Burton T. (et. al.) Wind energy Handbook

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will be at blade passing frequency and it is an unfortunate coincidence that this

often matches quite closely the natural frequency of oscillation of a small synchro-

nous generator connected to an electrical network.

In practice, synchronous generators are often ﬁtted with add itional cage damper

windings but it is not practical to provide the degree of damping required for wind

turbine applications. Also at higher ratings (above, say, 1 MW) second-or der effects

tend to reduce the damping available from induction generators (Saad-Saoud and

Jenkins, 1999). However, the basic principle remains that the damping provided by

induction generators is necessary for ﬁxed-speed wind turbines.

In contrast, the generators of variable-speed wind turbines are not connected

directly to the netw ork but are de-coupled through solid-state frequency converters.

Hence synchronous generators may be used.

6.10.1 Historical attempts to use synchronous generators

Induction generators are much less useful than synchronous generators for large-

scale power generation.

• The damping action results in higher energy losses in the rotor than with

synchronous generators. It is then, of course, necessary to arrange for the removal

of the heat dissipated in the rotor.

• All the reactive power necessary to energize the magnetic circuits must be

supplied from the network (or by local capacitors). If local capacitors are used

then there is the danger of self-excitation, when connection to the network is lost.

• There is no direct control over the terminal voltage or reactive power ﬂow.

• Induction generators do not produce sustained fault current for three-phase faults

on the network.

• They suffer from problems of voltage instabil ity. This was not an important issue

with limited wind generation but with very large wind farms is becoming of

concern.

Hence, in the early development of wind turbines considerable effort s were made

to use synchronous generators. These involved a number of innovative solutions to

the provision of damping. For example, both Westinghouse in the USA and

Howden in the UK used ﬂuid couplings in the drive train to provide damping. The

Wind Energy Group in the UK mounted a 250 kW synchronous generator using a

spring-damper system and connected a 3 MW synchronous generator through a

sophisticated variable-speed mechanical gearbox (Law, Doubt and Cooper, 1984).

However, these and other similar approaches using synchronous generators on

large prototype wind turbines are now of historical interest only.

TYPE OF GENERATOR 365

6.10.2 Direct-drive generators

There is considerable interest in the application of generators driven directly by the

wind-turbine rotor without a speed increasing gearbox and a number of manufac-

turers offer such wind turbines. However, the power output of any rotating

electrical machine may be generally described by (Laithwaite and Freris, 1980):

P ¼ KD

where D is the rotor diameter, L is the length, n is the rotational speed, and K is a

constant.

Thus it may be seen that if the rotatio nal speed is reduced then it is necessary

either to lengthen the generator in proportion or to increase the diameter. It is

cheaper to increase the diameter as this raises the power by the square rather than

linearly. Thus, direct-drive generators for wind turbines tend to have rather large

diameters but with limited length (Figure 6.20).

Induction generators require a rather small radial distance between the surface of

the rotor and the stator (known as the air-gap). This is necessary to ensure an

adequate air-gap magnetic ﬂux density as all the excitation is provided from the

stator. In contrast, synchronous generators have excitation systems on the rotor and

so can operate with larger air-gaps. It is difﬁcult to manufacture large diameter

electrical machines with small air gaps for mechanical and thermal reasons. Hence

direct-drive wind turbines use synchronous generators (either with permanent

magnet excitation or, more usually, with a wound rotor and electromagnets

providing the ﬁeld). The use of a synchronous generator, in turn, leads to the

requirement for solid-state frequency conversion equipment to de-couple the gen-

erator from the network and permit variable-speed operation.

6.11 Drive-train Mounting Arrangement Options

6.11.1 Low-speed shaft mounting

The functions of the low-speed shaft are the transmission of drive torque from the

rotor hub to the gearbox, and the transfer of all other rotor loadings to the nacelle

structure. Traditionally the mounting of the low-speed shaft on fore and aft

bearings has allowed these two functions to be catered for separately; the gearbox is

hung on the rear end of the shaft projecting beyond the rear bearing and the drive

torque is resisted by a torque arm. The front bearing is positioned as close as

possible to the shaft/hub ﬂange connection, in order to minimize the gravity

moment due to the cantilevered rotor mass, which usually governs shaft fatigue

design. The spacing between the two bearings will normally be greater than that

between front bearing and rotor hub in order to moderate the bearing loads due to

shaft moment (see Figure 6.15 for an illustration of a typical arrangement).

