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 V= 400kV AC Turbine Arrays Developer V= ??? AC NG Sub-Station 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 Sub-Station Cooperation and dialogue Grid companies Land owners Authorities Technology suppliers

Collecting the power from the turbines Typical inter array layout Turbine Arrays Developer V33kV AC Offshore Collector Station 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 V33kV AC Offshore Collection Developer then OFTO V132 kV AC Offshore Collector Station NG Sub-Station 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 V33kV AC Offshore Collection Developer then OFTO V 132kV AC Export Cable and DC Conversion Developer then Offshore Transmission Owner (OFTO) V= +/-320kV DC Cable LandfallOnshore Cable Route Onshore Converter Station Offshore Cable Route Offshore Converter StationOffshore Collector Station NG Sub-Station

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 StatementNational 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