1

Workshop on Renewable Energies

November 14-25, 2005

Nadi, Republic of the Fiji Islands

Module 2.2Module 2.2--22

WIND TURBINE TECHNOLOGYWIND TURBINE TECHNOLOGYElectrical SystemElectrical System

Gerhard J. Gerdes

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Contents Module 2.2Contents Module 2.2

Types of generator systems

Variable and fixed speed

Inverter technologies

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Converter type Application

induction (asynchronous) generator

applied in a large number of turbines (Danish type)

synchronous generator only used for a small number of small wind turbines, mainly for stand alone systems

asynchronous or synchronous generator with electronic inverter system

a widely used concept for variable speed machines, increasing applications with growing size of wind turbines

Types of generator systemsTypes of generator systems

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0

2

4

6

8

10

12

14

16

30 kW - <500 kW 500 kW - <1MW 1 MW - 2 MW

nu

mb

er

of

dif

fere

nt

typ

es

one speed

two speed

variable speed

Generator concepts of todayGenerator concepts of today’’s s

commercially available turbinescommercially available turbines

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ASG

induction generatorGear box

rotor

transformermain switch

fuse

contactor

thyristor startercompensation

system managementwind speed

pitch or stall

Electr. quantities

rotor

speed

Control

grid

Wind turbine with Induction Generator, direct Wind turbine with Induction Generator, direct

coupled to the gridcoupled to the grid

SG = synchronous generator, ASG = induction generator

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Typical induction generator Typical induction generator

(squirrel(squirrel--cage)cage)

1 shaft

2 ball bearings

3 rotor

4 aluminium sticks

5 aluminium ring

(4,5: squirrel-cage)

6 stator nuts with coils

7 stator

8 casing

9 coils of stator

10 ventilator

11 connection box

Squirrel-cage

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2 pole and 4 pole generator2 pole and 4 pole generator

Rotating field

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2 pole and 4 pole generator2 pole and 4 pole generator

Rotating field

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Synchronous Generator Speeds Synchronous Generator Speeds

(rpm) (rpm)

Pole number 50 Hz 60 Hz

2 3000 3600

4 1500 1800

6 1000 1200

8 750 900

10 600 720

12 500 600

a larger number of poles means a lower rotational

speed and thus lower gear box ratio

but it means also a larger dimension of the generator

and more weight

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Synchronisation of a.c. machinesSynchronisation of a.c. machines

synchronous machines can only be connected to the

grid, when

frequency

phase position

and voltage

are the same

induction generators do not have to be synchronised

synchronisation

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Induction generatorInduction generator

advantages:

cheap construction, no collector, no brushes

very low maintenance

no synchronisation

disadvantages:

requires an external 3-phase grid, no own grid building

capability

requires reactive power (usually compensated by

capacitor banks)

no voltage control

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Wind turbine with induction Wind turbine with induction

(asynchronous) generator (asynchronous) generator

rotational speed: fixed rotational speed plus slip (~ 2 %)

grid coupling: rigid, with low elasticity

excitation: by the grid

control: power control by stall or active stall

(also pitch in case of small turbines)

speed control by grid frequency

advantage: simple and cheap construction,

lower maintenance standard (stall)

no synchronisation with the grid required

disadvantage: production of reactive power,

generation of power peaks,

only low compensation of wind speed

fluctuations,

power generation not controllable (stall)

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Torque diagram Torque diagram –– grid connectedgrid connected

Mp: pull-out torque

linear torque function

under normal operation range

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Induction generator for wind Induction generator for wind

turbinesturbines

for a better matching of the torque characteristic of an

induction generator to that of a wind turbine (rotor) one

can:

vary the resistance of the rotor windings

vary applied grid voltage

use pole changing induction machines (i.e. two speed)

use a frequency converter with a squirrel-cage

induction generator

use a frequency converter with a slip ring induction

generator

use a double-fed induction generator

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Vary resistance of rotor windingsVary resistance of rotor windings

