WIND TURBINE TECHNOLOGY

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Workshop on Renewable Energies November 14-25, 2005

Nadi, Republic of the Fiji Islands Module 2.2

Module 2.2--22

WIND TURBINE TECHNOLOGY WIND TURBINE TECHNOLOGY

Electrical System Electrical System Gerhard J. Gerdes

4-Nov-05(11:23)rkshop on Renewable Energies

Contents Module 2.2 Contents Module 2.2

J Types of generator systems

J Variable and fixed speed

J Inverter technologies

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4-Nov-05(11:23)e7 / PPA Workshop on Renewable Energies

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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 systems Types of generator systems

0 2 4 6 8 10 12 14 16

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

number of different types

one speed two speed variable speed

Generator concepts of today Generator concepts of today’’s s commercially available turbines commercially available turbines

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5 ASG

induction generator Gear box

rotor

transformer main switch

fuse contactor

thyristor starter

compensation

system management wind 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 grid

coupled to the grid

SG = synchronous generator, ASG = induction generator

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 generator 2 pole and 4 pole generator

Rotating field

2 pole and 4 pole generator 2 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

J a larger number of poles means a lower rotational speed and thus lower gear box ratio

J but it means also a larger dimension of the generator and more weight

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

J synchronous machines can only be connected to the grid, when

Xfrequency Xphase position Xand voltage are the same

J induction generators do not have to be synchronised synchronisation

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Induction generator Induction generator

J advantages:

Xcheap construction, no collector, no brushes Xvery low maintenance

Xno synchronisation J disadvantages:

Xrequires an external 3-phase grid, no own grid building capability

Xrequires reactive power (usually compensated by capacitor banks)

Xno voltage control

Wind turbine with induction Wind turbine with induction (asynchronous) generator (asynchronous) generator

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

J grid coupling: rigid, with low elasticity

J excitation: by the grid

J control: power control by stall or active stall (also pitch in case of small turbines) speed control by grid frequency

J advantage: simple and cheap construction, lower maintenance standard (stall) no synchronisation with the grid required

J 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 connected grid connected

M_p: pull-out torque

linear torque function

under normal operation range

Induction generator for wind Induction generator for wind turbines

turbines

for a better matching of the torque characteristic of an induction generator to that of a wind turbine (rotor) one can:

J vary the resistance of the rotor windings

J vary applied grid voltage

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

J use a frequency converter with a squirrel-cage induction generator

J use a frequency converter with a slip ring induction generator

J use a double-fed induction generator

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Vary resistance of rotor windings Vary resistance of rotor windings

Vary applied grid voltage

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Pole changing induction machines Pole changing induction machines

Danish concept

Danish concept – – direct grid- direct grid - connection

connection

J stall controlled, three bladed rotor with fixed hub

J wing-tip air brakes for emergency braking

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

J generally two generators (small ~ .25 of large) or one generator with double windings (4 pole small, 6 pole large)

J induction generators deliberately designed for a larger slip

Xto avoid overload of generator during stall procedure Xto reduce mechanical stress through high torque

gradients during switching on and over the generators Xto reduce fluctuations of mechanical system and

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Danish concept

Danish concept - - schema schema

note: smaller turbines < 100 kW used belt drives

Danish concept

Danish concept – – load curve with load curve with two induction generators

two 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 kW kW

source: DEWI

Active power Reactive power Generator speed

Generator speed

Activepower, reactivepower

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 speed

speed

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Induction generator with variable Induction generator with variable slip slip

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

Induction generator with variable Induction generator with variable slip

slip - - characteristics characteristics

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25 generator

gear 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

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 pitch

speed control by power regulation of the inverter system

advantage: smoothing of power output,

compensation of wind speed fluctuations controllable power generation

controllable production of reactive power

operation at optimal power coefficient cpdue to variable speed

generation of voltage and frequency for autonomous energy systems

disadvantage: expensive construction

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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

Advantage variable speed Advantage 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

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6 pulse rectifier

6 pulse rectifier – – inverter system inverter system

rectifier

6-pulse

inverter transformer

source: DEWI

12 pulse rectifier

12 pulse rectifier – – inverter system inverter system

I1 and I2 before

and after the transformer two 6-pulse inverters in parallel

special transformer (three windings, with star/

triangle configuration)

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Harmonic distortion Harmonic distortion

6-pulse inverterwith high level of harmonic distortion

requires high efforts for filtering 12-pulse inverternormally 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

Inverter with pulse

Inverter with pulse- -width width- - modulation PWM

modulation PWM

rectifier

inverter with IGBT

transformer

source: DEWI

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Function of PWM inverter Function of PWM inverter

J the condenser between rectifier and PWM inverter acts like an ideal DC voltage source

J Insulated Gate Bipolar Transistors (IGBT) switch at high frequencies (up to 10 kHz) to create a sine wave

J practical no harmonic distortion up to ordinal number 19

J harmonics with higher ordinal numbers exist, but have no effect on the grid, because

Xinductance of the grid blocks higher frequencies Xcapacitance of grid acts like a short-cut for high

frequencies

J ideal for modern variable speed wind turbines

Xpower output can be controlled (power gradient) Xno harmonic distortion

Output of a PWM inverter with

Output of a PWM inverter with IGBTs IGBTs

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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)

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

Current/nominal current, % Sampling frequency of the PWM inverter

2. harmonic

3. harmonic

Frequency analysis of a wind with Frequency analysis of a wind with PWM inverter

PWM inverter

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Double fed induction generator Double fed induction generator

variable speed with pulse width modulated inverter variable 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

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 c_pdue to variable speed

disadvantage: expensive construction generation of harmonics

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39

Frequency converter (squirrel

Frequency converter (squirrel- -cage cage a.c.)

a.c.)

Frequency converter (slip ring a.c.)

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Double

Double- -fed induction generator fed induction generator (DFIG)

(DFIG)

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

Activepower/ratedpower Generator speed/ synchronousspeed

Active power Generator speed

Example: Grid connection of a Example: Grid connection of a variable speed double fed induction variable speed double fed induction generator

generator

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Rotational speed and power against Rotational speed and power against wind velocity

wind velocity

Wind Speed

Power

RPM Power

Wind Speed

Power Rotor Speed

RPM 2 RPM 1

Power

variable rotor speed WT fixed rotor speed WT, two stage

Time

Power Prated

Time Prated

variable rotor speed, pitch controlled WT

constant rotor speed, stall controlled WT

Typical instantaneous power Typical instantaneous power behaviour

behaviour

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45 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

Electricalpower, kW

Measured power curve and power Measured power curve and power curve given by manufacturer curve given by manufacturer