A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage...
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Transcript of A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage...
A Novel Control Scheme A Novel Control Scheme for a Doubly-Fed for a Doubly-Fed
Induction Wind Generator Induction Wind Generator Under Unbalanced Grid Under Unbalanced Grid
Voltage ConditionsVoltage Conditions
Ted Brekken, Ph.D.Ted Brekken, Ph.D.
Assistant Professor in Energy SystemsAssistant Professor in Energy Systems
Oregon State UniversityOregon State University
OutlineOutline
• Wind Energy OverviewWind Energy Overview• Research ObjectivesResearch Objectives• DFIG OverviewDFIG Overview• DFIG ControlDFIG Control• Unbalance and Induction MachinesUnbalance and Induction Machines• DFIG Unbalance CompensationDFIG Unbalance Compensation• Hardware ResultsHardware Results
Global Wind EnergyGlobal Wind Energy
• Almost 12 GW added between 2004 Almost 12 GW added between 2004 and 2005.and 2005.
Source: Global Wind Energy Outlook 2006, Global Wind Energy Council
New Installations - 2005New Installations - 2005
• Most of new installations continue to Most of new installations continue to be in US and Europe.be in US and Europe.
Source: Global Wind Energy Outlook 2006, Global Wind Energy Council
Wind Energy OverviewWind Energy Overview• GermanyGermany
USUSSpainSpainDenmarkDenmarkIndiaIndia
US Installed ProjectsUS Installed Projects
• Because of slow Midwest growth, the US Because of slow Midwest growth, the US still has huge potential.still has huge potential.
Source: American Wind Energy Association, www.awea.org/projects
Wind Energy OverviewWind Energy Overview• Wind generators and farms are getting larger.Wind generators and farms are getting larger.• 5 MW wind generators are now available with 7 MW in the 5 MW wind generators are now available with 7 MW in the
works.works.
(graphic from Vestas.com)
Wind Generator TopologiesWind Generator Topologies
• Direct connected.Direct connected.• Simplest.Simplest.• Requires switch to prevent motoring.Requires switch to prevent motoring.• Draws reactive power with no reactive control.Draws reactive power with no reactive control.
Wind Generator TopologiesWind Generator Topologies
• Doubly-fed.Doubly-fed.• The doubly-fed topology is the most common for high power.The doubly-fed topology is the most common for high power.• Rotor control allows for speed control of around 25% of synchronous.Rotor control allows for speed control of around 25% of synchronous.• Rotor converter rating is only around 25% of total generator rating.Rotor converter rating is only around 25% of total generator rating.• Reactive power control.Reactive power control.
Wind Generator TopologiesWind Generator Topologies
• Full-rated converter connected.Full-rated converter connected.• Lower cost generator than DFIG. Lower maintenance.Lower cost generator than DFIG. Lower maintenance.• Converter must be full-rated.Converter must be full-rated.• Full-rated converter allows for complete speed and reactive power Full-rated converter allows for complete speed and reactive power
control.control.• Could also be used with a synchronous generator.Could also be used with a synchronous generator.
Wind Generator TopologiesWind Generator Topologies
• Direct-drive.Direct-drive.• Eliminate the gearbox by using a very-high pole synchronous generator.Eliminate the gearbox by using a very-high pole synchronous generator.• Resulting generator design is relatively wide and flat.Resulting generator design is relatively wide and flat.• No gearbox issues.No gearbox issues.• Full-rated converter is required.Full-rated converter is required.• Full speed and reactive power control.Full speed and reactive power control.
Wind Energy IssuesWind Energy Issues
• Wind is intermittentWind is intermittent– Limits wind’s percentage of the energy mixLimits wind’s percentage of the energy mix
• Wind energy is often located in rural areasWind energy is often located in rural areas– Rural grids are often weak and unstable, and Rural grids are often weak and unstable, and
prone to voltage sags, faults, and unbalancesprone to voltage sags, faults, and unbalances• Unbalanced grid voltages cause many Unbalanced grid voltages cause many
problems for induction generatorsproblems for induction generators– Torque pulsationsTorque pulsations– Reactive power pulsationsReactive power pulsations– Unbalanced currentsUnbalanced currents
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Hardware ResultsHardware Results
Research ObjectivesResearch Objectives
• Research was carried out from 2002 to 2005 at Research was carried out from 2002 to 2005 at the U of M and at NTNU in Trondheim, Norway on the U of M and at NTNU in Trondheim, Norway on a Fulbright scholarshipa Fulbright scholarship
• Doubly-fed induction generators are the Doubly-fed induction generators are the machines of choice for large wind turbinesmachines of choice for large wind turbines
• The objective is to develop a control methodology The objective is to develop a control methodology for a DFIG that can achieve:for a DFIG that can achieve:– Variable speed and reactive power controlVariable speed and reactive power control– Compensation of problems caused by an unbalanced Compensation of problems caused by an unbalanced
gridgrid• Reduce torque pulsationsReduce torque pulsations• Reduce reactive power pulsationsReduce reactive power pulsations• Balance stator currentsBalance stator currents
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Hardware ResultsHardware Results
DFIG Overview - TopologyDFIG Overview - Topology
• Rotor control allows for speed and reactive Rotor control allows for speed and reactive power control. (Cage IG are fixed.)power control. (Cage IG are fixed.)
