a novel control scheme for a doubly-fed induction wind...

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?