Some electrical connection design issues for offshore

wind farms

Paul Gardner, Helen Snodin, & Nigel Scott

Garrad Hassan and Partners

nigel.scott@glasgow.garradhassan.co.uk

Presentation Rationale

Most UK projects quite advanced Grid connections and costs known

Issues of design choices Unknowns ?

Electrical Design

Can be split into 3 key areas:

Offshore array Transmission to shore Grid connection

will concentrate on the latter two.

Some Electrical Design Issues

Subsea vs land cable 33kV vs 132kV transmission to shore Export security

Parallel 132/33kV transformers Teed vs looped vs busbar connections

Grid Code changes

Electrical Design Diagram

Wind turbine

Other wind turbines

33kV cableonshore

33kV cableto shore

132/33kVtransformer132kV switch

circuit breaker

Offshore transformationto 132kV or continue at33kV ?

Arrangement ofoffshore array ?

Onshore Offshore

Continue withsubmarine cableonshore ?

Single or doubletransformerarrangement ?

Tee or loopconnection ?

Existing 132kVgrid line

132kV cable

33kV submarine cablewithin array

Wind turbinetransformer

Subsea vs Land Cables Offshore

33kV - use either EPR submarine cables or XLPE cable with additional waterproofing

132kV - use XLPE cables Better electrical insulator Sealed in watertight lead sheath - not land cable !

OnshoreSubsea cables can be used but only worth considering

for short and simple routes.Key issues are costs and practicability

33kV vs 132kV Transmission to Shore

Highest CostItem 132kV 33kVCable £Cable installation £Power losses £Substation £Power factor correction £O&M £Other costs / risks £

Export Security Teed vs Loop vs Busbar connections

Capital cost vs improved export security against grid faults, operations and maintenance outages

E.g. 100MW at £0.035/kWh and 35% load factor

Cost of Lost Production Time£1.2k 1 hour£29k 1 day

£206k 1 week£412k 2 weeks

Export Security - Tee & Loop

Wind farm

Circuitbreaker/switch

Existing grid line

Tee connectionarrangement

Loop connectionarrangement

Export Security Single or double 132/33kV transformers One transformer = all eggs in one basket Two transformers

Extra capital cost Improved security against transformer failure

new transformer or repairs could take months failure rates are low reduced export depending on transformer rating

Higher O&M costs

Grid Code Changes

Compliance in full or in part mandatory New Grid Code due in 2002 Possible requirements

Control of real and reactive power Frequency response Voltage regulation Stability

Implications - more cost ?

Summary

Impossible to cover all the issues, design choices and best practices

Hopefully raised awareness of one or two issues

The answers are often project specific