The opposite approach is to make the gearbox an integral part of the load path

between the low-speed shaft and tower top i.e., an ‘integrated gearbox’. The fore

366 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

and aft low-speed shaft bearings are absorbed within the gearbox, which moves to

the front of the nacelle in order to minimize the rotor cantilever distance, and the

gearbox casing then transmits the loads to the nacelle bedplate (Figure 6.21). Clearly

this approach requires a much more robust gearbox casing, which must not merely

resist the rotor loads, but do so without deﬂecting sufﬁciently to impair its function-

ing. Moreover its fore-aft length has to be increased in order to moderate the

bearing loads due to shaft moment. The beneﬁts lie in the reduced extent of the

bedplate and the elimination of separate bearings requiring separate provision for

lubrication, but a signiﬁcant disadvantage is that gearbox replacement requires the

removal of the rotor.

A conﬁguration which is becoming increasingly popular is one intermediate

between the two extremes described above, in which only the rear low-speed shaft

bearing is absorbed into the gearbox. The gearbox is usually set well back from the

front bearing in order to reduce the rear bearing loads, and is rigidly ﬁxed to

supporting pedestals positioned on either side of the nacelle. Typical arrangements

are shown in Figure 6.16, which shows a cross section through the nacelle of the

Nordex N-60 turbine, and in Figure 6.17. Note that the shaft tapers down in

diameter towards the rear reﬂecting the reducing bending moment. The advantage

of this arrangement is that the gearbox casing is not called upon to carry any

moments due to cantilevered rotor mass or rotor out-of-plane loadings.

Figures 6.18 and 6.19 are aerial views of the nacelle of a NEG-Micon 1.5 MW

machine with a similar drive train arrangement, after install ation of the low-speed

shaft.

Rotor hub

Front-

bearing

housing

Rear-

bearing

housing

Rotor

brake

Gearbox

Gearbox reaction arm

Generator

Low-speed shaft

Figure 6.15 View of Nacelle Showing Traditional Drive Shaft Arrangement

DRIVE-TRAIN MOUNTING ARRANGEMENT OPTIONS 367

In the case of wind turbines with direct-drive generators, the low-speed shaft

arrangement is dramatically different. The low-speed shaft, which now connects

the rotor hub to the rotor of the generator, is hollow, so that it can be mounted on a

concentric ﬁxed shaft cantilevered out from the nacelle bedplate (see Figure 6.20).

Rotor

hub

Hub mounting flange

Front-bearing

housing

Low-speed shaft

Gearbox

Gearbox mounting

Brake

High-speed

shaft

Cooler

Generator

Safety

coupling

Yaw drive

Yaw brake

Front

bearing

Nacelle

bedplate

Figure 6.16 Nacelle Arrangement for the Nordex N60 Turbine (Reproduced by permission

of Nordex)

Figure 6.17 Drive Train Side View. (From left to right the components visible through the

cut-out in the nacelle wall are: (1) low-speed shaft front bearing, (2) low-speed shaft, (3)

gearbox mountings, (4) gearbox, (5) high-speed shaft with brake, (6) generator. The fabricated

nacelle bedplate is also visible).

368

CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.11.2 High-speed shaft and generator mounting

The gen erator is normally mounted to the rear of the gearbox on an extension of the

nacelle bedplate and the connecting drive shaft – the ‘high speed shaft’ – is ﬁtted

with ﬂexible couplings at each end, to cater for small misalignments between the

generator and gearbox.

The generator axis is normally offset from the low-speed shaft axis. This is

because, except in the case of machines ﬁtted with a mechanical brake acting on the

rotor, access is required to the rear end of the low-speed shaft for actuation of

aerodynamic braking. Usually the generator is either offset to one side of the

nacelle, which introduces asymmetry into the nacelle bedplate, or it is offset

vertically upwards, which requires a vertical step in the bedplate.

A much more compact arrangement can be obtained by bolting the generator

rigidly onto the rear of the gearbox via an adaptor tu be (see Figure 6.21). The

surfaces of the mating interf aces have to be carefully machined to ensure shaft

alignment, and suitable access has to be provided to the coupling between the

generator and gearbox output shafts. Despite the neatness of this layout, it has only

been adopted by one or two manufacturers.

One consequence of locating the generator in the nacelle is that power cables

running down the tower are required to twist as the nacelle yaws. On some large

machines, the problems associated with the twisting of heavy cables have been

avoided by mounting the generator vertically in the top of the tower, and driving

Figure 6.18 Turbine Assembly in the Air (1). (View of nacelle of 1.5 MW NEG Micon turbine

after installation of low-speed shaft (front) and gearbox. The ring of bolt holes in the low-speed

shaft ﬂange for hub mountings are clearly visible). (Reproduced by permission of NEG Micon)

DRIVE-TRAIN MOUNTING ARRANGEMENT OPTIONS 369

the high-speed shaft via a bevel gear. An alternative solution to the problem of

heavy twisting cables, however, is to leave the generator in the nacelle and to

transform to a higher voltage there as well.

