Electrical Infrastructure Design Considerations

David FloodHead of Electrical Systems, Forewind

Stakeholder Workshops, April 2010

Starting points for design (1)

Approximate “Footprint” of offshore wind-farm location – from offshore exercise

Onshore connection point to UK Transmission Network• Location, Timing for this point set by National Grid• Developer has modest impact on choice of onshore

connection location

Forewind must develop the optimum connection strategy to link together these

two points

Starting points for design (2)

UK Transmission Network

National Grid

V= 400kV AC

Turbine Arrays

Developer

V= ??? AC

NG

Su

b-S

tatio

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Technical• Distance to existing grid or possible

connection point• Voltage level at existing grid (typically

400kV)• Available capacity on grid and at

connection point• Electrical losses Cost Environmental issues Visual impacts Health and Safety

Starting points for design (3)

UK Transmission Network

National Grid

V= 400kV AC

Turbine Arrays

Developer

V= ??? AC

NG

Su

b-S

tatio

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Cooperation and dialogue• Grid companies• Land owners• Authorities• Technology suppliers

Collecting the power from the turbines Typical inter array layout

Turbine Arrays

Developer

V≥33kV AC

Off

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Radial configuration• Most common solution• Adopted from onshore wind farms• “String” a number of turbines along a 33kV cable• Approximately 8 turbines on each array string

(max. 40MW)

Power collection added

UK Transmission Network

National Grid

V= 400kV AC

Turbine Arrays

Developer

V≥33kV AC

OffshoreCollection

Developerthen OFTO

V≥132 kV AC

Off

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Next step:• Connection to shore

Exporting the power to shore (1)

Two main choices of transmission to shore exist:• DC – Direct Current always flows in the same direction, but it

may increase and decrease• AC – Alternating Current flows one way, then the other way,

continually reversing direction

Mains electricity in the UK has a frequency of 50Hz AC

DC more suitable for transmission over long distances• Lower losses• Fewer cables• But requires large converter stations at each end

HVDC converters - What do these look like? [courtesy of ABB]

BorWin1 offshore converter station (400MW)

Murraylink HVDC Light, Berri station (220MW)

Power transmission added

UK Transmission Network

National Grid

V= 400kV AC

Turbine Arrays

Developer

V≥33kV AC

OffshoreCollection

Developerthen OFTO

V ≥132kV AC

Export Cable and DC Conversion

Developer then Offshore Transmission Owner (OFTO)

V= +/-320kV DC

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Cost/risk/consenting considerations in route selection (1)

Offshore AC array cables• Minimise array string lengths • Minimise losses• Avoid crossing array strings with export cable• Scour can cause lengths of hanging cable (risk of damage)

Offshore DC export cables• Minimise cable length if possible • Minimise number of crossings (pipelines, other cables etc)• Pair of DC cables can be “bundled” into single export cable• Need to have consented “corridor” to allow cable route to

deviate around archaeology, sensitive sites etc• Avoid areas where potential for damage to cable from

fishing/shipping activity

Cost/risk/consenting considerations in route selection (2)

Landfall point• Preferably chosen to minimise overall export distance• Focus on consentability of location to reduce cost and

consenting burden

Onshore DC export cables• Minimise cable length where possible – capital cost• Minimise onshore crossings – railways, rivers etc. All add

cost, time and consenting burden

Onshore converter station• 120m x 60m x 23m footprint (approx.), with associated impact• Needs to be in relatively close proximity to National Grid

Transmission Network connection

DESIGN EXERCISE

Inputs:• Wind-farm locations for each table informed by Offshore

Exercise output• Connection points to transmission network given by “

National Grid Offshore Development Information Statement”

Design the best connection routes to:• Minimise overall cable lengths• Ensure consentability of chosen route (onshore sub-station,

onshore cable route, landfall point, offshore cable route)• Minimise number of crossings (cables, pipelines, rivers,

railways etc)• Avoid sterilising areas of Dogger Bank Zone for future

development with chosen export cable route