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

Подождите немного. Документ загружается.

442 13.1 Grid-connected wind turbines

100

200

300

400

500

600

700

800

02468101214161820

Wind speed in m/s

Power in kW

Enercon E-40 / 6.44

Nordex N43

SÜDWIND S.46

Fig. 13-12 Power curves P(v) of three different wind turbines

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0 2 4 6 8 101214161820

Wind speed in m/s

Power coefficient c

P.tot

Enercon E-40 / 6.44

Nordex N43

SÜDWIND S.46

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0 2 4 6 8 101214161820

Wind speed in m/s

Power coefficient c

P.tot

Enercon E-40 / 6.44

Nordex N43

SÜDWIND S.46

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Fig. 13-13 Power coefficients c

P,tot

(v) of three different wind turbines

The dimensionless total power coefficients c

P.tot

reveal more. We compare first the

wind turbine of the Danish concept by NORDEX (two fixed rotor speeds) with the

ENERCON turbine which operates with variable rotor speed. For the range close

to the cut-in wind speed (3 m/s) the curves of c

P.tot

are nearly identical. Then the

NORDEX turbine shows two local optima which correspond to the design wind

speed of the “small” resp. “large” asynchronous generator. The first maximum of

13 Concepts of electricity generation by wind turbines 443

0.40 at a wind speed of approx. 5 m/s is significantly higher than the value of 0.35

for the variable-speed machine E-40. The full power conversion of the E-40

reduces its partial load efficiency significantly (at 10% of the rated power).

But for higher wind speeds the power coefficient of the E-40 which was de-

signed for variable speed is always slightly better than for the N43. At their rated

wind speed of 12 m/s the power is limited at both turbines. The N43 uses the stall

effect, the E-40 a blade pitching system. The curves of c

P.tot

are nearly identical in

this range. Since the control of the E-40 keeps the power output constant above

12 m/s there are no values shown for higher wind speeds.

The variable-speed wind turbine of SÜDWIND with a doubly-feeding asyn-

chronous generator and a converter in the rotor circuit cuts in only at 3.5 m/s,

which is barely visible in the power curve P(v). Up to a wind speed of approx.

6 m/s the curve of c

P.tot

is significantly below the curves of the N43 and E-40. In

the range between 6 and 9.5 m/s it is slightly above the values of the E-40. Both

reach a maximum efficiency of c

P.tot

= 0.45. Since the control limits the power in

the strong wind range above the rated wind speed the differences are negligible.

In general, it can be stated that despite the differences in the three wind turbine

concepts the c

P.tot

curves are astonishingly similar, except for differences in cut-in

wind speeds. The maximum power coefficient of the “primitive” Danish wind tur-

bine is only 2 percentage points below that of the wind turbines with a blade pitch-

ing system and variable rotational speed.

Thus, the relevant advantages of the modern variable-speed concepts are not

found in their efficiency, but in

- The reduction of power fluctuations during gusts,

- Significant relief for the structure in the strong wind range,

- The flexibility in adjusting the reactive power,

- The adaptation of the blade tip speed to local conditions “by pressing of a

button”, etc.

In summary: we gain increased flexibility with regard to the actual on-site condi-

tions.

13.2 Wind turbines for stand-alone operation

The most common stand-alone applications of wind turbines are the battery

charger and the wind pump. Stand-alone wind turbines eke out a niche existence

in Western Europe due to the omnipresent electrical grid. But they are of great

practical importance in the rest of the world. In Mongolia, e.g., the nomads oper-

ate thousands of battery chargers providing electricity for light and television.

444 13.2 Wind turbines for stand-alone operation

13.2.1 Battery chargers

Battery chargers have small power ratings. Typical values range from a few watts

to approx. 1.5 kW. This implies small rotor diameters (0.5 to 3.0 m), and relatively

high rotor speeds. The gearbox which is usually necessary between generator and

rotor is not needed here. Directly driven synchronous generators with a medium or

large number of poles (8 to 20 poles) are used.

For the selection of the aerodynamic profiles the low Reynolds numbers of the

flow around the blades have to be taken into account (Re = w c/

< 100 000). Suit-

able profiles for this range are found in catalogues of aerodynamic profiles for

model airplanes (e.g. [4] in chapter 5 of this book).

