Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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3.2 Drive train

Torque support

Spur gear, 1st stage

Planet

Fast shaft

Fixed

hollow wheel

Hollow shaft (connection to

rotor shaft)

Sun wheel

Spur gear, 2nd stage

Torque support

Spur gear, 1st stage

Planet

Fast shaft

Fixed

hollow wheel

Hollow shaft (connection to

rotor shaft)

Sun wheel

Spur gear, 2nd stage

Fig. 3-39 Three-stage gearbox for wind turbines with fixed hollow wheel (Metso)

Fig. 3-40 Two-stage gearbox for wind turbines: one step stage epicyclic gear stage with rotating

hollow wheel and one spur gear stage (Renk)

3 Wind turbines - design and components

The advantage of the planetary gear is that the tangential force per teeth contact is

reduced by the number of planets. With three planet wheels it is F/3 under 120° at

the sun wheel and under 180° (sun and hollow wheel contact) at each planet

wheel, table 3.4. The design with a rotating hollow wheel is more complicated, but

less noise propagates directly into the casing than in the case of a fixed hollow

wheel [14].

The gearbox has to meet various demands: it should certainly operate without

any problems, have small weight and dimensions, have a low noise emission level

(above all without tonality), survive even greater damages and also be mainte-

nance-friendly [e.g. 15]. Moreover, the required lubrication has to be assured

under very different operating states, also at very low speeds (idling, start-up) [10]

and under difficult climatic conditions. These demands partially conflict in course

of the design procedure and require a very high effort when calculating statics,

dynamics, material strength and service life [15…17]. This is even increased by

the insights gained in the course of operating experience and can only be per-

formed by extensive PC based numerical calculations.

Table 3.4 Comparison of spur gears and planetary gears

Type Spur gear Planetary gear

Scheme

Planet

wheel

Rotating

planet

carrier

Sun

wheel

Hollow

wheel

Rotating

hollow

wheel

Planet

wheel

Rotating

planet

carrier

Sun

wheel

Hollow

wheel

Rotating

hollow

wheel

Hollow wheel fixed Planet carrier fixed

Transmission

ratio

1:5 < 1:7

Force per teeth

contact

F F/3 (with 3 planets)

Teeth contact

frequency

1  n 3  n

Weight high low

Dimensions big small

Noise emission high low

Efficiency

approx. 98% per

stage

approx. 99% per stage

Costs

Favourable for P <

500 kW

Minimum 1 planetary stage

favourable for P > 500 kW

3.2 Drive train

Sun

Planet

Hollow wheel

Sun

Planet carrier

Sun

Planet

Hollow wheel

Sun

Planet carrier

Fig. 3-41 Relations in a planetary gear with fixed hollow wheel and rotating planet carrier

3.2.3 Couplings and brakes

Due to the enormous torque, there is a rigid coupling between rotor shaft and slow

gearbox shaft. In drive trains with a three-point support of the rotor shaft, the latter

is fixed in the hollow shaft of the gearbox by an interference fit assembly (Fig.

3-40), either by shrinkage or force fitting which is easier to dismantle.

Between the fast shaft of the gearbox and the generator, only a slender shaft is

required to transmit the smaller toque. But these two shafts require an elastic cou-

pling since there may be a misalignment between the gearbox and the generator.

Moreover, both drive train components are elastically mounted on noise and vibra-

tion absorbing damper elements. So the coupling used is torsion proof but elastic

with respect to bending (multi-disk clutch, disks or steel bolts in rubber eyes),

Figs. 3-42 and 3-43. In order to protect gearbox and generator, an overload protec-

tion is often integrated in the fast shaft coupling (slipping clutch or shear bolts).

Fig. 3-42 Coupling shaft between gearbox and generator with brake disc and two couplings, left:

in leakage current isolating version, right: flexical GKG link type coupling (Winergy)

The certification guidelines of the German Lloyd [15] require two independent

braking systems. At least one of them has to act on the aerodynamic side at the

rotor. At stall-controlled wind turbines this is done by the tip brakes, i.e. the turnable

3 Wind turbines - design and components

blade tips (Figs. 3-2 and 3-13), whereas pitch-controlled wind turbines rotate the

entire blade (cf. chapter 3.1). The second braking system tends to be a mechanical

disk brake. At smaller wind turbines (< 600 kW) it is either found at the fast shaft

or at the low speed shaft (Fig. 3-38) which has the advantage that the loads do not

pass the gearbox when braking the rotor. But with increasing wind turbine size the

braking torque increases strongly, as does the required disk diameter. One of the

biggest commercial wind turbines with the mechanical disk brake on the low-

speed side of the gearbox is the TW-600 manufactured by Tacke (today GE

Wind).

