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

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3.3 Auxiliary aggregates and other components

Fig. 3-48 Lightning protection zones (LPZ) of a wind turbine [acc. to 21]

3.3.4 Lifting devices

Most of the larger wind turbines are equipped with winches to lift tools and

smaller spare parts up into the nacelle. In Multi-MW wind turbines; there is often

also a slewing crane or overhead crane inside the nacelle to move heavier objects,

Figs. 3-30, 3-34 and 3-35. For the exchange of large components, mobile cranes

are needed, like for the erection of wind turbines. In the nacelles of offshore wind

turbines the integrated cranes are often larger since the costs for the (external)

crane ships are extremely high. Fig. 3-49 shows such a “nacelle crane” which is

suitable for the replacement of entire drive train components (rotor bearing, gear-

box and generator).

3 Wind turbines - design and components

Fig. 3-49 Left: nacelle with additional crane, GE 3.6 (GE Wind); right: model (Liebherr)

Fig. 3-50 Left: Acceleration sensor (horizontal and vertical direction, company Mita Teknik) at

the rubber bearing of the gearbox, right: revolution speed sensor at the generator back (REpower)

3.3.5 Sensors

The supervisory and control system of the wind turbine (see chapter 12) continu-

ally processes numerous data on the operating and ambient conditions which are

provided by numerous sensors in the wind turbine and on the nacelle. The follow-

ing operating data is acquired permanently:

x Wind speed and direction (Fig. 3-32)

x Rotor and generator speed (Fig. 3-50)

x Temperatures (ambient, bearings, gearbox, generator, nacelle)

x Pressure (gearbox oil, cooling system, pitch hydraulics)

3.4 Tower and foundation

x Pitch and yaw angle (Figs. 3-27 and 3-45)

x Electrical data (voltage, current, phase)

x Vibrations and nacelle oscillation (e.g. acceleration sensor, Fig. 3-50, as

well as proximity switches for emergency stop)

The applied type of sensor and its mounting may vary considerably for different

wind turbine types. In the case of additional monitoring sensors it is often dis-

cussed whether they are “just another additional electrical component” which

increases the failure risk and down times, producing yield loss and increased costs

of operation and maintenance (O&M). Or, whether they provide really useful

information for detecting imminent damages, scheduling (preventive) O&M and,

therefore, help increasing performance and yield and reducing O&M costs.

Correct and continuous operating data is essential for monitoring and improv-

ing wind turbine performance. Experience gained from the operating data partially

reveals a lower component service life than assumed during design [23] which

may present the operator and insurance with financial problems. Severe damages

often cause consequent damages at other components, moreover e.g. a gearbox re-

placement requires the costly removal of the rotor with a mobile crane. Not only

the maintenance and repair costs, but the yield loss due to the increased standstill

time until repair (bad weather and accessibility) is also an issue. Therefore, at

larger wind turbines and especially those with difficult accessibility like the offshore

wind farms, the number of sensors is increased and e.g. condition-monitoring

systems (CMS) are installed. These are certified systems [24] for surveying the

condition of bearings and gearbox by frequency analysis of the measured vibrations.

Remote monitoring and analysis then observes the evolution of potential damages,

so scheduling the repair measures is easier, and severe damages with consequent

damages are prevented. At offshore wind turbines extensive load monitoring is

also needed for insurance reasons. Despite all sensors and monitoring systems it is

important to inspect wind turbines periodically. Some damage types and their

causes – e.g. the detection of avoidable additional mass unbalance or aerodynamic

unbalance [25] as well as alignment errors of the drive train - have not been inte-

grated in CMS to date and are revealed only by additional measurements.

3.4 Tower and foundation

3.4.1 Tower

When designing a horizontal axis wind turbine, the mechanical engineer often

under-estimates the construction complexity of the tower and foundation. However,

the stability of the entire wind turbine structure is the most important issue when

3 Wind turbines - design and components

applying for a construction permit from the building authority. Therefore, in the

course of assembling the large amount of certification documents the manufac-

turer is “forced” to investigate the construction in detail (cf. section 9.1.4). Besides

the static stability, the dynamic behaviour of the wind turbine’s tower is of similar

importance, see chapter 8.

