Cost Benefit Methodology for Optimal Design of Offshore Transmission Systems

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Predrag Djapic and Goran Strbac

Annex 25: Grid Integration of Offshore Wind Monday 18th _{June 2007, DTI London}

DTI Centre for Distributed Generation and

Sustainable Electrical Energy

Cost Benefit Methodology for Optimal

Design of Offshore Transmission Systems

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Content

• Scope

• Methodology

• Key results

• Application to Round II

• Impact on the regulatory regime design

Reports:

(1)Cost Benefit Methodology for Optimal Design of Offshore Transmission Systems

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Scope

Wind Generator Offshore Grid Entry Point Shoreline Onshore TO / DNO Onshore system (Transmission or Distribution) Offshore Transmission System Onshore Grid Entry Point or

User System Entry Point

Offshore TO Generator

Ratings: 1500MW

Distance: up to 100km

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Examples of direct configurations

132 kV 33 kV Onshore network 132 kV 220kV 33 kV 220 kV Onshore network Windfarm electrical system Windfarm electrical system

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Example of shared AC connection

132 kV 33 kV

400 kV 132 kV

Onshore transmission network 400 kV

132 kV 33 kV

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132 kV 33 kV 400 kV 132 kV 400 kV 132 kV 33 kV 132 kV 33 kV AC DC AC DC AC DC DC AC DC AC DC AC 275 or 400 kV

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Scope of cost-benefit analysis

• Offshore networks

– Switchgear reliability, installation cost, platform cost, ratings, maintenance requirements

– Cable reliability, installation cost, maintenance requirements, ratings

• Transmission mode (AC vs DC)

– Compensation requirements – Losses

• Windfarms

– Wind resource characteristics

– Typical turbine ratings, availability, cost – Windfarm size

– Windfarm distances from shore

• Future value of energy and ROCs

• Impact on onshore system operation

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Concept of

cost

-b

enefit analysis

Network Capacity and Redundancy

Cost

Investment

&Maintenance

Costs

Cost of

curtailed

wind energy

and losses

Total Cost

Optimal network

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Wind characteristics

0 5 10 15 20 25 30 35 40 45 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Normalized Wind Output

Fr e q ue nc y ( % ) Non-diverse wind source Diverse wind source

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Curtailment and loss factors

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Power (p.u.) C o ns tr a ine d E n e rgy ( p .u .) _{Load factor: 0.4} Diversified profile Non-diversified profile

Profile Load factor (%) Loss factor (%)

Non-diversified 40 29

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Key results

• Offshore platform (AC or HVDC)

– Flexibility is of high value

– Offshore platform capacity should be about 95% of the maximum export capacity of the wind farm connected.

– For wind farms with a capacity of 120MW or greater, following an outage (planned or unplanned) of any offshore platform

transformer, there should be, at a minimum, 50% of the installed platform transformer capacity remaining.

• Offshore cable network

– Optimal capacity installed can be lower than the maximum export capacity of the wind farm connected (X factor)

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Design of cable networks

X factor (%) Cable length (km)

Wind farm profile Condition - Increase in 25 50 75 100 Cable rating >95 Number of cables >91 >88 >86 >84 Cable rating >90 Number of cables >85 >82 >79 >77 Diversified wind profile Non-diversified wind profile

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Robustness of design (cable network)

Length (km) 25 50 75 100 Fixed value

MTTR (months) Energy cost (£/MWh)

23 24.5 25.5 26 50

14.5 15 15.5 15 75

10 10.5 10.5 10.5 100

Energy cost (£/MWh) MTTR (months)

310 320 310 310 2

Energy cost case MTTR case

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Robin Rigg Shell Flats West Duddon Walney Barrow Ormonde Hillhouse 3x220kV 1x220kV Rhyl Flats Gwynt Y Mor North Hoyle Burbo Bank 2x220k V New substation Rh yl 1x132kV 33k V 2x33kV 2x132kV 1x132k V 1x132k V

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Scroby Sands Teeside Westermost Rough Humber Gateway Triton Knoll Race Bank Docking Shoal Lincs Dugeon East Sheringham Shoal Inner Dowsing Cromer Sall Earlham 2 x220kV 2 x132kV New 2 x275 line Lynn 4 x220kV 2 x220kV 2 x220kV Garton 33kV Caister 33kv Redcar 33kV 33kV Skegness 33kV or 132kV 2 x132kV

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Kentish flats Gunfleet Sands 1X132kV Clacton-on-Sea Bramford Gunfleet Sands II Greater Gabbard 3x132kV London Array 3x220kV Thanet 2x132kV Richborough Power Station 1x33kv Herne Bay

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Impact on the regulatory regime design

(compensation scheme)

• On shore:

– Generators that obtain Firm Access Rights by purchasing TEC (at TNUoScharges)

– TEC provides compensation for constrained off generation

• Offshore: different regime proposed

• No fundamental difference in approach to design

• Network design based on long term average value of wind energy curtailed (large number of farms operated over long period of time)

• Impact of technology limitations: large wind farms supplied with multiple cables (600MW wind farm = 8% curtailment after a loss of a single cable) • Risk to individual small projects potentially higher

• Different treatment will be inconsistent with the standards developed

• Cost of constraints

– All Round II projects: £10.8m and £13.1m per annum (post BETTA constraint costs £100m)

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Predrag Djapic and Goran Strbac

Annex 25: Grid Integration of Offshore Wind Monday 18th _{June 2007, DTI London}