31338.pdf

8/10/2019 31338.pdf

1/8

-------------------------------------------------------------------------------------------------------------------

----------

IQPC GmbH|Friedrichstr. 94 | D-10117 Berlin, Germany

t: +49 (0) 30 2091 3274 | f: +49 (0) 30 2091 3240 | e: [email protected] | w: www.iqpc.de

Visit IQPC for a portfolio of topic-related events, congresses, seminars and conferences: www.iqpc.de

The challenges of moving from 33 kV to 66 kV for offshore

wind farms

By Colin Pawsey

The development of large wind turbines, 5MW and upwards, installed further out to

sea requires the development of the entire wind farm infrastructure to support

them. Stepping up the inter-array voltage from 33 kV to 66 kV is increasingly being

considered as a crucial component of future wind farm design, and has the potential

to contribute significantly to cost reductions and higher yields. However, there are

several challenges for the industry to negotiate to implement this change, and the

effects on turbine structure, switchgear, transformers and cables must all be taken

into account.

Driving down costs

One of the key issues for the wind industry is the reduction of cost, both in terms of

wind farm construction and cost of energy. Back in 2008 the Caron Trust set up theOWA (Offshore Wind Accelerator) R&D programme with the aim of reducing the cost

of offshore wind by 10% by 2015. The programme is a joint industry project which

involves nine offshore wind developers Dong Energy, E.ON, Mainstream

Renewable Power, RWE Innogy, Scottish Power Renewables, SSE Renewables,

Statkraft, Statoil, and Vattenfall who between them own 72% of the UKs licensed

capacity.

The OWA Research, Development and Demonstration Programme is focusing on five

key research areas to achieve cost reduction, and a move to 66 kV forms part of the

overall strategy. These five areas of development are:

Foundations developing new turbine foundation designs for 30-60m water

depths that are cheaper to fabricate and install.

8/10/2019 31338.pdf

2/8

-------------------------------------------------------------------------------------------------------------------

----------




Access systems Developing improved access systems to transfer

technicians and equipment onto turbines for operations and maintenance inheavier seas.

Wake effects improving the layout of large wind farms to reduce wake

effects and optimize yields.

Electrical systems Developing new electrical systems to reduce transmission

losses and increase reliability.

Cable installation improving cable installation methods.

The OWA estimates that a change to 66 kV for the inter-array voltage of wind farms

will result in a considerable cost saving, and is actively promoting the development

of the infrastructure required to support this change.

Source: The Carbon Trust

Advantages of moving to 66 kV

There are several advantages to using higher voltage arrays on offshore wind farms,

including the ability to use electrically larger substation transformers. These

substation transformers are significantly lighter than 33 kV transformers of the

same rating, and there is also the potential to reduce the number of substation

8/10/2019 31338.pdf

3/8

-------------------------------------------------------------------------------------------------------------------

----------




transformers. Higher voltage will provide the opportunity to connect turbines in a

ring network to improve availability, while lower losses in array cables can also beexpected.

Analysis carried out by the Carbon Trust in 2012 into higher voltages included a

detailed comparison of 33 kV radial and ring array systems and 66 kV radial and

ring array systems. This included an analysis of all key technical components of the

system such as cables, switchgear, transformers and offshore substations, as well

as optimizing and comparing the inter-array designs. A detailed cost-benefit

analysis was carried out in order to compare the systems, which included CAPEX

(Capital Expenditure), operation and maintenance, cost of system losses, and cost

of losses due to cable failure for an assumed wind farm lifetime of 25 years.

The study concluded that higher voltage systems were found to exhibit NPV (Net

Present Value) improvements over the 33 kV systems, and that a key benefit of

moving to a higher voltage is the ability to implement ring array systems

economically and with viable electrical designs. In addition it was found that at 66

kV there is the possibility to remove a platform and connect all the turbines to a

single platform. This may prove to be an even greater advantage as wind farms

move further offshore and into deeper water and offshore platforms become more

expensive.

The report also concluded that housing equipment externally was found to have a

CAPEX impact that outweighed any operation and maintenance improvements, and

therefore recommended that equipment should be housed within the tower.

However, it was noted that there may be other advantages in housing equipment

externally, such as greater flexibility for wind farm developers to specify their own

electrical equipment.

The final recommendation of the analysis was that the offshore inter-array voltage

should move to 66 kV, and that the optimal solution is for wind turbine equipment

to be accommodated within the tower. In order to implement the 66 kV system, itwas recommended that a qualification test or tests should be encouraged for wet

type 66 kV designs, and that further engagement with wind turbine manufacturers

should be carried out to ensure that 66 kV wind turbines are developed.

