A Novel Control Scheme for a
Doubly-Fed Induction Wind
Generator
Under Unbalanced Grid
Voltage Conditions
Ted Brekken, Ph.D.
Assistant Professor in Energy Systems
Oregon State University
Outline
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Wind Energy Overview
Research Objectives
DFIG Overview
DFIG Control
Unbalance and Induction Machines
DFIG Unbalance Compensation
Hardware Results
Global Wind Energy
• Almost 12 GW added between 2004 and
2005.
Source: Global Wind Energy Outlook 2006, Global Wind Energy Council
New Installations - 2005
• Most of new installations continue to be in
US and Europe.
Source: Global Wind Energy Outlook 2006, Global Wind Energy Council
Wind Energy Overview
• Germany
US
Spain
Denmark
India
US Installed Projects
• Because of slow Midwest growth, the US still has
huge potential.
Source: American Wind Energy Association, www.awea.org/projects
Wind Energy Overview
• Wind generators and farms are getting larger.
• 5 MW wind generators are now available with 7 MW in the
works.
(graphic from Vestas.com)
Wind Generator Topologies
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Direct connected.
Simplest.
Requires switch to prevent motoring.
Draws reactive power with no reactive control.
Wind Generator Topologies
• Doubly-fed.
• The doubly-fed topology is the most common for high power.
• Rotor control allows for speed control of around 25% of
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synchronous.
Rotor converter rating is only around 25% of total generator rating.
Reactive power control.
Wind Generator Topologies
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Full-rated converter connected.
Lower cost generator than DFIG. Lower maintenance.
Converter must be full-rated.
Full-rated converter allows for complete speed and reactive power control.
Could also be used with a synchronous generator.
Wind Generator Topologies
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Direct-drive.
Eliminate the gearbox by using a very-high pole synchronous generator.
Resulting generator design is relatively wide and flat.
No gearbox issues.
Full-rated converter is required.
Full speed and reactive power control.
Wind Energy Issues
• Wind is intermittent
– Limits wind’s percentage of the energy mix
• Wind energy is often located in rural areas
– Rural grids are often weak and unstable, and prone to
voltage sags, faults, and unbalances
• Unbalanced grid voltages cause many problems
for induction generators
– Torque pulsations
– Reactive power pulsations
– Unbalanced currents
Outline
• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results
Research Objectives
• Research was carried out from 2002 to 2005 at the U of
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M and at NTNU in Trondheim, Norway on a Fulbright
scholarship
Doubly-fed induction generators are the machines of
choice for large wind turbines
The objective is to develop a control methodology for a
DFIG that can achieve:
– Variable speed and reactive power control
– Compensation of problems caused by an unbalanced grid
• Reduce torque pulsations
• Reduce reactive power pulsations
• Balance stator currents
Outline
• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results
DFIG Overview - Topology
stator
grid
DFIG
rotor
DC link
AC
DC
DC
AC
• Rotor control allows for speed and reactive power
control. (Cage IG are fixed.)
DFIG Overview – Variable Speed
Control
• Higher Cp means
more energy
captured
• Maintain tip-speed
ratio at nominal
value
(graphic from Mathworks)
DFIG Overview – Reactive Power
Control
*
Rr ' Re Vr I r 
P
2 R '
Ps  I r '

 Ir ' r  r
s
s
s
s
2
Qs 
Vs
2
Xm

Im Vr I r 
0.2  s  0.2
s

Vs
2
Xm

Qr
s
Outline
•
•
•
•
•
•
•
•
Wind Energy Overview
Research Objectives
DFIG Overview
DFIG Control
Unbalance and Induction Machines
DFIG Unbalance Compensation
Simulation Results
Hardware Results
DFIG Control
• Control is done by transforming three-phase to
two-phase
DFIG Control – Machine Flux
Oriented
• q-axis controls reactive power (flux)
• d-axis controls torque
DFIG Control – Grid Flux Oriented
• Align d-axis with
voltage, instead of
flux
• Easier, more stable
• d-axis -> torque
• q-axis -> reactive
power (Qs)
DFIG Control
• d-axis controls torque, hence speed
DFIG Control
• q-axis controls reactive power (Qs)
DFIG Control – Stability
• DFIGs naturally
have complex
poles near the
RHP, near the
grid frequency
(ird/vrd transfer function)
Outline
• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results
3 Phase Voltage Unbalance
• Causes torque puslations, reactive power
pulsations, unbalanced currents, possible
over heating
• Unbalance can be seen as the addition of
a negative sequence
• Unbalance factor (VUF, IUF) is the
magnitude of the negative sequence over
the magnitude of the positive sequence
Unbalance – Second Harmonic
balanced
unbalanced
• Therefore,
1+0.2 sin(2 x-30 /180)
1.2
1.1
1
0.9
0.8
0
1
2
3
4
x
5
6
compensate for the
second harmonic in
the dq system
Outline
• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results
Unbalance Compensation
• Intentionally injecting a disturbance with an auxiliary
controller to drive the disturbance to zero
d-axis Inner Loop
• Compensation controller looks like a bandpass and leadlag filter

 s  z  1 
s 0 Q filt
Cd ,comp  Cd ,comp ,bp Cd ,comp ,ll  k  2
 s  s  Q   2 
 s   1 
0
0
filt
p



