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1
Power Generator Technologies for
Wind Turbine
Mehrdad Ghandhari
1 2
References
1. “Wind Power Plants”, ABB, Technical
Application Papers No.13.
2. “WECC Wind Power Plant Dynamic Modeling
Guide”, WECC Renewable Energy Modeling
Task Force.
3. Wind Turbine Plant Capabilities Report”,
Australian Energy Market Operator, AEMO
4. “Understanding Inertial and Frequency
Response of Wind Power Plants”, NREL/CP-
5500-55335 3
Basic structure
Wind
speedMech.
parts ~ Power
System
Elec.
devices
4
Synchronous Machines (SM)
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
5
• a, b and c denote the stator three windings.
• They are 120 electrical degrees apart.
a-axis
b-axis
c-axis
m
a a
b
b c
c
d-axisq-axis
fi
6
• The stator is represented by three magnetic
axes a, b and c each corresponding to one of
the phase windings.
a-axis
b-axis c-axis
m
d-axis q-axis
ci
fu
fi
bubi
au
ai
cu
n
2
7
• The field winding.
• It carries a direct current to produce a magnetomotive force (mmf) which drives the field flux around the magnetic circuit.
a-axis
b-axis
c-axis
m
a a
b
b c
c
d-axisq-axis
fi
8
• Since the turbine with a torque (Tm) rotates
the rotor shaft,
– the field winding on the rotor (which carries a direct
current) produces a rotating flux in the air gap which
induces voltages over the stator windings.
– Slip rings and brushes will be needed to supply the
field winding with dc current.
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
9
• The rotor is represented by two axes. The
direct axis (d-axis), which is the magnetic
axis of the field winding.
a-axis
b-axis c-axis
m
d-axis q-axis
ci
fu
fi
bubi
au
ai
cu
n
10
• The quadrature axis (q-axis) which is located
90 electrical degrees behind the d-axis.
a-axis
b-axis c-axis
m
d-axis q-axis
ci
fu
fi
bubi
au
ai
cu
n
11
• The mechanical rotor angle.
• It defines the instantaneous position of the
rotor d-axis with respect to a stationary
reference.
a-axis
b-axis c-axis
m
d-axis q-axis
ci
fu
fi
bubi
au
ai
cu
n
m mt
12
a-axis
b-axis
c-axis
m
a a
b
b c
c
d-axisq-axis
fi
2
2 260
2
( ) ( )120
120 ( )( )
r
m m
r m
f
nf
p
pf Hz n rpm
f Hzn rpm
p
2m r
pt
m mt
3
13
a-axis
b-axis
c-axis
m
a a
b
b c
c
d-axisq-axis
fi
14 15
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
16
• The turbine with a torque (Tm) rotates the
rotor shaft and thereby the field winding on
the rotor (which carries a direct current)
produces a rotating flux in the air gap.
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
17
• When the generator is loaded, currents
flowing in the stator windings also produce
rotating flux in the air gap.
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
18
• Then, the resultant (or combined) flux
across the air gap provides an electro-
magnetic torque (Te) which opposes the
torque of the turbine (Tm).
Rotor
Stator
Stator
mmT eT
Shaft
mP
Generator
Mech.
parts
Power
System
Elec.
devices
4
19
m e
dM P P
dt
20
Modelling
SGI
U
Power System
SG
djxqE
IPower System
U
21
jq q
q
d
E E e
E UI
jx
*3Re
3 sin cos
e e
e e e q
q qq
d d
P Q
S P jQ E I
E U Ej E U
x x
22
3 cosq
e qd
EQ E U
x
a-axis
b-axis
c-axis
m
a a
b
b c
c
d-axisq-axis
fi
23
Induction Machines (IM)
as-axis
sai
ar-axis
sci
sbirai
rbi
rci
24
Squirrel-Cage IM (SCIM)
as-axis
sai
ar-axis
sci
sbi
5
25 26
Squirrel-cage IM
27
1r ss
s r
s
s
28
Modelling (steady-state)
U
sjX sIsR rjX rIrR
mjX1
r
sR
s
agP
shP
2 213 3
loss
sh
ag sh loss
r r r r
PP
P P P
sR I R I
s
29
Modelling (steady-state)
U
sjX sIsR rjX rIrR
mjX1
r
sR
s
agP
shP
213sh r r
sP R I
s
2 2
1
sh sh she
mr s
P P PT
sp p
30
Modelling (steady-state)
U
sjX sIsR rjX rIrR
mjX1
r
sR
s
agP
shP
213sh r r
sP R I
s
232 2
1
sh re r
ss
P p RT I
ss
p
6
31
Modelling (steady-state)
U
sjX sIsR rjX rIrR
mjX1
r
sR
s
agP
shP
U
sjX sIsR rjX rI
mjXrR
s
ThU
ThjXThR rjX rI
rR
s
agP ??
??
??
Th
Th
Th
U
R
X
32
Modelling (steady-state)
rI
232
re r
s
p RT I
s
ThU
ThjXThR rjX rI
rR
s
agP
/
Th
Th r Th r
U
R R s j X X
33
Modelling (steady-state)
2
2
2 2
32
32 /
re r
s
Thr
s Th r Th r
p RT I
s
Up R
s R R s X X
ThU
ThjXThR rjX rI
rR
s
agP
U
sjX sIsR rjX rIrR
mjX1
r
sR
s
agP
shP
s r
s
s
2
2
2 2
32
32 /
re r
s
Thr
s Th r Th r
p RT I
s
Up R
s R R s X X
100 1sh
ag
Ps
P
max
max
22
max
0
???
e
ee
rT
Th Th r
e
dTT
ds
Rs
R X X
T
as-axis
sai
ar-axis
sci
sbirai
rbi
rci36
Wound-rotor IM
as-axis
sai
ar-axis
sci
sbirai
rbi
rci
External
Electrical
Devices
7
Comparisons (wind power applications)
In general
• +: IM is cheap, reliable, and readily
available in a wide range of electrical sizes.
