Briefing: The North Sea grid

It is estimated that ~33 GW of offshore wind capacity will be installed in the North Sea by 2020, and this is expected to increase to 83 GW by 2030. The North Sea grid is a concept that is intended to transfer the electricity generated from offshore wind farms installed in the North Sea to land, interconnect the grids of adjacent countries and facilitate the creation of a European internal electricity market. This briefing note explains the North Sea grid concept and the basic principles of high-voltage direct current submarine power transmission


Introduction
The North Sea grid is a concept that is intended to transfer power generated from offshore wind farms installed in the North Sea to land, interconnect the grids of adjacent countries and facilitate the creation of a European internal electricity market.Several proposals of the North Sea grid concept exist in the literature.The Airtricity Foundation Project (Airtricity, 2006) proposed 10 GW offshore wind farms to be connected to the grids of the UK, Germany and the Netherlands.Greenpeace (2008) reported that about 65 GW of offshore wind capacity could be connected to the grids of seven countries around the North Sea.The Friends of the Supergrid (FOSG, 2011) proposed to develop the North Sea grid in phases.The first phase is to integrate 23 GW of offshore wind capacity from the UK, German and Belgian offshore wind farm clusters into the grids of four countries (the UK, Germany, Belgium and Norway).The European Network of Transmission System Operators for Electricity (ENTSO-E, 2011) estimated that 33 GW of offshore wind capacity will be installed in the North Sea by 2020 and 83 GW by 2030.In 2010, ten countries (Sweden, Denmark, Germany, the Netherlands, Luxembourg, France, the UK, Ireland, Norway and Belgium) signed a memorandum of understanding to develop an integrated North Sea grid and formed the North Sea Countries Offshore Grid Initiative (NSCOGI, 2011).
The proposed North Sea grid would use both high-voltage direct current (HVDC) and high-voltage alternating current (HVAC) for submarine electrical power transmission.HVAC transmission is mature and well understood.HVDC has better control capabilities, reduced asset footprint and lower power losses.In addition, HVDC can interconnect power systems operating at different frequencies and phase angles.At transmission distances beyond 70-80 km and at a voltage of 150kV and above, HVAC is not practical due to the capacitance and hence charging current of the submarine cable.ENTSO-E (2011) estimated that the ratio of HVDC to HVAC submarine power cable capacities to be installed in the North Sea will be ~4:1 by 2030.This briefing note explains the North Sea grid concept and the basic principles of HVDC submarine power transmission.

Submarine electrical power systems
The electrical system of an offshore wind farm consists of a medium-voltage electrical collection network and a highvoltage electrical transmission connection.Figure 1 shows the simplified electrical system of an offshore wind farm in the North Sea.The collection grid uses transformers in each wind turbine to step up the generation voltage of the wind turbines from 690 V to a medium voltage of 25-40 kV.A network of medium-voltage alternating current (AC) cables connects the offshore wind turbines to an offshore AC substation.The transmission connection uses the offshore AC substation to transform the medium voltage to a high voltage of 130-150 kV for connection to an offshore converter station.
Remote offshore wind farms use offshore converter stations to transform the AC generated from the offshore wind turbines into direct current (DC).These offshore converter stations are mounted on offshore converter platforms.HVDC submarine power cables connect the offshore converter platforms to shore as shown in Figure 1.At the other end of the submarine cables, onshore converter stations receive the power from the wind farms and convert it back to AC, which is fed into the terrestrial power grid.The three key components of the HVDC networks of the proposed North Sea grid are offshore converter platforms, submarine power cables and onshore converter stations.The HVDC submarine power cables can also interconnect the grids of two or more countries, thereby creating an offshore grid.

Offshore converter platforms
The two main components of an offshore converter platform are the topside and the foundation support structure.Topsides house the offshore HVDC converter stations.Foundation support structures host the topsides.The three possible foundation support structures are fixed, mobile jack-up and gravity base.The fixed platforms use jacket support structures that are attached to the seabed through piles.The topsides and jackets are installed by lifting from a barge using a heavy-lift crane vessel.The topside of a 1000 MW HVDC converter platform could weigh up to 10 000 t and this will require a large crane vessel.This has implications for both costs and availability and multiple offshore lifts (ABB, 2012a).
A mobile jack-up platform has a self-installing topside that is mounted on a substructure.These topsides house offshore converter platforms, which have an embedded jack-up system.The substructure is formed by steel piles that are installed around 50 m deep the seabed.The floating topside is towed into position directly above the substructure and raised up to about 20 m above sea level by the embedded jack-up system.This approach does not require a large crane vessel.This concept was applied to the 864 MW Sylwin1 converter platform with dimensions of 83 × 56 × 40 m (length × width × height) and a total weight of 25 000 t (Siemens, 2014).
The gravity-based platform consists of a topside welded to a gravity-based support (GBS) structure.These GBS platforms are constructed onshore, towed into position and secured on the seabed by their own weight and ballasting.This approach eliminates the need for heavy-lift vessel or offshore jack-up operations.The 900 MW DolWin2 project under construction will use the self-installing gravity-based structure platform for efficient production and ease of installation (ABB, 2012a).

Submarine power cables
According to the ENTSO-E (2014) 10-year network development plan, about 20 000 km of HVDC subsea power cables are required by 2030, of which 14 000 km (i.e. about 70%) are to be installed in the North Sea.Cable manufacturers would need to expand their production capabilities and more cable-laying vessels would be required to meet the predicted demand.
The two HVDC submarine power cable technologies available on commercial terms are mass-impregnated (MI) paper cables and extruded cross-linked polyethylene (XLPE) plastic cables.The central conductor of these cables is made either of copper or aluminium.The insulation of MI paper cables consists of clean paper impregnated with a high-viscosity compound based on mineral oil.The next generation of MI paper cables would use paper polypropylene laminate as insulation to achieve ratings of 650 kV and 1500 MW per cable.A single-core MI paper cable could have a conductor size of up to 2500 mm 2 and weigh about 37 kg/m (ABB, 2014).HVDC submarine cables have a sheathed and armoured layer for protection against harsh conditions associated with offshore installation and service (ABB, 2014).Table 1 is a summary of the latest HVDC submarine power cables (FOSG, 2011;National Grid, 2012).

