July/August 2013 | Renewable Energy Focus14

While the social, economic and

political drivers for wind energy pick

up momentum, there are also huge

opportunities for technical innovation

in the industry, particularly as wind

farms increase both in physical size

and generation capacity.

Often overlooked by the casual ob-

server, cable plays a vital role in wind

power. A single wind turbine might

contain as many as 50 to 100 individu-

al types of cable, housed in its nacelle,

tower and base, each taking a role

in power transmission, distribution,

control or communications. Cable

manufacturers have a responsibility to

ensure that their customers achieve

more from infrastructure investment.

Turbine design dictates cable confi guration

The arrangement of electrical

components in a wind turbine dictates

AS THE wind turbine sector develops and

matures, effi cient use of cable is one area

where turbine OEMs can streamline their

processes, says Thibaut Zumsteeg from

cable supplier Nexans.

Cables play a vital role in the wind power industry and it’s one area where turbine

manufacturers can streamline

the type and confi guration of cables

to be used. Housed in the nacelle,

the generating components include

the generator, gearbox, drive train

and brake assembly. They are linked

to the tower via a loop or looping

cable, named because it loops into

the nacelle as it turns into the wind.

The loop cable transmits power to

the lower section of the tower, where

cable is fi xed to the tower structure,

before connecting to the grid.

Turbine cables are typically a

mixture of low voltage (LV) and me-

dium voltage (MV) cables, with their

selection depending on the voltage

rating and design of the generator and

it is the location of the transformer

that is the deciding factor.

Performing the role of stepping up

the voltage from the LV generator to

20-36kV for transmission to the grid,

the transformer can be located in

the nacelle itself, the central or lower

sections of the tower or even outside

the tower.

For models where the transformer

is in the nacelle, a MV cable rated at

36kV connects the transformer to the

distribution point at the base of the

tower. This allows turbines to oper-

ate at higher voltages and therefore

deliver more power, a popular trend in

onshore turbines today.

Conversely, when the transformer

is housed in the central or lower

section of the tower, an LV cable rated

at 0.6-1kV connects the generator to

the transformer.

In the case of a Double Fed

Induction Generator (DFIG), both LV

and ‘low’ MV cables can be installed

as loop cables between the nacelle and

the tower.

Beyond the basic confi guration

of the turbine, cable type is deter-

mined by the location and role, with

all cable required to meet the UL

and CSA standards. Loop cables, in

particular, must pass stringent testing

to meet the demands of their specifi c

application.

Four types of turbine cableNacelle power cables• : LV cables in

the nacelle need to withstand the

tough environment of the nacelle,

which demands a small bending

radius, resistance to aggressive

chemicals and ozone, and the

ability to withstand temperatures

as high as 105˚C and as low as

-40˚C. LV cables in the nacelle

tend to be specifi ed with a

specialist insulation and sheath in

rubber or another compound with

similar performance. Single core

MV cables also perform a role in

the nacelle, often as output

connections from the winding

bars of Class H generators and

current converter cabinets. These

need to carry high levels of

current in extremely hot conditions

of up to 180˚C and are silicone

insulated.

Loop cables from nacelle to tower• :

Moving down from the nacelle, the

MV and LV cables used as loop

cables need to be both light and

fl exible so that they can withstand

the torsional stress applied by up

to four full rotations of the nacelle

Cable cost cutters for the wind sector

Renewable energy • technology update

focus:Technology

REF0413_Focus_Technology_Nexans 14 29-07-13 14:26:39

15July/August 2013 | Renewable Energy Focus

About: Thibaut Zumsteeg is Global Marketing Manager for Wind Energy, Nexans

in either direction. Depending on

the requirements, loop cables can

come in single-, three- or four-core

versions. The insulation and sheath

of loop cables need to be resistant

to oil, abrasion, UV, ozone and tem-

peratures ranging from -40 to

90˚C. Low Smoke halogen free

(LS0H) insulation and sheath

materials are also increasingly in

demand and lifetime testing con-

fi rms that cables are fi t for purpose

for the design life of the turbine.