Compensation Controller Design
(Cd,comp)
(d-axis loop gain)
Outline
• Wind Energy Overview
• Research Objectives
• DFIG Overview
• DFIG Control
• Unbalance and Induction Machines
• DFIG Unbalance Compensation
• Hardware Results
Hardware Pictures
Hardware Results (15 kW)
• Transient activation of compensation
• VUF = 0.04
Generator Stator Reactive Power
reactive power (per unit)
Generator Torque
torque (per unit)
-0.5
-1
-1.5
0.2
0.4
0.6
0.8
time (seconds)
Generator Torque 100 Hz Magnitude
1
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
time (seconds)
0.8
1
0.1
0
-0.1
-0.2
0
1.2
reactive power (per unit)
torque (per unit)
0
0.2
1.2
0.2
0.4
0.6
0.8
1
time (seconds)
Generator Stator Reactive Power 100 Hz Magnitude
1.2
0.2
0.1
0
0
0.2
0.4
0.6
time (seconds)
0.8
1
1.2
Hardware Results (15 kW)
Stator Voltage and Current Unbalance Factor
0.3
stator
rotor
0
0.25
-0.5
-1
0
active power (per unit)
VUF
IUF
0.2
0.4
0.6
0.8
time (seconds)
Generator Total Active Power
1
1.2
total
0
unbalance factor
active power (per unit)
Generator Stator and Rotor Active Power
0.2
0.15
0.1
-0.5
-1
0.05
0
0.2
0.4
0.6
time (seconds)
0.8
1
1.2
0
0.2
0.4
0.6
time (seconds)
0.8
1
1.2
Rotor d-Axis Voltage
Stator Current
1
voltage (per unit)
current (per unit)
0.2
isa
isb
isc
0
-1
0
0.2
0.4
0.6
0.8
time (seconds)
Stator Current 50 Hz Magnitude
1
0.1
0
-0.1
-0.2
1.2
0
0.2
0.4
0.6
time (seconds)
Rotor q-Axis Voltage
0.8
1
1.2
0
0.2
0.4
0.6
time (seconds)
0.8
1
1.2
1
voltage (per unit)
current (per unit)
0.2
isa
isb
isc
0.8
0.6
0
0.2
0.4
0.6
time (seconds)
0.8
1
1.2
0.1
0
-0.1
-0.2
Hardware Results (15 kW)
• Steady
Stator Reactive Power 100 Hz Component
no comp (hardware)
w/comp (hardware)
no comp (simulation)
w/comp (simulation)
torque (per unit)
0.3
0.25
y=9.3e+000*x+0.01
no comp (hardware)
w/comp (hardware)
no comp (simulation)
w/comp (simulation)
0.25
reactive power (per unit)
0.35
y=6.8e+000*x-0.00
0.2
0.15
0.2
y=6.2e+000*x-0.00
y=6.6e+000*x-0.01
0.15
0.1
0.1
0.05
0.05
y=3.2e-001*x+0.02
y=2.9e-001*x+0.00
y=5.9e-001*x-0.00
0
0
0.01
0.02
0.03
0.04
stator voltage unbalance factor (VUF)
0.05
0.06
0
0
0.01
y=3.5e-001*x-0.00
0.02
0.03
0.04
0.05
0.06
stator voltage unbalance factor (VUF)
Stator Current Unbalance Factor (IUF)
Reduction, Simulation:
Torque -> 11.5
Qs -> 17.7
IUF -> 7.4
no comp (hardware)
w/comp (hardware)
no comp (simulation)
w/comp (simulation)
0.25
0.2
unbalance factor
state
Torque 100 Hz Component
0.4
y=7.1e+000*x-0.01
y=6.1e+000*x-0.00
0.15
0.1
Reduction, Hardware:
Torque -> 29.1
Qs -> 22.8
IUF -> 5.5
y=1.3e+000*x+0.02
0.05
y=8.2e-001*x-0.00
0
0
0.01
0.02
0.03
0.04
stator voltage unbalance factor (VUF)
0.05
0.06
Thank You!
Questions?