6.12 Drive-train Compliance

The rotational dynamics of the drive train can have a major effect on loading. The

effect is very different in ﬁxed- and variable-speed turbines, but in each case the

consequence of ignoring drive-train dynamics at the design stage can be very

severe.

Figure 6.19 Turbine Assembly in the Air (2). (View of low-speed shaft and front bearing after

installation on 1.5 MW NEG Micon turbine). (Reproduced by permission of NEG Micon)

370

CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

Figure 6.20 Direct-drive Generator Arrangement

Figure 6.21 Integrated Gearbox on the Zond Z-750 turbine. (The gearbox is mounted on a

circular nacelle bedplate, with the hub to the left and generator at the rear. An electrically

driven yaw drive can be seen beneath the generator).

DRIVE-TRAIN COMPLIANCE 371

In the variable-speed case, the dynam ics may be quite simple: the drive train may

be modelled as a rotor and a generator inertia, separated by a torsional spring.

Typically the natural frequency of this resonant system is quite high, of the order of

3–4 Hz. However, this mode is subject to very little damping, especially above

rated where the generator torque is held constant. (Below rated the torque will be

varied as a function of rotational speed, thus providing a small amount of

damping.) There is very little aerodynamic dampi ng from the rotor, and this mode

of vibration can potentially generate very large gearbox torque osc illations. Chapter

8 explains how the control system can be used to damp this mode by appropriate

control of the generator torque, but it is important to ensure that the resonant

frequency does not coincide with a signiﬁcant forcing frequency such as 6P, which

can make it very difﬁcult to achieve sufﬁcient damping through the control system.

In the ﬁxed-speed case, the directly-coupled induction generator provides a lot of

damping since the air-gap torque increases steeply with generator speed. The

torque–slip curve completely changes the dynamics compared to the variable-

speed case, resulting in a much lower ﬁrst mode frequency, typically closer to 1 Hz.

The damping factor is strongly dependent on the generator slip: a generator with

0.5% rated slip can give a peak dynamic magniﬁcation of perhaps 2 to 5 at the

resonant frequency, whereas with 2 percent slip the peak magniﬁcation may be no

more than 1 to 1.5. The position of the peak with respec t to blade-passing frequency

is critical: if the blade-passing frequency is close to the peak, very large gearbox

torque and electrical power oscillations will occur at this frequency. Pitch control

may further exacerbate these.

With a two-bladed turbine the blade-passing frequency tends to be closer to the

resonant peak; with a three-bladed turbine the blade-passing frequency is typic ally

higher, where the dynamic magniﬁcation is much lower. Nevertheless, even for

three-bladed turbines it is not uncommon for power and torque oscillations at the

blade passin g frequency to be as large as 50–100 percent during pitch controlled

operation in high winds.

The use of a high-slip generator greatly improves the situation, but there are two

main drawbacks. First, eac h 1 percent of slip corresponds to 1 percent of extra

losses, which signiﬁcantly reduces the energy yield below rated wind speed.

Second, these extra losses equate with heat dissipation in the generator, making it

more difﬁcult to keep the generator cool, especially in large machines.

An alternat ive to high generator slip which has occasionally been used is a ﬂuid

coupling between the gearbox and the generator. This is also a device which

generates a torque proportional to slip speed, and it suffers from the same draw-

backs as a high-slip generator.

Another technique which has sometimes been used is to reduce the resonant

frequency by introducing additional torsional ﬂexibility into the drive train. This

can be done by means of a quill shaft, a ﬂexible low-speed coupling, or ﬂexible

mounts for the gearbox or even for the whole bedplate. The frequency reduction is,

however, accompanied by a further loss of damping, and it may therefore be

necessary to incorporate additional me chanical damping with the torsional ﬂex-

ibility, which is not always easy to engineer. Torsional ﬂexibility in the high-speed

shaft is not usually practical because of the large angular movement required to

achieve the necessary ﬂexibility: half a revolution may be necessary, compared to

372 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

just one or two degrees at the low-speed shaft. An interesting variant (Leithead and

Rogers, 1995) is to mount the generator on ﬂexible mounts. This system can be

tuned to absorb energy at the blade-passing frequency through an addition al mode

of vibration of the generator casing against its mountings. This mode also affects

the generator slip speed (the difference between rotor and casing speeds) and is

therefore damped by the slip curve. Nevertheless, generator casing displacements

would still need to be of the order of 10–158, which is still not easy to engineer.

