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
• 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
USSpainDenmarkIndia
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
• 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
synchronous.• Rotor converter rating is only around 25% of total generator rating.• Reactive power control.
Wind Generator Topologies
• 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
• 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 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
• Rotor control allows for speed and reactive power control. (Cage IG are fixed.)
stator
rotor
grid
AC
DC
DC
AC
DFIG
DC link
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
*2 2Re' '' 'r rr r r
s r r
V IR R PP I Is s s s
2 2Im r rs s rs
m m
V IV V QQX s X s
0.2 0.2s
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
• Therefore, compensate for the second harmonic in the dq system0 1 2 3 4 5 6
0.8
0.9
1
1.1
1.2
x
1+0.2 sin(2 x-30 /180)
balanced unbalanced
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 lead-lag filter
0, , , , , 2 2
0 0
11
filt zd comp d comp bp d comp ll
filt p
s Q sC C C ks s Q s
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)
0 0.2 0.4 0.6 0.8 1 1.2
-1.5
-1
-0.5
torq
ue (p
er u
nit)
time (seconds)
Generator Torque
0 0.2 0.4 0.6 0.8 1 1.20
0.1
0.2
0.3
0.4
torq
ue (p
er u
nit)
time (seconds)
Generator Torque 100 Hz Magnitude
0 0.2 0.4 0.6 0.8 1 1.2
-0.2
-0.1
0
0.1
0.2
reac
tive
pow
er (p
er u
nit)
time (seconds)
Generator Stator Reactive Power
0 0.2 0.4 0.6 0.8 1 1.20
0.1
0.2
reac
tive
pow
er (p
er u
nit)
time (seconds)
Generator Stator Reactive Power 100 Hz Magnitude
• Transient activation of compensation• VUF = 0.04
Hardware Results (15 kW)
0 0.2 0.4 0.6 0.8 1 1.2
-1
-0.5
0
activ
e po
wer
(per
uni
t)
t ime (seconds)
Generator Stator and Rotor Active Power
statorrotor
0 0.2 0.4 0.6 0.8 1 1.2
-1
-0.5
0
activ
e po
wer
(per
uni
t)
t ime (seconds)
Generator Total Active Power
total
0 0.2 0.4 0.6 0.8 1 1.2-0.2
-0.1
0
0.1
0.2
volta
ge (p
er u
nit)
time (seconds)
Rotor d-Axis Voltage
0 0.2 0.4 0.6 0.8 1 1.2-0.2
-0.1
0
0.1
0.2
volta
ge (p
er u
nit)
time (seconds)
Rotor q-Axis Voltage
0 0.2 0.4 0.6 0.8 1 1.2
-1
0
1
curre
nt (p
er u
nit)
t ime (seconds)
Stator Current
isaisbisc
0 0.2 0.4 0.6 0.8 1 1.20.6
0.8
1
curre
nt (p
er u
nit)
t ime (seconds)
Stator Current 50 Hz Magnitude
isaisbisc
0 0.2 0.4 0.6 0.8 1 1.2
0.05
0.1
0.15
0.2
0.25
0.3
time (seconds)
unba
lanc
e fa
ctor
Stator Voltage and Current Unbalance Factor
VUFIUF
Hardware Results (15 kW)• Steady
state
0 0.01 0.02 0.03 0.04 0.05 0.060
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
y=9.3e+000*x+0.01 y=6.8e+000*x-0.00
y=3.2e-001*x+0.02
y=5.9e-001*x-0.00
stator voltage unbalance factor (VUF)
torq
ue (p
er u
nit)
Torque 100 Hz Component
no comp (hardware)w/comp (hardware)no comp (simulation)w/comp (simulation)
0 0.01 0.02 0.03 0.04 0.05 0.060
0.05
0.1
0.15
0.2
0.25
y=6.6e+000*x-0.01
y=6.2e+000*x-0.00
y=2.9e-001*x+0.00
y=3.5e-001*x-0.00
stator voltage unbalance factor (VUF)
reac
tive
pow
er (p
er u
nit)
Stator Reactive Power 100 Hz Component
no comp (hardware)w/comp (hardware)no comp (simulation)w/comp (simulation)
0 0.01 0.02 0.03 0.04 0.05 0.060
0.05
0.1
0.15
0.2
0.25
y=7.1e+000*x-0.01 y=6.1e+000*x-0.00
y=1.3e+000*x+0.02
y=8.2e-001*x-0.00
stator voltage unbalance factor (VUF)
unba
lanc
e fa
ctor
Stator Current Unbalance Factor (IUF)
no comp (hardware)w/comp (hardware)no comp (simulation)w/comp (simulation)
Reduction, Simulation:Torque -> 11.5Qs -> 17.7IUF -> 7.4
Reduction, Hardware:Torque -> 29.1Qs -> 22.8IUF -> 5.5
Thank You!
Questions?