Electrical Infrastructure
Design Considerations
David Flood
Head 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
NG Sub-Station
V= 400kV AC
 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
Turbine
Arrays
Developer
V= ??? AC
Starting points for design (3)
UK Transmission
Network
Turbine
Arrays
National Grid
Developer
NG Sub-Station
V= 400kV AC
 Cooperation and dialogue
•
•
•
•
Grid companies
Land owners
Authorities
Technology suppliers
V= ??? AC
Collecting the power from the turbines
Typical inter array layout
Turbine
Arrays
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)
Developer
V≥33kV AC
Offshore Collector Station
•
•
•
•
Power collection added
Offshore
Collection
Turbine
Arrays
National Grid
Developer
then OFTO
Developer
V≥132 kV AC
V≥33kV AC
NG Sub-Station
V= 400kV AC
 Next step:
• Connection to shore
Offshore Collector Station
UK Transmission
Network
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]
Murraylink HVDC Light, Berri
station (220MW)
BorWin1 offshore converter
station (400MW)
Power transmission added
UK Transmission
Network
Export Cable and DC
Conversion
Offshore
Collection
Turbine
Arrays
National Grid
Developer then Offshore
Transmission Owner (OFTO)
Developer
then OFTO
Developer
V= +/-320kV DC
V ≥132kV AC
V≥33kV AC
Offshore Collector Station
Offshore Converter Station
Offshore Cable Route
Cable Landfall
Onshore Cable Route
Onshore Converter Station
NG Sub-Station
V= 400kV AC
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