Onshore converter stations
There are two main HVDC converter technologies: linecommutated converter (LCC) and self-commutated voltage source converter (VSC).Table 2 is a summary of the status of HVDC converters (Cao and Cai, 2013;National Grid, 2012).

Energy
Briefing: The North Sea grid Adeuyi and Wu LCC-HVDC is a mature technology and suitable for longdistance bulk power transfers.VSC-HVDC is a more recent development and has independent control of active and reactive powers, improved black start capability and occupies less space than LCC-HVDC.Therefore, VSC-HVDC is the key technology for offshore wind power transmission and the North Sea grid.MI paper cables are suitable for both LCC and VSC applications.Extruded XLPE insulation cables are suitable for VSC applications and are available at voltages of up to 500 kV.

National strategies
Within the proposed 83 GW of offshore wind capacity to be installed in the North Sea by 2030, 38•5 GW will be off the coast of the UK and 24 GW off the coast of Germany (ENTSO-E, 2011).In the UK, most offshore transmission circuits will be connected to the shore using VSC-HVDC platforms each rated at 1 GW and ±320 kV (National Grid, 2012).
In Germany, offshore wind farms have been grouped into 13 clusters, and most of the offshore VSC-HVDC platforms are each rated at up to 900 MW and ±320 kV (BSH, 2012).In Belgium, the total power from offshore wind farms will be aggregated through two offshore HVAC platforms with a combined capacity of 2•3 GW.The two platforms will be intertied and connected to an onshore substation using 220 kV AC submarine cables.This design includes future interconnectors with France and the UK through an international HVDC platform rated at up to 3 GW and above ±500 kV (Elia, 2013).From Norway, new HVDC interconnectors are planned to the EU, rated at up to 1•4 GW and ±500 kV (STATNETT, 2011).

Development of interconnectors in the North Sea
Interconnectors use submarine power cables to connect the electricity transmission systems of adjacent countries.Interconnection could allow electricity to flow from one country to another according to the market prices on either side of the interconnector.At present, four countries (Great Britain, the Netherlands, Denmark and Norway) have 3•4 GW of interconnection capacity through six HVDC interconnectors in the North Sea. Figure 2 shows the existing and proposed HVDC interconnectors to be installed in the North Sea by 2020.Table 3 is a summary of the existing and proposed subsea interconnection capacities to be installed in the North Sea by 2020.
It is estimated that ten HVDC subsea interconnectorshaving a total capacity of about 9•3 GW and a total route length of about 3800 kmwill be installed in the North Sea by 2020.

Visions of the future North Sea grid
Existing HVDC subsea cables of the North Sea grid are pointto-point circuits, and each circuit provides a single service either for interconnecting transmission grids or connecting offshore generators to onshore grids (Lévêque et al., 2012).
Although the topology of the future North Sea grid has not been agreed, ENTSO-E (2011) has proposed two possible topologies: (a) local coordination and (b) fully integrated.
The local coordination topology assumes a continuation of existing offshore grid development regimes.This will result in a multiplication of point-to-point circuits in the North Sea.The fully integrated topology is intended to interconnect several point-to-point circuits and offshore wind power generation units.This will create a multi-terminal HVDC system, in which any unused transmission capacity when wind farms are operating below their peak generation can be used for balancing and energy trading between the grids of different countries (Lundberg et al., 2012).However, reliable operation of such multi-terminal HVDC schemes will require high-power DC circuit breakers and DC flow control devices, which are still being developed.Manufacturers of DC circuit breakers have announced the results of prototype tests in which DC exceeding 3 kA was interrupted in <3 ms (ABB, 2012b;Alstom, 2013).The next step is to deploy such DC breakers into real HVDC networks.

Conclusions
The North Sea grid concept is intended to transfer the power generated from offshore wind farms installed in the North Sea to land and interconnect the grids of adjacent countries.HVDC will be the key technology for submarine electrical power transmission in the proposed North Sea grid.LCC-HVDC is a mature technology and suitable for interconnection of transmission girds of different countries.VSC-HVDC is suitable for offshore wind power transmission.The major components of the HVDC networks of the proposed North Sea grid are offshore converter platforms, submarine power cables and onshore converter stations.
Strategic development of the North Sea grid sets a number of challenges involving technical, economic, financial, environmental and socio-political aspects (FOSG, 2014).To address some of these challenges, ENTSO-E, NSCOGI and the Agency for the Cooperation of Energy Regulators are to design regulatory frameworks for coordinated development and management of the North Sea grid (NSCOGI, 2012).The proposed North Sea grid could help to lower electricity supply prices, reduce the cost of delivering security of supply and support the decarbonisation of electricity supplies in the EU.
Energy Briefing: The North Sea grid Adeuyi and Wu

Figure 1 .
Figure 1.Simplified electrical system of an offshore wind farm.Copyright Alstom

Figure 2 .
Figure 2. Existing and proposed HVDC interconnectors in the North Sea by 2020.Copyright d-maps.com

Table 1 .
Status of HVDC Cables a Converters have one pole LCC, line-commutated converter; VSC, self-commutated voltage source converter

Table 2 .
Status of HVDC converters

Table 3 .
Subsea interconnection capacities in the North Sea by 2020