In order to determine the stationary operating behaviour, the familiar equivalent

circuit diagram of the AC synchronous machine is extended to include the load

and the rectifier (RF). Fig. 13-14b shows the resulting load curves. Below the

threshold speed, the generator voltage is lower than the sum of the voltage pro-

vided by battery charger and rectifier. Hence, there is no power output. Above the

threshold speed, there is a steep increase in power transfer, similar to the power

curve of the synchronous machine with a resistive load, see section 11.1, Fig.

11-9.

For a battery charger with a constant load, two types of operation can be distin-

guished. For low wind speeds, the battery provides the largest part of the required

power. The wind turbine operates at its power optimum (A) if it is a well designed

machine. For high wind speeds or small loads, the energy output of the wind tur-

bine exceeds the demand of the load, and has to be limited (B), therefore.

For a generator with an excitation winding, this can be achieved by adjusting

the excitation current so that the critical battery voltage is reached, but not ex-

ceeded. Fig. 13-14b shows that the power consumption is then limited, and that

the wind turbine gradually transitions to idling for higher wind speeds. If the rotor

is designed with a high tip speed ratio, limiting measures for the rotational speed

are required, e.g. by tilting, see Fig.12-19 in chapter 12.

Small battery chargers which use permanent magnets for the excitation have no

need for control of the charging current. In this case, the charging power is simply

limited by the value of the generator inductance (Fig. 11-9) or by an additional in-

ductance connected in series, Fig. 13-15. For these components, the impedance

= :L

add

) increases with the frequency. In this way the battery charging cur-

rent is limited. This arrangement is shown in Fig. 13-15. If the battery storage is

large in relation to the installed generator capacity, an overcharging of the battery

is hardly possible.

In order to make better use of the advantages of excitation with permanent

magnets (good efficiency and low wear), devices from power electronics have to

be incorporated. Due to the constant excitation of the synchronous generator, the

generated voltage is proportional to the rotational speed.

13 Concepts of electricity generation by wind turbines 445

For high speeds, this voltage exceeds the rated charging voltage of the battery sub-

stantially. A step-down converter [4] is used to convert this high DC voltage to the

lower voltage level needed for the battery. Generator and battery voltage are then

decoupled, and the battery charging current can be adjusted to reasonable values

in accordance with the available power and the needs of the battery

RF step down converter consumer

Battery DC grid

storage

Field controller

CO 2

CO 1

P [W]

500

600 n [rpm]

; T

increases

A: P

load

> P

Operation with constant

excitation close to the

turbine optimum power

B: P

load

< P

Limiting the power input when the battery is

full

CO1: Reduction of the exciting current (I

)

CO2: Reduction of operating time (T

)

Charging

begins

RF step down converter consumer

Battery DC grid

storage

Field controller

CO 2

CO 1

P [W]

500

600 n [rpm]

; T

increases

A: P

load

> P

Operation with constant

excitation close to the

turbine optimum power

B: P

load

< P

Limiting the power input when the battery is

full

CO1: Reduction of the exciting current (I

)

CO2: Reduction of operating time (T

)

Charging

begins

RF step down converter consumer

Battery DC grid

storage

Field controller

CO 2

CO 1

P [W]

500

600 n [rpm]

; T

increases

A: P

load

> P

Operation with constant

excitation close to the

turbine optimum power

B: P

load

< P

Limiting the power input when the battery is

full

CO1: Reduction of the exciting current (I

)

CO2: Reduction of operating time (T

)

Charging

begins

Fig. 13-14 Battery charger, a) Block diagram; CO 1 = Controller for a generator with a field

winding excitation, CO 2 = Controller for a permanent-excited generator, b) Load curves of a

battery charger

446 13.2 Wind turbines for stand-alone operation

add

Thermo

switch

N S

add

Thermo

switch

N S

Fig. 13-15 Battery charger with a thermo element to switch in an additional inductance at high

power levels; synchronous generator with excitation by permanent magnets

13.2.2 Resistive heaters with synchronous generators

Resistive heaters can be used to convert electric energy into thermal energy, for

instance to make hot water. The block diagram in Fig. 13-16 shows the rather

simple layout of such a system. It consists of a wind turbine rotor with only a sim-

ple speed limitation, of a synchronous generator (SG) with gearbox, and of the

corresponding resistors. How well the optimum load curve of the wind turbine is

matched can be influenced through the excitation and the load resistors.

For a machine with constant excitation, the relationships derived in chapter 11

for the alternator remain valid (cf. Fig. 11-9). For low rotational speeds the load

curve is almost a parabola. After reaching the rated torque, the torque decreases,

but power, voltage and current continue to rise, albeit more slowly. The power

output of the wind turbine is well utilized in a wide range, Fig. 13-16, curve a.

