Module 2.22.2-2
WIND TURBINE TECHNOLOGY
Electrical System
Gerhard J. Gerdes
4-Nov-05 (11:23)
Workshop on Renewable Energies
November 14-25, 2005
Nadi, Republic of the Fiji Islands
Contents Module 2.2
J
J
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Types of generator systems
Variable and fixed speed
Inverter technologies
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Types of generator systems
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
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Generator concepts of today’
today’s
commercially available turbines
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number of different types
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one speed
two speed
variable speed
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10
8
6
4
2
0
30 kW - <500 kW
500 kW - <1MW
1 MW - 2 MW
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Wind turbine with Induction Generator, direct
coupled to the grid
rotor
Gear box
induction generator
transformer
main switch
contactor
ASG
fuse
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grid
rotor
speed
thyristor starter
compensation
pitch or stall
Electr. quantities
wind speed
system management
Control
SG = synchronous generator, ASG = induction generator
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Typical induction generator
(squirrel-cage)
1
2
3
4
5
shaft
ball bearings
rotor
aluminium sticks
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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3
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2 pole and 4 pole generator
Rotating field
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2 pole and 4 pole generator
Rotating field
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Synchronous Generator Speeds
(rpm)
Pole number 50 Hz
2
3000
4
1500
6
1000
8
750
10
600
12
500
J
J
60 Hz
3600
1800
1200
900
720
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. machines
J
synchronous machines can only be connected to the
grid, when
X frequency
X phase position
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X and voltage
J
are the same
induction generators do not have to be synchronised
synchronisation
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Induction generator
J
advantages:
X cheap construction, no collector, no brushes
X very low maintenance
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X no synchronisation
J
disadvantages:
X requires an external 3-phase grid, no own grid building
capability
X requires reactive power (usually compensated by
capacitor banks)
X no voltage control
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Wind turbine with induction
(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 – grid connected
Mp: pull-out torque
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linear torque function
under normal operation range
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Induction generator for wind
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
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Vary applied grid voltage
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Pole changing induction machines
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Danish concept – direct gridconnection
J
J
J
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J
J
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
X to avoid overload of generator during stall procedure
X to reduce mechanical stress through high torque
gradients during switching on and over the generators
X to reduce fluctuations of mechanical system and
electrical output
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Danish concept - schema
note: smaller turbines < 100 kW used belt drives
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Danish concept – load curve with
two induction generators
from small to large generator
from large to small generator
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Rotor speed – power diagram 600
kW
source: DEWI
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Switching from lower to higher rotor
speed
Example: Stall controlled wind turbine
switching from lower to higher rotor speed (asynchronos generator)
Generator speed
Active power, reactive power
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n2
n1
Active power
Reactive power
Generator speed
2
4
6
8
10
Time, s
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14
16
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20
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Induction generator with variable
slip
induction generator with
additional rotor winding
Thyristor
filter
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 variable
slip - characteristics
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24
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Wind turbine with inverter system in the main
power circuit (variable speed)
rotor
gear
box
generator
excitation unit
inverter system
transformer
main switch
SG or
ASG
fuse
grid
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rotor speed
capacitors if
asynchronous
generator
filter
pitch or stall
electr. quantities
wind speed
system management
control
SG = synchronous generator, ASG = induction generator
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Wind turbine with inverter system in the main
power circuit(variable speed)
rotational speed:
grid coupling:
storage
the
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excitation:
control:
system
advantage:
disadvantage:
variable
soft, not coupled to grid frequency,
elasticity produced by using the rotational energy
capacity of the rotor at acceleration or deceleration of
rotational speed
self-excitation (i.e. exciting dynamo)
power control by pitch (seldom stall)
speed limitation by pitch
speed control by power regulation of the inverter
smoothing of power output,
compensation of wind speed fluctuations
controllable power generation
controllable production of reactive power
operation at optimal power coefficient cp due to variable
speed
generation of voltage and frequency for
autonomous energy systems
expensive construction
generation of harmonics,
higher level of maintenance
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Wind turbine with variable speed
rotor
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power – rotor
speed curve =
always in
aerodynamic
optimum
source: DEWI
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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
source: DEWI
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6 pulse rectifier – inverter system
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rectifier
6-pulse
inverter
transformer
source: DEWI
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12 pulse rectifier – inverter system
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two 6-pulse
inverters in parallel
I1
and
I2
before
special transformer
(three windings, with star/
triangle configuration)
and after the
transformer
source: DEWI
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Harmonic distortion
6-pulse inverter
12-pulse inverter
source: DEWI
6-pulse inverter with high level of
harmonic distortion
requires high efforts for filtering
12-pulse inverter normally two 6pulse 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
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Inverter with pulse-widthmodulation PWM
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rectifier
inverter
with
IGBT
transformer
source: DEWI
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Function of PWM inverter
J
J
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J
J
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
X inductance of the grid blocks higher frequencies
X capacitance of grid acts like a short-cut for high
frequencies
J
ideal for modern variable speed wind turbines
X power output can be controlled (power gradient)
X no harmonic distortion
Output of a PWM inverter with IGBTs
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Power switch cabinet
(300 kW) with IGBTs
(Enercon)
Enercon)
IGBT modules
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IGBT control
board (MPU)
AC out
DC in
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Frequency analysis of a wind with
PWM inverter
Frequency analysis of a wind with pulse width modulated inverter
frequency resolution: 6,25 Hz
Current/nominal current, %
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1.5
Sampling frequency of the PWM inverter
1.0
2. harmonic
3. harmonic
0.5
0.0
2
3
4
5
6
7
Frequency, kHz
8
9
10
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Double fed induction generator
variable speed with pulse width modulated inverter
rotor
gear
box
Slip ring
generator
transformer
inverter system
ASG
main switch
fuse
grid
filter
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rotor speed
pitch or stall
electr. quantities
wind speed
system management
control
ASG = induction generator
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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 (squirrel-cage
a.c.)
39
Frequency converter (slip ring a.c.)
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Double-fed induction generator
(DFIG)
Example: Grid connection of a
variable speed double fed induction
generator
1.2
Active power/rated power
Generator speed / synchronous speed
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1.0
0.8
Generator speed
0.6
0.4
Active power
0.2
0.0
0
10
20
30
40
50
60
70
80
90
100
Time, s
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fixed rotor speed WT,
two stage
Power
Power
Power
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variable rotor speed WT
Power
RPM 2
RPM
RPM 1
Rotor Speed
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Rotational speed and power against
wind velocity
Wind Speed
Wind Speed
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Typical instantaneous power
behaviour
constant rotor speed,
stall controlled WT
Power
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variable rotor speed,
pitch controlled WT
P rated
P rated
Time
Time
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Measured power curve and power
curve given by manufacturer
1800
Electrical power, kW
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1600
1400
1200
1000
800
600
400
Calculated power curve
200
Measured power curve
0
0
5
10
15
20
25
Wind speed at hub height, m/s
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