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.


• 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.


• 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.


• 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








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







• Simulation ResultsSimulation 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))


• d-axis controls torque, hence speedd-axis controls torque, hence speed


• 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








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








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








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


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


• 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


reac

tive

pow

er (

per

unit)

Stator Reactive Power 100 Hz Component

no comp (hardware)


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


unba

lanc

e fa

ctor

Stator Current Unbalance Factor (IUF)

no comp (hardware)


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?

A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage...

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