6.13 Rotor Position with Respect to Tower

6.13.1 Upwind conﬁguration

The upwind conﬁguration is the one most commonly chosen. The principal

advantage is that the tower shadow effect is much less for the same blade–tower

spacing, reducing both dynamic loads on the blade and rhythmic noise effects. Set

against this is the need to take great care to avoid the risk of blade–tower strikes

with upwind machines, requiring accurate prediction of blade deﬂections under

turbulent wind loading.

The clearance between the undeﬂected blade and the tower can be increased by

tilting the low-speed shaft upwards or by increasing the rotor overhang. It is

desirable to keep the rotor overhang small in order to minimize low-speed shaft

and nacelle bedplate bending moments, so the low-speed shaft is normally tilted

upwards by 58 or 68 to provide the necessary blade–tower clearance, at the cost of a

very small reduction in power output.

6.13.2 Downwind conﬁguration

The wind velocity deﬁcit behind a wind-turbine tower is much greater than that in

front of it, to the extent that Powles (1983) has reported a turbulent region with

essentially no forward velocity extending up to four tower diameters downstream

of an octago nal tower. Beyon d this distance, recovery is relatively rapid, with the

deﬁcit reduced to about 25 percent at seven tower diameters downstream.

In addition to the mean wind-speed velocity deﬁcit behind the tower, vortex

shedding results in additional wind-speed ﬂuctuations over and above those

already present due to turbulence. The two effects combine to present a harsh

environment to the blades immediately behind the tower. The blades are subjected

to a large negative impulsive load each time they pass the tower, which contributes

signiﬁcantly to blade fatigue damage, and the audible tower ‘thump’ that results is

liable to be unwelcome. Designers usually mitigate both effects by positioning the

rotor plane well clear of the tower, but this inevitably increases nacelle costs

somewhat.

An important beneﬁt of the downwind conﬁguration is that it allows the use of

very ﬂexible blades without the risk of tower strike. Such blades beneﬁt by being

ROTOR POSITION WITH RESPECT TO TOWER 373

less severely unloa ded by the tower shadow, because wind loading deﬂects them

further from the tower in the ﬁrst place.

6.14 Tower Stiffness

A key consideration in wind turbine design is the avoidance of resonant tower

oscillations excited by rotor thrust ﬂuctuations at rotational or blade-passing

frequency. The damping ratio may be only 2–3 percent for tower fore-aft oscilla-

tions and an order of magn itude less for side-to-side motion, so unacceptably large

stresses and deﬂections could develop if the blade-passing frequency and tower

natural frequency were to coincide. Rotational frequency is less of a concern,

because cyclic loadings at this frequency only arise if there are geometrical

differences between blades.

Wind-turbine towers are customarily categorized according to the relationship

between the tower natural frequency and the exciting frequencies. Towers with a

natural frequency greater than the blade-passing frequency are said to be stiff,

while those with a natural frequency lying between rotational frequency and blade-

passing frequency are said to be sof t. If the natural frequency is less than rotatio nal

frequency, the tower is described as soft–soft.

If the tower is designed to meet strength requirements and no more, its frequency

category is primarily determined by the ratio of to wer height to turbine diameter,

with the higher ratios producing the softer towers. The principal beneﬁts of stiff

towers are modest – they allow the turbine to run up to speed without passing

through resonance, and tend to radiate less sound. However, since stiff towers

usually require the provision of extra mater ial not otherwise required for strength,

soft towers are generally preferred.

6.15 Personnel Safety and Access Issues

An integral part of wind-turbine design is the inclusion of the necessary safety

provisions for operation and maintenance sta ff. Minimum requirements include the

following:

• ladder access to the nacelle – this needs to be ﬁtted with a fall-arrest device,

unless ladder runs are short and protected by intermediate landings; careful

attention needs to be paid to the route between the tower top and nacelle to avoid

hazards arising from sudden yawing movements;

• an alternative means of egress from the nacelle, for use in case of ﬁre in the tower

– this can take the form of an inertia-reel device , enabling personnel to lower

themselves through a hatch in the nacelle ﬂoor;

• locking devices for immobilizing the rotor and the yawing mechanism – rotor

brakes and yaw brakes are not considered sufﬁcient, because of the risk of

374 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES