC technology offers several advantages:

• As opposed to LCC, the VSC technology offers “black start capability”, i.e. the DC connection can be used to start up the Danish grid (area DK1) in case of a total blackout.

• One of the advantages of the VSC technology is that it enables control of reactive power and hence contributes to controlling the voltage level in the AC grid. This advantage makes the VSC technology economically attractive in connection with the development of re-newable energy resources as operating the large power plants is not feasible in situations with sufficient wind power.

• In the past, VSCs had so large electrical losses that the technology was not competitive compared to the LCC technology. However, the technology has improved. The losses in Interconnectors based on VSC technology today are only marginally larger than with the LCC technology. So, with this and other operating ad-antages, the VSC technology is an attractive alternative, and the VSC technology is seen as the future technology in HVDC.  

The COBRAcable project includes a research project with two PhD students who work on developing a multi-terminal HVDC concept in case it becomes feasible to connect one or more offshore wind farms to the COBRAcable.

The converter station in Denmark

The converter station in Denmark is built close to the existing transformer station at Endrup, east of the town of Esbjerg. Offshore wind farms Horns Rev 2 and 3 are also connected here.

 

The converter station is an extension of the existing HVAC substation and includes a 2-breaker converter that connects the COBRAcable to the Danish high-voltage grid:

• Converter hall where the cables are connected and the VSC equipment is installed. The building’s total length is 121 meters and its height 19.5 meters. Architects have been involved in the design to ensure that the structure blends into the landscape.

• Extension of the existing HVAC substation to connect the converter to the existing HV grid

• A new HVAC yard for switchgear, transformers, cooling plant etc.

• A building for the converter’s controls, auxiliary supply, cooling pumps etc.

• A building for spare parts and maintenance equipment.

The cables

The total cable route is 329 km of which 307 km are offshore and 22 km are onshore. Two cables are installed in parallel making the total length of cables 658 km.

 

The cable is a 320-kV PEX DC cable with a 2,500-mm2 aluminium conductor.

 

The submarine cables will be installed as bundled, whereas the onshore part will be installed in cable ducts at a distance of 70 cm.

 

Time Schedule

Task	Start	Finish
Land cable installation, DK	Jan. 2017	Dec. 2017
Land Cable Installation, NL	June 2018	July 2018
Cable installation, Nearshore, DK	Aug. 2017	Oct. 2017
Cable installation, Nearshore, NL	Feb. 2018	Aug. 2018
Cable installation offshore	July 2017	Oct. 2017
Cable installation offshore	Feb. 2018	July 2018
Civil construction, converter Building, DK	Oct. 2016	June 2018
Civil construction, converter Building, NL	Sept. 2016	April 2018
Installation of converters etc.,DK	Sept. 2017	June 2018
Installation of converters etc.,NL	Sept. 2017	July 2018
Commercial operation	Q3 2019