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Vary applied grid voltageVary applied grid voltage

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Pole changing induction machinesPole changing induction machines

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Danish concept Danish concept –– direct griddirect grid--

connectionconnection

stall controlled, three bladed rotor with fixed hub

wing-tip air brakes for emergency braking

direct grid connection through thyristors (“soft-starter”)

generally two generators (small ~ .25 of large) or one

generator with double windings (4 pole small, 6 pole

large)

induction generators deliberately designed for a larger

slip

to avoid overload of generator during stall procedure

to reduce mechanical stress through high torque

gradients during switching on and over the generators

to reduce fluctuations of mechanical system and

electrical output

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Danish concept Danish concept -- schemaschema

note: smaller turbines < 100 kW used belt drives

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Danish concept Danish concept –– load curve with load curve with

two induction generatorstwo induction generators

from small to large generator

from large to small generator

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Rotor speed Rotor speed –– power diagram 600 power diagram 600

kWkW

source: DEWI

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2 4 6 8 10 12 14 16 18 20

Active power

Reactive power

Generator speed

Ge

nera

tor

sp

eed

Acti

ve

po

wer,

reacti

ve

po

wer

Time, s

n1

n2

Example: Stall controlled wind turbine

switching from lower to higher rotor speed (asynchronos generator)

Switching from lower to higher rotor Switching from lower to higher rotor

speedspeed

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Induction generator with variable Induction generator with variable

slipslip

rotor speed partly variable (~2 to 10 % of nominal speed)

elastic grid coupling through increased slip

reduction of power peaks, no synchronisation needed

generates reactive power, reduced efficiency under part load

induction generator with

additional rotor winding

Thyristor filter

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Induction generator with variable Induction generator with variable

slip slip -- characteristicscharacteristics

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generatorgear

box

rotor

transformer

fuse

filter

system management

wind speed

pitch or stall

electr. quantities

rotor speed

excitation unit

SG or

ASG

inverter system

capacitors if

asynchronous

generator

main switch

control

grid

Wind turbine with inverter system in the main Wind turbine with inverter system in the main

power circuit (variable speed)power circuit (variable speed)

SG = synchronous generator, ASG = induction generator

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Wind turbine with inverter system in the main Wind turbine with inverter system in the main

power circuit(variable speed)power circuit(variable speed)

rotational speed: variable

grid coupling: soft, not coupled to grid frequency, elasticity produced by using the rotational energy

storage capacity of the rotor at acceleration or deceleration of

the rotational speed

excitation: self-excitation (i.e. exciting dynamo)

control: power control by pitch (seldom stall)speed limitation by pitchspeed control by power regulation of the inverter

system

advantage: smoothing of power output, compensation of wind speed fluctuationscontrollable power generationcontrollable production of reactive poweroperation at optimal power coefficient cp due to variable speedgeneration of voltage and frequency for autonomous energy systems

disadvantage: expensive constructiongeneration of harmonics,higher level of maintenance

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Wind turbine with variable speed Wind turbine with variable speed

rotor rotor

power – rotor

speed curve =

always in

aerodynamic

optimum

source: DEWI

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Advantage variable speedAdvantage variable speed

fluctuations of wind speed

converted into torque

fluctuations

high mechanical strain on

the drive train

fluctuations of wind speed

converted into rotor speed

increase

rotor stores energy

smoothens output

source: DEWI

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6 pulse rectifier 6 pulse rectifier –– inverter systeminverter system

rectifier

6-pulse

inverter transformer

source: DEWI

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12 pulse rectifier 12 pulse rectifier –– inverter systeminverter system

I1

and

I2

before

and after the

transformer

two 6-pulse

inverters in parallel

special transformer

(three windings, with star/

triangle configuration)

source: DEWI

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Harmonic distortionHarmonic distortion