Commercial wind turbines of more than 500 kW tend to have the disk brake on

the fast shaft, Fig. 3-43 and Figs. 3-31 to 3-35. The brake is dimensioned for ex-

treme load cases with emergency stops where the rotor has to be brought from full

load operation or over speeding to standstill within seconds. During normal opera-

tion (no emergency stop) the braking procedure activates the aerodynamic brake at

first and then the mechanical brake at small remaining torque, in order to stop the

rotor completely.

In principle, wind turbines with individual blade pitch systems do not need a

mechanical brake because the aerodynamic braking system is redundant due to

autonomous pitch drives. Basically, pitching only one blade to feather is enough to

brake the rotor from full load to standstill.

However, a mechanical brake and a rotor lock with fastening bolts are required

for maintenance and repair at the rotor and in the nacelle. In all wind turbines,

whenever staff are working at the rotor or in the hub, the rotor lock must be used

for safety purposes.

Fig. 3-43 Disc brake at the fast shaft of a gearbox (Svenborg)

3.3 Auxiliary aggregates and other components

3.2.4 Generators

A detailed discussion of the generators and their electrical characteristics is found

in chapter 11. So this section treats only aspects of the generator types relevant for

the drive train design. The pole number, together with the grid frequency, deter-

mines the generator speed and whether a gearbox is required or not. Asynchronous

generators which are coupled directly to the grid, employed in stall-controlled

wind turbines usually have 4, 6 or 8 poles. From a grid frequency of 50 Hz then

follows a super synchronous generator speed slightly above 1500, 1000 or 750

rpm (depending on the slip). Suitable gearbox types and their transmission ratio

were discussed in chapter 3.2.2.

Doubly-fed asynchronous generators have a variable speed but the operating

speed range is in a similar order. Only low-speed multi-pole ring generators

(separately excited or permanently magnetic excited synchronous generators) may

operate without a gearbox, Fig. 3-30.

A hybrid type is the medium-speed generator of the Multibrid M5000 with a

moderate pole number driven by a planetary gearbox, with a generator speed of

approx. 150 rpm, cf. Fig. 3-37.

With generator power of more than 1 MW, air cooling reaches its limits. So,

water cooled generators are also applied. The ring generators of gearless wind

turbines, Fig. 3-30, are large enough for effective air cooling, but the noise genera-

tion in the small gap between rotor and stator of the generator has to be

minimized.

3.3 Auxiliary aggregates and other components

3.3.1 Yaw system

In the case of historical windmills, major effort was required for the orientation of

the rotor perpendicular to the wind. The miller had the troublesome task of

pushing the tailpole in order to adjust the rotor to the variable wind direction, see

chapter 2. Only in the middle of the 18

century, did the invention of the fantail

allow the yawing process to be automated. Even today the yaw system is a “non-

trivial” functional subsystem of the wind turbine.

Wind turbines with a horizontal rotor axis orientation allow either

x passive yaw systems - e.g. autonomous yawing of a turbine with a

downwind rotor or windvanes at upwind rotor turbines - or

3 Wind turbines - design and components

x active yaw systems are applied - e.g. a fantail or yaw drives driven by

external energy (also known as azimuth drives).

As already explained in section 3.1, the downwind rotor (leeward in relation to

the tower, e.g. SÜDWIND; Fig. 3-18) is suitable for autonomous passive yawing

of the wind turbine because if the wind direction is not parallel to the rotor axis,

the force from the wind pressure on the rotor causes a yaw moment around the

tower axis which adjusts the rotor to the wind direction, similar to a windvane. But

for wind turbines with a high tip speed ratio, which have a relatively low solidity

of the rotor area, this works only when the rotor is turning. Hence, for low wind

speeds either the nacelle housing itself has a passive function as a kind of addi-

tional “windvane”, or an active drive is required.