8/10/2019 31338.pdf

4/8

-------------------------------------------------------------------------------------------------------------------

----------




Challenges:

Turbine structures

The impact on turbine structures of the move to 66 kV is one factor that must be

considered. As larger turbines are developed for deep water, foundations and

turbine construction are developing to create new designs which are more cost-

effective to build, to install, and to maintain. As part of the Carbon Trusts analysis

work was also carried out to assess the increase in structural costs associated with

moving to a higher voltage inter-array system. Three options were considered:

1.

All equipment placed inside the wind turbine tower.

2.Transformer outside and high and low voltage switchgear inside the tower.

3.Transformer and low voltage switchgear outside and high voltage switchgear

inside the tower.

It was assumed that any equipment outside the tower would need to be housed in

an external marine container, and although this gives the advantage of flexibility for

wind farm developers in equipment specification, the option to specify larger

equipment types, and improved access to HV equipment, it was found that the costof the container to house the equipment would be prohibitive. Despite concerns that

high voltage equipment may not fit inside the tower, components were identified

that could be incorporated into the tower, and as such any impact on structural

design would be minimal. However, the housing of equipment externally may still

become a viable economic option as the design of turbines and high voltage

equipment develops.

Switchgear and transformers

Although there is a slight cost penalty in terms of equipment, as transformers andswitchgear for 66 kV are more expensive, these costs should be outweighed by the

overall benefits. Hybrid GIS/AIS switchgear appears to be the best option for 66 kV

systems, and the Carbon Trusts cost analysis found that it would be considerably

8/10/2019 31338.pdf

5/8

-------------------------------------------------------------------------------------------------------------------

----------




cheaper than full GIS equipment, which is specified on a project basis and can have

complex arrangements.

A slim type transformer was identified as the most promising transformer type, as

they are already in common use in 33 kV systems and fit within the wind turbine

tower. For 66 kV systems, a force-cooled transformer was identified and was found

to have reasonable costs when compared with 33 kV transformers.

Transformers and switchgear are commercially available for 66 kV, although there is

still potential for more efficient and cost effective components to be developed.

However, the lack of 66 kV cable is seen as the biggest barrier to moving to higher

voltage arrays.

Cables

Source: Narec

8/10/2019 31338.pdf

6/8

-------------------------------------------------------------------------------------------------------------------

----------




Cable availability is the key issue the industry has to overcome to move to 66 kV

inter-array systems. Wet type cables without a lead sheath are currently used for33 kV array systems, which are significantly cheaper than dry type cables which

incorporate a water-blocking lead sheath. The water blocking for wet type cables is

performed by XLPE or EPR insulation, and at present there are no commercially

available inter-array cables of this type that can operate at voltages higher than 36

kV.

The OWA estimates that over 6000km of cable will be required to build round 3 of

the UKs wind power development, but the industry faced a chicken and egg

situation in that cable manufacturers were reluctant to invest in the certification

process of a new 66 kV cable without guarantees of demand, while developers areunable to specify a new array cable voltage when cables are not yet certified. To

resolve this issue and encourage development the Carbon Trust has awarded

funding of 400,000 between three cable manufacturers to qualify 3 different cost-

effective cable designs , as well as share the results from the qualification of a

fourth cable.

JDR will qualify a 3-core 630mm2, copper conductor, wet design, 66 kV cable.

Nexans will qualify a 3-core 630mm2, aluminium conductor, dry design, 66 kV

cable, and will share findings from their qualification of a 3-core 630mm2

,copper conductor, dry design, 66 kV cable.

Pyrsmian will qualify a 3-core 800mm2, aluminium conductor, 66 kV cable.

Test results from all four cables will be available in 2015, with extended test

programmes for some prototype cables expected to be complete by 2017. The OWA

estimates that although there will be a 12% increase in cable cost for 66 kV, there

will also be a 100% increase in transmittable power, so it is essential that cable is

developed to support 66 kV arrays.

8/10/2019 31338.pdf

7/8

-------------------------------------------------------------------------------------------------------------------

----------




Testing and demonstration sites

For the move to 66 kV to become a reality there is also a need for testing and

demonstration of the system, and the National Renewable Energy Centre (Narec)

delivered a feasibility assessment, supported by ABB and the Crown Estate, to

identify potential UK sites to demonstrate 66 kV inter-array technologies.