stator
rotor
grid
AC
DC
DC
AC
DFIG
DC link
DFIG Overview – Variable DFIG Overview – Variable Speed ControlSpeed Control
• Higher CHigher Cpp means more means more energy energy capturedcaptured
• Maintain tip-Maintain tip-speed ratio at speed ratio at nominal valuenominal value
(graphic from Mathworks)
DFIG Overview – Reactive DFIG Overview – Reactive Power Control Power Control
*2 2Re' '
' 'r rr r r
s r r
V IR R PP I I
s s s s
2 2Im r rs s r
sm m
V IV V QQ
X s X s
0.2 0.2s
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Simulation ResultsSimulation Results
• Hardware ResultsHardware Results
DFIG ControlDFIG Control
• Control is done by transforming three-Control is done by transforming three-phase to two-phasephase to two-phase
DFIG Control – Machine Flux DFIG Control – Machine Flux OrientedOriented
• q-axis controls reactive power (flux)q-axis controls reactive power (flux)
• d-axis controls torqued-axis controls torque
DFIG Control – Grid Flux DFIG Control – Grid Flux OrientedOriented
• Align d-axis with Align d-axis with voltage, instead of voltage, instead of fluxflux
• Easier, more Easier, more stablestable
• d-axis -> torqued-axis -> torque
• q-axis -> reactive q-axis -> reactive power (Qpower (Qss))
DFIG ControlDFIG Control
• d-axis controls torque, hence speedd-axis controls torque, hence speed
DFIG ControlDFIG Control
• q-axis controls reactive power (Qq-axis controls reactive power (Qss))
DFIG Control – StabilityDFIG Control – Stability
• DFIGs DFIGs naturally naturally have complex have complex poles near poles near the RHP, near the RHP, near the grid the grid frequencyfrequency
(ird/vrd transfer function)
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Hardware ResultsHardware Results
3 Phase Voltage Unbalance3 Phase Voltage Unbalance
• Causes torque puslations, reactive Causes torque puslations, reactive power pulsations, unbalanced power pulsations, unbalanced currents, possible over heatingcurrents, possible over heating
• Unbalance can be seen as the Unbalance can be seen as the addition of a negative sequenceaddition of a negative sequence
• Unbalance factor (VUF, IUF) is the Unbalance factor (VUF, IUF) is the magnitude of the negative sequence magnitude of the negative sequence over the magnitude of the positive over the magnitude of the positive sequencesequence
Unbalance – Second Unbalance – Second HarmonicHarmonic
• Therefore, Therefore, compensate for the compensate for the second harmonic in second harmonic in the dq systemthe 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
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Hardware ResultsHardware Results
Unbalance CompensationUnbalance Compensation
• Intentionally injecting a disturbance with an Intentionally injecting a disturbance with an auxiliary controller to drive the disturbance to zeroauxiliary controller to drive the disturbance to zero
d-axis Inner Loopd-axis Inner Loop
• Compensation controller looks like a bandpass Compensation controller looks like a bandpass and lead-lag filterand lead-lag filter
0, , , , , 2 2
0 0
1
1filt z
d comp d comp bp d comp llfilt p
s Q sC C C k
s s Q s
Compensation Controller Compensation Controller DesignDesign
-40
-20
0
20
40From: In(1) To: ird
Mag
nitu
de (
dB)
100
101
102
103
104
-135
-90
-45
0
45
90
Pha
se (
deg)
Bode Diagram
Frequency (Hz)
w ith comp
w ithout comp
-80
-60
-40
-20
0
20
40
Mag
nitu
de (
dB)
100
101
102
103
104
-90
0
90
180
Pha
se (
deg)
Bode Diagram
Frequency (Hz)
(Cd,comp) (d-axis loop gain)
OutlineOutline
• Wind Energy OverviewWind Energy Overview
• Research ObjectivesResearch Objectives
• DFIG OverviewDFIG Overview
• DFIG ControlDFIG Control
• Unbalance and Induction MachinesUnbalance and Induction Machines
• DFIG Unbalance CompensationDFIG Unbalance Compensation
• Hardware ResultsHardware Results
Hardware PicturesHardware Pictures
Hardware Results (15 kW)Hardware Results (15 kW)
0 0.2 0.4 0.6 0.8 1 1.2
-1.5
-1
-0.5
torq
ue (
per
unit)
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 (
per
unit)
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 (
per
unit)
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 (
per
unit)
time (seconds)
Generator Stator Reactive Power 100 Hz Magnitude
• Transient activation of compensationTransient activation of compensation
• VUF = 0.04VUF = 0.04
Hardware Results (15 kW)Hardware Results (15 kW)
0 0.2 0.4 0.6 0.8 1 1.2
-1
-0.5
0
activ
e po
wer
(pe
r un
it)
time (seconds)
Generator Stator and Rotor Active Power
stator
rotor
0 0.2 0.4 0.6 0.8 1 1.2
-1
-0.5
0
activ
e po
wer
(pe
r un
it)
time (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 (
per
unit)
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 (
per
unit)
time (seconds)
Rotor q-Axis Voltage
0 0.2 0.4 0.6 0.8 1 1.2
-1
0
1
curr
ent
(per
uni
t)
time (seconds)
Stator Current
isa
isb
isc
0 0.2 0.4 0.6 0.8 1 1.20.6
0.8
1
curr
ent
(per
uni
t)
time (seconds)
Stator Current 50 Hz Magnitude
isa
isb
isc
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
VUF
IUF
Hardware Results (15 kW)Hardware Results (15 kW)
• SteadSteady y statestate
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+0.01y=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 (
per
unit)
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 (
per
unit)
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!Thank You!
Questions?Questions?