The windvane for passive yawing of upwind rotor turbines is a characteristic of

the Western mill, cf. Figs. 2-10, 3-1 and chapter 12. Due to its simple system de-

sign and the fact that neither external energy nor control is required, it is also

commonly used in other small wind turbines, especially battery chargers.

Passive yaw systems have to be designed in such a way that sudden changes in the

wind direction do not provoke fast yaw movements producing strong additional

loads due to gyroscopic forces. At single- and two-bladed rotors the situation is

even worse. The inertia against yawing depends on the angular rotor blade posi-

tion which increases the strong dynamic loads. Therefore, the application of pas-

sive yaw systems is limited in general to a rotor diameter of up to 10 m.

Active yaw systems position the nacelle using drive units and are applied in

wind turbines of both upwind and downwind rotor configuration. No external

energy is needed if the wind itself is driving the fantail, orientated perpendicular

to the rotor as in the Dutch smock windmills, Figs. 2-8 and 2-12. The torque from

this small auxiliary rotor (rosette), Fig. 3-44, is transmitted using a worm gear

with a high transmission ratio (up to 4,000) to the rotating assembly of the yaw

system.

A yaw system with one or more electrical or hydraulic yaw drives is most

common for larger wind turbines. They are controlled using the signal from a

small windvane on top of the nacelle, Figs. 3-30 and 3-32, and act on the spur gear

of the big rotating assembly at the tower-nacelle connection, Figs. 3-44 right and

3-45 right. Multi-MW wind turbines may have up to eight yaw drives.

Since there is always an unavoidable clearance in this spur gear system, nacelle

oscillations, which would increase wear of the teeth flanks, have to be prevented.

For this purpose, the nacelle is either fixed by yaw brakes which are released only

during a yawing movement, and/or additional friction brakes are permanently act-

ing. The yaw drive then has to work against this friction force. If several electrical

yaw drives are installed, another possibility is locking the yaw system “on the

electrical side”: when the yaw movement is completed, half of the drives get the

signal to “try to turn” into the other direction. The drives work against each other,

and the fixing torque assures in each yaw gear teeth contact on a defined flank.

When designing the wind turbine, the fact that the active yaw system couples

3.3 Auxiliary aggregates and other components

nacelle and tower rigidly has to be taken into account. Torsional oscillations of the

tower are transferred into the nacelle.

Fig. 3-44 Left: wind turbine with rosette for yawing (Allgaier) [19]; right: yaw bearing with yaw

drive and yaw brakes (Wind World)

Yaw drives Yaw brakes

Nacelle frame

Yaw angle

sensor

Yaw drives Yaw brakes

Nacelle frame

Yaw angle

sensor

Yaw drives Yaw brakes

Nacelle frame

Yaw angle

sensor

Fig. 3-45 Left: section of a yaw drive with multi-stage planetary gear, motor removed

(Liebherr); right: electrical yaw drive, yaw brakes and yaw angle sensor (REpower)

3 Wind turbines - design and components

3.3.2 Heating and cooling

Wind turbines have to operate within a large temperature range based on the cli-

matic conditions on site. Moreover, the heat losses of gearbox and generator heat

up the nacelle. The temperatures should not exceed the admissible operating range

of the installed components, above all the sensible electronics. For a defined heat

removal, the air flow is directed by special ventilation systems. Their design has to

consider also the minimisation of noise emission and airborne sound propagation.

Apart from the cooling system for the nacelle itself, there are separate coolers

e.g. for the drive train components gearbox and generator, Figs. 3-34 and 3-36.

Since the admissible temperature level of the generator is often higher than that of

the gearbox, a combined oil-water-cooling cycle may be used, Fig. 3-46. The

water-air cooler is often located outside the nacelle, which may then be encapsulated

completely, e.g. for air conditioning in offshore applications. If the gearbox oil is

overheated, the lubrication properties may degrade due to the destruction of the

additives, and the service life of the gearbox may be reduced significantly.