It is often observed that wind turbines, which drive a generator, that has a per-

manent magnetic field, require wind speeds to be quite high, before they start turn-

ing. The culprit behind this is the “magnetic latching”, i.e. the rotor appears to be

locked into a rest position and needs torque to break loose. This possible short-

coming can be dealt with during the design of the generator.

The demand for quickly increasing electrical power is another detriment to

start-up at low wind speeds. This can be remedied by initially disconnecting the

load.

Self-excited synchronous machines do not have this problem, since excitation

only starts above a minimum rotational speed. Their load curve is steeper since the

excitation current is caused by the stator voltage which increases with the rota-

13 Concepts of electricity generation by wind turbines 447

tional speed. The exact curve is somewhat influenced by the selected controller

type. It will, however, always be quite similar to the curve b in Fig. 13-16.

Fig. 13-16 Wind turbine for heating, speed control by centrifugal pitch, block diagram and load

curves: a) generator with permanent magnetic field, b) self-excited generator

448 13.2 Wind turbines for stand-alone operation

13.2.3 Wind pump system with electrical power transmission

Wind pump systems with electrical power transmission have some advantages

compared to systems with direct mechanical coupling, but a lower wind pump ef-

ficiency has to be accepted. However, a more suitable site for the wind turbine can

now be chosen because it is possible that well and turbine are at separate loca-

tions. Moreover, for deep well pumps (multi-stage centrifugal pumps) electrical

power transmission is more suitable than mechanical power transmission.

A distinction is usually made between systems with variable speed

(Fig. 13-17), and systems where the wind turbines operate at a more or less con-

stant speed (Fig. 13-18). In variable speed systems, the blade pitch limits the speed

during strong winds. In order to determine the operational behaviour, the equiva-

lent circuit diagram of the synchronous machine is used and extended by the

equivalent circuit diagram of the stationary asynchronous motor (ASM),

Fig. 11-22.

In this diagram the asynchronous motor appears as a resistive-inductive load.

The diagram leads us to the behaviour of a synchronous machine in a stand-alone

operation. However, it should be noted, that the parameters which represent the

asynchronous motor are in part a function of the rotational speed.

Fig. 13-17 shows the load curve of a wind pumping system operating at vari-

able speeds with a self-excited synchronous generator. Four ranges of operation

can be distinguished:

A Low-speed idling of the rotor, power consumption

determined by the friction in the bearings and gearbox, etc.

B Generator delivers voltage: pump acts as fluid friction dynamometer since

the rotational speed is not sufficient to overcome the geodetic head

C Water pumping: operation of the wind pump close to optimal wind

turbine power output

D Speed and power limitation: by a centrifugal pitch control

Fig. 13-18 shows a wind pumping system with electrical power transmission that

operates in a very narrow range of speeds. The pitch-controlled wind turbine

MAN-Aeroman was basically designed for grid-connected operation. As the speed

range is very narrow, the electrical machine and the centrifugal pump always work

close to their rated operating points. The layout of the system is, therefore, com-

paratively easy, but the engineering and control effort is substantial.

When the wind speed changes, pumps are switched on or off in order to adapt

to the current supply of energy. Above 10 m/s, the fast pitch control leads to a

constant power output. As an alterative to pitch control, a stall control may be

used to limit the power. In that case, the wind turbine’s speed is limited by con-

necting a controllable heater, if the maximum allowed pump power would other-

wise be exceeded.

13 Concepts of electricity generation by wind turbines 449

Fig. 13-17 Wind pumping system operating with variable rotor speeds, a) Block diagram,

b) Load curve

450 13.2 Wind turbines for stand-alone operation

Fig. 13-18 Wind pumping system operating at nearly constant rotor speed by switching the

pumps, a) Block diagram, b) Load curve

13 Concepts of electricity generation by wind turbines 451

13.2.4 Stand-alone wind turbines for insular grids

When feeding insular or local grids, a wind turbine supplies a fairly constant fre-

quency and voltage for isolated consumers, e.g. alpine huts, farms, or rural com-

munities in the Third World. In this application, the supply of both active and re-

active power is required, and voltage control is necessary. Therefore, synchronous

generators (SG) are suitable. A fairly stable frequency is achieved by switching

consumers on and off. A stall-controlled turbine combined with an adjustable

heater or a fast pitch control is used.

Fig. 13-19 Wind turbine with pitch control and consumer control system for stand-alone opera-

tion, a) Block diagram, b) Load curve