Moreover, the tower is of great relevance for the economic efficiency of a wind

turbine for several reasons: On one hand, it produces a significant portion of the

wind turbine’s initial costs, approx. 15 to 20%, and also determines the costs for

transport and erection. On the other, the energy yield (i.e. the profit) significantly

depends on the turbine’s hub height. The wind speed increases with height loga-

rithmically at most sites, and a hub height above the atmospheric surface boundary

layer delivers a constantly higher energy yield (see chapter 4). Therefore, the

choice of the optimum hub height (respectively tower height) has to be made for

each site individually. To facilitate certification and selection, the manufacturers

offer each wind turbine with several set tower heights. This helps calculating the

optimum cost-benefit ratio.

100

120

140

160

0 500 1.000 1.500 2.000 2.500 3.000

Nennleistung in kW

Nabenhöhe in m

Small towers

(coast)

Tall towers

(inland)

0 500 1000 1500 2000 2500 3000

Hub height [m]

160

140

120

100

Rated power [kW]

100

120

140

160

0 500 1.000 1.500 2.000 2.500 3.000

Nennleistung in kW

Nabenhöhe in m

Small towers

(coast)

Small towers

(coast)

Tall towers

(inland)

Tall towers

(inland)

0 500 1000 1500 2000 2500 3000

Hub height [m]

160

140

120

100

Rated power [kW]

Fig. 3-51 Hub height versus rated power for commercial wind turbines

At the coast (short roughness length and lower turbulent intensity) the wind speed

picks up fast with the height above ground, so mostly smaller towers are suitable,

Fig. 3-51. The inland surface boundary layer (higher roughness length and turbu-

lent intensity) is larger, which means that higher towers are more suitable. The

ratio of hub height to rotor diameter is between 1.0 and 1.4 for wind turbines at

3.4 Tower and foundation

coastal sites in Germany and between 1.2 and 1.8 for those at inland sites. The

higher values are valid for smaller wind turbines, below approx. 300 kW, the

larger values for the MW-class wind turbines.

The tower’s structural design (see the Campbell diagrams in chapter 8) is either

x soft or

x stiff.

A stiff tower design means that first natural bending frequency of the tower is

above the exciting rotor speed n, respectively the corresponding rotational fre-

quency. A soft tower design, on the contrary, means that the first natural bending

frequency of the tower is below the rotational frequency of the rated speed. At

these wind turbines during start-up, the passage of the tower’s natural frequency

has to be controlled in order to prevent resonance effects with increasing vibra-

tions of the system.

Fig. 3-52 shows the study of different tower designs for the wind turbine

WKA-60. The rotational frequency of 0.3833 Hz is for all designs below the first

tower’s natural frequency, i.e. for all is the frequency ratio f

0.1

/ n > 1. But the

blade passing frequency of 1.15 Hz (rotational frequency multiplied by the blade

number) is always above the first tower’s natural frequency. Moreover, it turned

out in the design study that for all steel tower designs the second natural bending

frequency was very close to this exciting blade frequency. Therefore, the WKA-60

was erected with a concrete tower.

The dynamic design of speed-variable wind turbines is a very tricky issue, see

chapter 8. While the towers for small and medium sized wind turbines of less than

500 kW are mostly stiff (i.e. rigid) constructions, the towers of the big wind tur-

bines have almost always a soft tower design in order to save on material and

costs.

Another tower design criterion is whether it is

x a self-supporting tower or

x a guyed mast (or lattice tower).

The stiffness against tilting and torsion of self-supporting towers is quite high,

but requires a high mass if it is to be stiff stiff against bending as well.

The lattice tower needs the lowest material mass for a stiff tower design: it may

save up to 50% compared to an equivalent tubular steel tower, cf. Fig. 3-52.

Moreover, due to having many connections, the structural damping is higher than

that of the steel tower. This was the reason why a lot of Danish wind turbines of

the first generation were designed with a lattice tower, cf. Fig. 3-2. Later, they

were less frequently used due to their visual impact on the landscape and also be-

cause of costs, given the higher share of labour cost involved in manufacture and

erection which is a disadvantage e.g. in Northern Europe. There is a higher degree

of automation attainable in the manufacture tubular steel towers with bending and

welding machines, Fig. 3-53, top.

3 Wind turbines - design and components

Not pre-

stressed: 135

prestressed:

185

Not pre-

stressed: 115

Prestressed:

160

100200175185230

Relative costs for

supporting structure

>%@

54045543063 + guy

wires

8790114

Tower mass >t@

3.50

8.10

300

3.30

5.40

300

3.50

stepped

520/250

2.70

3.10

4.30

3.20

7.50

3.50

stepped

35–20

 top >m@

 bottom >m@

Wall thickness >mm@

0.96

2.50

0.96

2.50

0.65

1.70

0.55

1.44

0.55

1.44

0.56

1.46

0.55

1.44

tower natural bending

frequency f

0.1

>Hz@

Frequency ratio f

0.1

/ n >p@

on-site con-

crete, conical

on-site

concrete

Pre-fabricated

segments

Guyed lattice

tower

Lattice

tower

Conical

bottom

Cylindri-

cal tower

Wind turbine:

WKA-60-II

Rotor:

3 blades,  60 m

Rotor speed:

n = 23 rpm =

0.3833 Hz

Blade frequency:

3 x n = 1.15 Hz

Head mass: 207 t

hub height: 50 m

ConcreteSteelMaterial:

Not pre-

stressed: 135

prestressed:

185

Not pre-

stressed: 115

Prestressed:

160

100200175185230

Relative costs for

supporting structure

>%@

54045543063 + guy

wires

8790114

Tower mass >t@

3.50

8.10

300

3.30

5.40

300

3.50

stepped

520/250

2.70

3.10

4.30

3.20

7.50

3.50

stepped

35–20

 top >m@

 bottom >m@

Wall thickness >mm@

0.96

2.50

0.96

2.50

0.65

1.70

0.55

1.44

0.55

1.44

0.56

1.46

0.55

1.44

tower natural bending

frequency f

0.1

>Hz@

Frequency ratio f

0.1

/ n >p@

on-site con-

crete, conical

on-site

concrete

Pre-fabricated

segments

Guyed lattice

tower

Lattice

tower

Conical

bottom

Cylindri-

cal tower

Wind turbine:

WKA-60-II

Rotor:

3 blades,  60 m

Rotor speed:

n = 23 rpm =

0.3833 Hz

Blade frequency:

3 x n = 1.15 Hz

Head mass: 207 t

hub height: 50 m

ConcreteSteelMaterial:

Fig. 3-52 Various tower designs for WKA-60 [26]

However, the lattice towers are extensively used in countries which have a differ-

ent structure of the production costs (lower labour costs compared to material

costs). Moreover, they are an option for very tall towers (hub height above 100 m)

at German inland sites where their low specific tower weight is advantageous. A

lattice tower product line has lower certification costs. The on-site assembly of the

tower segments, Fig. 3-53 bottom, reduces the component dimensions and there-

fore circumvents the transportation limits for the maximum tower diameter (clear-

ance below bridges and road curve radius) existing for tubular steel towers and

concrete towers, see below and chapter 15.