In the winter time, the ambient temperatures may fall below 0°C, even outside

of cold climate sites. After a longer standstill, the gearbox oil also becomes cold

and its viscosity increases, causing lubrication problems during the wind turbines’

start-up. To assure good lubrication, additional heaters are then installed. More-

over, the temperatures in the electronic cabinets should not go below the allowed

range. An encapsulation of the nacelle or entire wind turbine e.g. with shutters at

all ventilation openings is suitable, but the effective cooling of the nacelle should

not be disturbed in summertime.

Moreover, rotor blade heating systems are installed, if there is increased risk of

icing. Ice on the blades changes the aerodynamic characteristics drastically and

causes increased vibrations due to the additional mass unbalance of the rotor.

Negative influences of condensation water in the blade are also reduced by the

rotor blade heating, which is either done electrically with heating wires in the

blade laminate or by blowing hot air from the nacelle into the hub and blades.

Heated anemometers and wind vanes are also useful under icing conditions. A

frozen anemometer would cause e.g. yield loss because the wind turbine cannot

start despite good wind speed. A frozen windvane may suggest, depending on the

relative angle to the rotor axis, a permanently occurring deviation of the wind

direction causing permanent yawing - or it may hide existing wind direction devia-

tions causing yaw angle errors with additional loads. Both pose a potential

dangerous to the wind turbine, see e.g. [20].

3.3 Auxiliary aggregates and other components

Gear oil

Cooling water

Plate heat exchanger

Oil - water

Gearbox

Generator

Water - air - Cooler

Pump station

Gear oil

Cooling water

Plate heat exchanger

Oil - water

Gearbox

Generator

Water - air - Cooler

Pump station

Fig. 3-46 Combined Oil - water - cooling cycle for gearbox and generator (Nordex)

3.3.3 Lightning protection

Lightning is a potential danger to wind turbines because of their large total height

and their installation at exposed sites. Accordingly, there are guidelines for the de-

sign of the lightning system, e.g. IEC 61400-24, see table 9.1. Statistically, a wind

turbine in Germany is only struck once every 10 years by lightning [21, 22], but at

exposed sites in the German highlands this number is significantly higher. Often,

the flash strikes the turbine at the highest point, i.e. the blade tip. However, in the

case of large wind turbines of the MW class, a flash may be propagate upwards

from the turbine into the clouds.

Rotor blades are often equipped with special lightning receptors in the blade tip

area. These are metal disks of approx. 5 cm diameter integrated into the blade sur-

face. Other manufacturers install several receptors along the blade radius. Or the

blade tip as well as the leading and trailing edge are made from aluminium. The

high lightning currents (up to 1000 kA) are conducted in a defined way e.g. by

metal cables inside the rotor blade. If condensation water in the rotor blade were

to heat up and evaporate by lightning energy, the blade would explode. Many

blade manufacturers install lightning registration cards at each blade root to record

3 Wind turbines - design and components

the currents. Later analysis provides the basis for detecting potential blade

damage.

In order to protect the entire wind turbine against lightning damage the currents

have to be conducted further down into the ground. It is important to protect the

bearings as the high currents would point-weld together the balls and the running

surface.

Fig. 3-47 Components of the lightning protection system for the protection of the bearings; left:

sliding brush contact on the main shaft, right: on the yaw drive, combined with spark gaps (RE-

power)

To prevent this, spark gaps and/or sliding contacts with carbon brushes are in-

stalled, Fig. 3-47. The first crucial point in a wind turbine with a pitch system is

the blade bearing between rotor blade and hub, the second is the rotor bearing be-

tween hub and nacelle frame, and the third is the yaw bearing between nacelle

frame and tower. The scheme of the lightning protection system with the different

lightning protection zones (LPZ) is shown in Fig. 3-48. LPZ 0, e.g., means that the

object (rotor, nacelle top with wind sensors) may be directly hit by the strike.

From inside the tower down to the earth rod, the required protection measures are

the same as for technical buildings. Electronic components, the control, switch-

gear, transformers, etc. have additional surge and lightning protection.

Only approx. 30% of the lightning damages at wind turbines in Germany result

from direct strikes, while approx. 60% are caused by strikes into the power and

telecommunication lines. Therefore, an entire lightning protection concept accord-

ing to the standards is required [21]. A periodic inspection of the grounding resis-

tance is required to assure that the lightning protection system functions properly

under operation from the rotor blade down to the ground.