• +: Construction of IM is simple and robust.
• +: SCIM needs little maintenance.
• -: SCIM takes reactive power from PS.
37
Comparisons (wind power applications)
• +: SM is more efficient (at least the large
ones).
• +: SM can more easily be controlled to
keep the system frequency constant.
• +: SM can supply its own reactive power,
and thereby it is able to control its terminal
voltage.
• -: More complicated construction, and
needs more maintenance.
38 39
Wind
speedMech.
parts ~ Power
System
Elec.
devices
P
Rotor speed
2 31
2w r wP r v
m p wP C P
( , )pC f
r r
w
r
v
40
P
Rotor speed
P
Wind speed41
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
TYPE 1
• Gearbox: wind turbine 40 to 400 (rpm),
generator 1000 to 1500 (rpm)
• Use squirrel-cage induction machines
directly connected to the power grid, and
operate with less than 1% variation in rotor
speed (fixed-speed wind turbines)
42
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
TYPE 1
(1 )
0 0.01
r ss
s
P
Rotor speed
8
43
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
TYPE 1
• To keep fixed speed: gear of the gearbox,
the number of poles of the electrical
generator, and blade control.
• They are simple, robust, reliable and
relatively cheap.
44
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
TYPE 1
• But they consume reactive power, have high
mechanical stresses and limited controls on
the output power (fed into PS).
• Variations of wind speed lead to a change in
the mechanical torque, which results in a
fluctuation of the output power.
45
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
TYPE 1
46 47
TYPE 2
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
• A wound-rotor whose rotor windings are
connected to thyristor-controlled variable
resistance.
• Below the rated wind speed, its behavior is
the same as TYPE1.
48
TYPE 2
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
• Above the rated wind speed, the resistance
is controlled to maintain the output power
constant (normally to rated power).
• This control allows a “variable” speed
operation of up to 10%, i.e. 0 0.1s
9
49 50 51
TYPE 2
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
• Application of variable resistance results in
developing heat in the rotor which must be
dissipated. Therefore, in practice pitch
control is used to minimize the heat
generated.
52
TYPE 2
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
• Compared to TYPE1, TYPE 2 is more
efficient to capture wind power, and gives
rise to less output power and voltage
fluctuations. However, it requires more
maintenance due to the slip rings and
brushes.
• It also consumes reactive power. 53
TYPE 2
Rotor
Stator
mmT eT
mP
Power
System
Stator Cap
IM
54
10
55
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
• It is also referred to as Doubly-Fed Induction
Generators (DFIG).
• The rotor windings and the power system are
connected through a back-to-back VSC.
• This configuration allows not to lose the
power dissipated as heat in TYPE 2. 56
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
• A VSC is able to control active and reactive
power in both directions, independently.
• VSC1 modulates the rotating magnetic field
to control the rotating speed of the rotor. It is
also able to control the turbine reactive
power or the terminal voltage.
57
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
• The rating of VSCs is normally at around
25–35% of the generator rating.
• Then, it is possible to obtain 25-35% speed
variation above or below the synchronism
speed by VSC1.
58
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
• Another advantage is that the mechanical
drive train is largely decoupled from the
electrical system via VSCs. Thus, the wind
speed variations do not have a pronounced
impact on PS.
59
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
U
sjX sIsR rjX rI
mjX
rR
s
rU
s
60
TYPE 3
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
• The objective of VSC2 is to supply or absorb
the active power exchange of VSC1 by
controlling the DC voltage over the DC
capacitor.
• The reactive power exchange of VSC2 is
normally set to zero (reactive power control).
11
61
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2IM
62 63
TYPE 4
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2
SM
• SM with filed winding or permanent magnet
(PM).
• With low-speed conversion and a more
number of poles, the gearbox can be
removed which implies less mechanical
losses and higher reliability.
64
TYPE 4
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2
SM
• The back-to-back VSC completely decouples
the generator and PS which allows allows
operation of the generator at any speed to
maximum rated speed.
• However, the VSCs have the same rating as
the generator.
65
TYPE 4
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2
SM
66
TYPE 4
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2
SM
• With PM there is no need of slip-rings and
brushes. It also reduces the excitation losses
and the size of the generating unit.
12
67
Rotor
Stator
mmT eT
mP
Power
System
Stator
VSC1 VSC2
SM
Comparisons
68
TYPE 1 TYPE 2 TYPE 3 TYPE 4
Speed range E D C A
Reactive power E E C B
Initial cost A B C D
Maintenance cost A C C B
Numbers installed D D B C
A Excellent
B Very Good
C Good
D Moderate
E Poor
69
Power system stability
• Rotor angle stability
• Voltage stability
• Frequency stability
70
Transient
stability
Malin - Round Mountain MW Flow
2300
2400
2500
2600
2700
2800
2900
3000
0 3 6 9 12 16 19 22 25 28 31 34 37 40 43 47 50 53 56 59 62 65 68 71 74
Time in Seconds
71
POD
WSCC August 10, 1996
Disturbance
72
• Generators in Southern Finland oscillating
against those of Southern Sweden and
Norway (frequency 0.3 Hz)
• Limits the transmission capacity from
Finland to Sweden
13
73
Voltage
stability
74 75
76 77 78