3.4 Tower and foundation

Fig. 3-53 Top: production of steel tube tower segments; left: bending of plate on 3-roll bending

bench: right: welding together the plate of the tower segment (BWE); Bottom: FL 2500 in

Laasow with 160 m lattice tower; left: assembling of the bottom tower segments; right: complete

assembled wind turbine (Seeba, W2E, Fuhrländer)

Tubular towers are manufactured with a round or a polygonal section. The section

area grows conically (or in cylindrical steps) from the top to the tower bottom tak-

ing into account the increasing bending moment, above all from growing lever of

the rotor thrust. Moreover, at the tower bottom, a material saving tube design with

3 Wind turbines - design and components

large diameter and small wall thickness does not disturb the aerodynamics of the

rotor. The already mentioned transportation limits for wind turbines of the Multi-

MW class is the maximum clearance below bridges of 4.0 to 4.2 m. The maximum

tower bottom diameter of a Vestas V-80 (2MW) is exactly 4.0 m. Tubular towers

are mostly manufactured from steel. However, spun concrete towers are also

applied which have lower production costs but higher transport and erection costs

because a significantly larger mass is needed.

In order to simplify transport and erection there is the alternative of on-site

production of the concrete tower using a jacking framework, Fig. 3-54. But in this

case the quality control is difficult. Work has to be carried out very carefully, if

the concrete is processed in 100 m height at low temperatures in the wintertime .

Thus, another alternative was developed, e.g. by the manufacturer ENERCON,

with its tower assembly from concrete segments produced in the workshop under

defined conditions. This process has cost advantages compared to the on-site con-

crete method especially for a larger serial production. Concrete towers have a

higher structural damping than steel towers, but they require tension anchors and

steel ropes for pre-stressing the concrete which has only a limited capability of

bearing tensile forces.

The hybrid tower is a tower type which combines the advantages of both the

tower materials, concrete and steel. The tower bottom is made from concrete in

order to avoid the transportation limits, and the upper tower part is from steel. Fig.

3-55 shows a hybrid tower with only a short concrete base. The tension anchors

connecting the two tower parts and pre-stressing the concrete are inside the tower

and freely accessible to allow the tension to be controlled and readjusted.

Fig. 3-54 Tower made of in-situ concrete with jacking formwork (Pfleiderer)

3.4 Tower and foundation

100

Concrete

Steel

Concrete

Steel

Fig. 3-55 Hybrid tower (GE Wind)

500

1000

1500

2000

2500

3000

20 40 60 80 100 120

Nabenhöhe in m

Spezifische Turmmassen in kg/m

500

1000

1500

2000

2500

3000

20 40 60 80 100 120

Nabenhöhe in m

Spezifische Turmmassen in kg/m

Hub height in m

Specific tower weight in kg/ m

500

1000

1500

2000

2500

3000

20 40 60 80 100 120

Nabenhöhe in m

Spezifische Turmmassen in kg/m

500

1000

1500

2000

2500

3000

20 40 60 80 100 120

Nabenhöhe in m

Spezifische Turmmassen in kg/m

Hub height in m

Specific tower weight in kg/ m

Fig. 3-56 Specific tower weight versus hub height [27]

Fig. 3-56 gives an overview of the typical specific tower weight versus the hub

height for different types of self-supporting towers. The large spreading of the

values for a certain hub height is attributable to different types and manufacturers.

Masts with guy wires are a common tower type of the smaller wind turbines,

e.g. Aerosmart 5 (Fig. 3-57 left) and SÜDWIND 1237 (Fig. 3-18). They are

lightweight towers and are suitable for erection with a winch, i.e. without any

auxiliary crane, which reduces significantly the costs for transport and erection,

3 Wind turbines - design and components

101

Fig. 3-58, right. The guyed masts need a defined tension in the guy wires which

has to be checked periodically. An interesting model of this mast was developed

for a container hybrid system (comprising wind, solar, diesel, battery, etc.), Fig. 3-

57b. The mast and guy wires are anchored to the container, so there is no require-

ment for an additional foundation and anchoring in the soil.

Other special tower types for small wind turbines also geared towards easy

mounting and lowering of the turbine (for O&M or during storm) are the

“A-shaped” mast. The latter needs only anchoring pegs instead of concrete founda-

tions due to the big base area between its legs.

Fig. 3-57 Left: guyed pole (Aerosmart 5); right: mobile hybrid system (Terracon Energy Con-

tainer)

Fig. 3-58 Left: sail wind turbine with guyed A-pole; right: erection with a winch