Tower cables• : Connecting the

nacelle to the switchgear at the

base of the tower, whether the

transformer is located in the

nacelle or the body of the tower,

tower cables are an extension of

the loop cable. In the case of tur-

bines where the transformer is in

the nacelle, the MV loop cable

extends as a single length to the

switchgear. Conversely, when the

transformer is in the tower itself,

the LV loop cable is connected to

the transformer via a fi xed instal-

lation cable made of highly conduc-

tive copper or aluminium for

effi cient transmission. Whatever

the arrangement, LV and MV

cables need to have proven resist-

ance to oil, abrasion, UV and ozone.

They also need to operate reliably

from -40 to 90˚C and are often

specifi ed in a LS0H design.

Communication cables• : The set of

cables is not complete without con-

trol and data transmission cables.

Comprising anything from two to

100 cores, control cables are fl exi-

ble, usually shielded for EMC pro-

tection and carry small currents of

300 V to 1 kV to control the tur-

bine’s motor drive, braking, posi-

tioning and rotor speed. On the

other hand, electronic and data

transmission cables carry the sig-

nals that control all electronic and

mechanical devices and carry data

from sensors that measure wind

speed, temperature and perform-

ance. Increasingly these are EMC

shielded and include thermoplastic

modifi ed 2 – 5 core sensor multi-

core and multipair cables to carry

data. Two-core Fieldbus cables are

used in parallel with power cables

to carry signals to control elec-

tronic and mechanical devices.

Cable manufacturers have a responsibility to ensure customers achieve more from their investments

Two-core Profi bus cable is also

used, delivering up to 12 Mbits for

complex control and data trans-

mission cables that deliver speeds

comparable to Industrial Ethernet.

Finally, fi bre-optic cables deliver

high capacity for the transmission

of monitoring and control data.

Loop cable testingOf all the cables in a wind turbine,

it is the loop cable that experiences

the highest levels of mechanical

stress. During the 20-year lifetime

of a typical turbine, a loop cable

will undergo thousands of cycles of

twisting as the nacelle rotates. Cable

manufacturers carry out testing on

cable to ensure their torsion and

strength performance.

Normal practice is to test 10 metre

lengths of cable and it needs to with-

stand a minimum rotational angle per

meter over at least 2,000 cycles that

the turbine will experience through-

out its lifetime (or more if specifi ed

by the OEM). While the goal is to ac-

commodate up to four full rotations of

the nacelle before the operator’s main-

tenance staff must step in, the reality

is that turbines usually only reach up

to two and a half complete rotations.

InstallationOEMs have a number of options

when ordering cable, the fi rst of which

is to order cable by the drum and cut

it themselves or use harness makers to

prepare lengths for installation. Alter-

natively, cable lengths can be cut to size

by the cable manufacturer and deliv-

ered ready for installation. There is a

new trend for OEMs to order cable kits

from the manufacturer, which include

pre-cut cable lengths, stripped and fi t-

ted with the appropriate connectors, to

speed up the installation process.

Because a turbine’s tower can ex-

tend to 140 metres in height, it is im-

possible to transport as a single unit.

Instead, four or fi ve tower sections of

up to 25 metres are preinstalled with

cable lengths, which are connected

on-site, with short lengths of up to

60 centimetres at either end of each

tower section allowing for straightfor-

ward connection by specialist electri-

cal installers on-site.

With this in mind, Nexans is working on solutions for the next

generation of wind turbine projects,

where the industry will be able to

order precise lengths of cable set out

in plans, minimising waste and time

needed for costly on-site installation.

One major international wind tur-

bine manufacturer, Nordex, has been

benefi tting from Nexans’ approach to

supplying cable sets and kits.

A framework agreement sees

Nexans supplying customised power

cable sets (cable cut to length) and

kits (pre-terminated cables custom-

ised for each turbine) for high ef-

fi ciency wind turbines. Ordered direct

from Nexans by Nordex’s tower man-

ufacturers, the cables, connectors and

kits are supplied ready to install for

onshore wind projects across Europe

and kits have already been supplied to

tower manufacturers worldwide.

As the wind turbine sector devel-

ops and matures, effi cient use of cable

is one area where turbine OEMs can

streamline their processes. Cable and

accessories need to off er guaranteed

performance and deliver power but

new developments in the cable indus-

try mean that installation can be faster

and more effi cient than ever before.

Technology

REF0413_Focus_Technology_Nexans 15 29-07-13 14:26:40