Initial investigation assessed all planned UK offshore wind demonstration sites and

identified two potential candidates: Hunterston, North Ayrshire and Blyth,

Northumberland. Both were subsequently investigated for technical feasibility, cost

versus 33 kV implementation, and viability for LVRT (Low Voltage Ride Through).

Hunterston showed promise for demonstration purposes, but the existingconnection is 33 kV so the site would require a step up transformer resulting in

higher costs, and LVRT testing at 66 kV would not be practical. Blyth showed a

potential cost reduction for 66 kV (particularly for radial configurations) but at the

least cost parity, while LVRT testing would also be achievable.

Continuation in the development of a test site can be expected throughout

2014/2015, and demonstration will be crucial to the development of 66 kV systems.

Fukushima case study

Global demonstration and testing has already begun with the development of the

Fukushima floating offshore wind farm demonstration project. Funded by Japans

ministry of Economy, Trade and Industry, the project initially involved the

installation of one 2MW floating wind turbine, a 25MVA floating substation and a 66

kV underwater cable. The second phase planned for 2015 will include the

installation of two 7MW floating wind turbines. The project has been initiated to

assess economic feasibility and results will be made publicly available, providing

much needed information regarding 66 kV systems.

Summary

The move from 33 kV to 66 kV appears to be the natural one as turbines become

much larger and move further out to sea. However, there are many challenges for

the industry to consider, not least the design of certified cables for inter-array

8/10/2019 31338.pdf

8/8

-------------------------------------------------------------------------------------------------------------------

----------



Visit IQPC for a portfolio of topic related events congresses seminars and conferences: www iqpc de

systems. Yet with cost reduction at the forefront of the wind industry in terms of

development and cost of energy, the move to 66 kV will be a vital component ofcost-effective wind energy in the future.

Colin Pawsey is a freelance technical journalist, focusing on new trends and

technologies in the renewable energy and automotive sectors. He is a regular

contributor and writing consultant to IQPC and Automotive IQ, and is also the

founder of copywriting agency Pure Copy.

Sources:

http://www.all-energy.co.uk/__novadocuments/30379?v=635060409652770000

http://proceedings.ewea.org/annual2012/allfiles2/1161_EWEA2012presentation.pdf

http://www.carbontrust.com/our-clients/o/offshore-wind-accelerator

http://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-

Breanne-Gelattly.pdf

http://www.narec.co.uk/narec.co.uk/documents/presentations/7._narec_pmck_reta

_event_december_2013_final_-_paul_mckeever.pdf

http://www.abb.co.uk/cawp/seitp202/c3f68b2486248f5bc1257a520046d762.aspx

http://www.fukushima-forward.jp/pdf/pamphlet3.pdf
http://www.all-energy.co.uk/__novadocuments/30379?v=635060409652770000http://proceedings.ewea.org/annual2012/allfiles2/1161_EWEA2012presentation.pdfhttp://www.carbontrust.com/our-clients/o/offshore-wind-acceleratorhttp://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-Breanne-Gelattly.pdfhttp://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-Breanne-Gelattly.pdfhttp://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-Breanne-Gelattly.pdfhttp://www.narec.co.uk/narec.co.uk/documents/presentations/7._narec_pmck_reta_event_december_2013_final_-_paul_mckeever.pdfhttp://www.narec.co.uk/narec.co.uk/documents/presentations/7._narec_pmck_reta_event_december_2013_final_-_paul_mckeever.pdfhttp://www.abb.co.uk/cawp/seitp202/c3f68b2486248f5bc1257a520046d762.aspxhttp://www.fukushima-forward.jp/pdf/pamphlet3.pdfhttp://www.fukushima-forward.jp/pdf/pamphlet3.pdfhttp://www.abb.co.uk/cawp/seitp202/c3f68b2486248f5bc1257a520046d762.aspxhttp://www.narec.co.uk/narec.co.uk/documents/presentations/7._narec_pmck_reta_event_december_2013_final_-_paul_mckeever.pdfhttp://www.narec.co.uk/narec.co.uk/documents/presentations/7._narec_pmck_reta_event_december_2013_final_-_paul_mckeever.pdfhttp://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-Breanne-Gelattly.pdfhttp://www.carbontrust.com/media/508349/Carbon-Trust-OWA-All-Energy-2014-Breanne-Gelattly.pdfhttp://www.carbontrust.com/our-clients/o/offshore-wind-acceleratorhttp://proceedings.ewea.org/annual2012/allfiles2/1161_EWEA2012presentation.pdfhttp://www.all-energy.co.uk/__novadocuments/30379?v=635060409652770000

Documents

31338.pdf