6-pulse inverter with high level of

harmonic distortion

requires high efforts for filtering

12-pulse inverter normally two 6-

pulse inverter in parallel

one inverter with star-star, the

other with star-triangle

connection

special three-winding transformer

harmonic distortion greatly

reduced

12-pulse inverter more complex,

requires less efforts for filtering

6-pulse inverter

12-pulse inverter

source: DEWI

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Inverter with pulseInverter with pulse--widthwidth--

modulation PWMmodulation PWM

rectifier inverter

with

IGBT

transformer

source: DEWI

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Function of PWM inverterFunction of PWM inverter

the condenser between rectifier and PWM inverter acts

like an ideal DC voltage source

Insulated Gate Bipolar Transistors (IGBT) switch at

high frequencies (up to 10 kHz) to create a sine wave

practical no harmonic distortion up to ordinal number

19

harmonics with higher ordinal numbers exist, but have

no effect on the grid, because

inductance of the grid blocks higher frequencies

capacitance of grid acts like a short-cut for high

frequencies

ideal for modern variable speed wind turbines

power output can be controlled (power gradient)

no harmonic distortion

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Output of a PWM inverter with Output of a PWM inverter with IGBTsIGBTs

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Power switch cabinet Power switch cabinet

(300 kW) with (300 kW) with IGBTsIGBTs

((EnerconEnercon))

DC in

AC out

IGBT modules

IGBT control

board (MPU)

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Frequency analysis of a wind with pulse width modulated inverter

frequency resolution: 6,25 Hz

0.0

0.5

1.0

1.5

2 3 4 5 6 7 8 9 10

Frequency, kHz

Cu

rre

nt/

no

min

al c

urr

en

t, %

Sampling frequency of the PWM inverter

2. harmonic3. harmonic

Frequency analysis of a wind with Frequency analysis of a wind with

PWM inverter PWM inverter

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Double fed induction generatorDouble fed induction generatorvariable speed with pulse width modulated invertervariable speed with pulse width modulated inverter

Slip ring

generator

gear

box

rotor

transformer

fuse

filter

system management

pitch or stall

electr. quantities

rotor speed

ASG

inverter system main switch

control

grid

wind speed

ASG = induction generator

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Double fed induction generator Double fed induction generator

rotational speed: variable

grid coupling: soft, elasticity produced by using the rotational

energy storage capacity of the rotor at acceleration

or deceleration of the rotational speed

excitation: by the grid

control: power control by stall or pitch

speed limitation by pitch

speed control by power regulation of the

inverter system

advantage: smoothing of power output

compensation of wind speed fluctuations

controllable power generation

operation at optimal power coefficient cp due to

variable speed

disadvantage: expensive construction

generation of harmonics

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Frequency converter (squirrelFrequency converter (squirrel--cage cage

a.c.)a.c.)4-

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Frequency converter (slip ring a.c.)Frequency converter (slip ring a.c.)

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DoubleDouble--fed induction generator fed induction generator

(DFIG)(DFIG)4-

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70 80 90 100

Time, s

Acti

ve

po

wer/

rate

dp

ow

er

Gen

era

tor

sp

eed

/ s

yn

ch

ron

ou

ssp

eed

Active power

Generator speed

Example: Grid connection of a Example: Grid connection of a

variable speed double fed induction variable speed double fed induction

generatorgenerator

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Rotational speed and power against Rotational speed and power against

wind velocity wind velocity

Wind Speed

Po

wer

RPM

Power

Wind Speed

Po

we

r

Ro

tor

Sp

eed

RPM 2

RPM 1

Power

variable rotor speed WT fixed rotor speed WT,

two stage

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Time

Po

we

r

Prated

Time

Prated

variable rotor speed,

pitch controlled WT

constant rotor speed,

stall controlled WT

Typical instantaneous power Typical instantaneous power

behaviourbehaviour

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0

200

400

600

800

1000

1200

1400

1600

1800

0 5 10 15 20 25

Calculated power curve

Measured power curve

Wind speed at hub height, m/s

Ele

ctr

ica

lp

ow

er,

kW

Measured power curve and power Measured power curve and power

curve given by manufacturer curve given by manufacturer