Voltage grid support of DFIG wind turbines during grid faults

Gabriele MichalkeUniversity of Technology Darmstadt, Germany

Anca D. Hansen Risø National Laboratory, Denmark

EWEC Milan 7-10 May 2007

2

Outline

• Background

• DFIG wind turbine – modelling, control issues in case of grid faults:

• Drive train and pitch control system• DFIG system control and protection

• DFIG wind turbine – voltage grid support control

• Power transmission system test model

• Case study - simulation results

• Conclusions

3

Background

• Projects:• Ph.D project ”Variable Speed Wind Turbines - Modelling, Control and Impact on

Power Systems” funded by ”Stiftung Energieforschung Baden-Württemberg”• ”Simulation platform to model, optimise and design wind turbines” – funded by

Danish Energy Agency

• Participants:• Darmstadt Technical University• Risø National Laboratory• Aalborg Technical University

• Overall goal: • Wind farms interaction with the power system during grid faults• Advanced control design of wind farms according to the new grid codes

• Focus in this presentation: • Voltage grid support of DFIG wind turbines during grid faults

4

DFIG wind turbine – modelling, control issues in case of grid faults:

Crowbar

DFIG

~=

~ ~ ~

Power converter control

refQrefP

Control mode :• normal operation• fault operation

~=

RSC GSC

Fault detection

Drive train with gearbox

Wind turbine

Pitch angle control

DFIG system – control and protection

Aerodynamics

k

c

5

Drive train and pitch control system

ngear

J rot

Trot

J gen

Tgenc

k

• 2 mass mechanical model

eqosc J

kf

2

1

gengearrot

gengearroteq JnJ

JnJJ

2

2

Free – free frequency:

Equivalent inertia:

• Pitch control system

+

-

ref

ref

Gain schedullingKPI

+

- servoT

1

dt

d

s

1PI

Pitch angle controls the speed

Prevent over-speed both in: - normal operations - grid faults operations

Rate of change limitation important during grid faults

6

DFIG system control (normal operation)

ACDC AC

DC

RSC GSC

Power converter

PI PI PI PI dcU

GSCQ

DCrefUGSC

refQ

gridP

gridQ

gridrefP

gridrefQ

Slow control (power)

Power converter control • RSC controls Pgrid and Qgrid independently!• GSC controls UDC and QGSC=0 !

Reference signals:• Active power for RSC is defined by

MPT:

P

MPT

Maximum power tracking point

gridrefP

gridrefPPI PI PI

RSCqrefI RSC

drefI GSCqrefI GSC

drefI

RSCdIRSC

qI GSCdI

GSCqI

Fast control (current)

PI

• DC voltage is set to constant value

• GSC is reactive neutral

• Reactive power for RSC - certain

value or zero

7

DFIG system control and protection during grid faults

Power converter is very sensitive to grid faults !!!• Protection system monitors DFIG signals • Crowbar protection:

external rotor impedance

Severe grid faults triggers crowbar:• RSC disabled• DFIG behaves as SCIG• GSC can be used as a STATCOM

Damping controller

-1 -0.5 0 0.5 1 1.5 2 2.5 3-3

-2

-1

0

1

2

3

Speed [p.u.]

Ele

ctro

ma

gn

etic

to

rqu

e [

p.u

.] crowbarcrowbarcrowbar RRR 321

• Increased crowbar: improved dynamic stability of the generator reduces reactive power demand

New grid codes require:• Fault ride-through capability: wind turbine has to remain connected to the grid during grid faults

-1 -0.5 0 0.5 1 1.5 2 2.5 3-25

-20

-15

-10

-5

0

Speed [p.u.]

Re

act

ive

po

we

r [M

var]

. . . .

crowbarcrowbarcrowbar RRR 321

8

Fault Ride Through – Damping of Torsional oscillations during grid faults

10.007.505.002.500.00 [s]

1.150

1.125

1.100

1.075

1.050

1.025

10.007.505.002.500.00 [s]

3.0E+4

2.0E+4

1.0E+4

0.0E+0

-1.0E+4

DIgS

ILENT

Generator speed [pu]

Without damping controller With damping controller

Mechanical torque [Nm]

[sec]

During grid faults:• Unbalance between the torques, which act at the ends of the drive train• Drive train acts like a torsion spring

that gets untwisted• Torsional oscillations excited in the drive train

Wind speed

ref

gridrefP

-+

Damping controller

PIOptimal speed

Damping controller:• designed and tuned to damp torsional oscillations• provides active power reference for RSC control

9

DFIG wind turbine – voltage grid support control

Third stage (voltage grid support)

DFIG control structure – normal operation

GSCrefQgrid

refPgridrefQ

• During grid faults DFIG controllability is enhanced by a proper co-ordination of three controllers:

co-ordination GSC Reactive Power Boosting

GSC reactive power boostingDampingController

Damping controller

RSC Voltage Controller

RSC voltage controller

• Damping controller damps actively the torsional oscillations of the drive train system during grid faults

gridrefacU ,

gridacU

gridrefQ+

-

RSC voltage controller

PI

• RSC voltage controller controls grid voltage as long as the protection device is not triggered

• GSC reactive power boosting controls grid voltage when RSC is blocked by the protection device

10

Power transmission system test model Power transmission system model:

• delivered by the Danish Transmission System Operator Energinet.dk

• contains: busbars 0.7kV to 400kV 4 conventional power plants consumption centres lumped on-land local wind turbine 165 MW offshore active stall wind farm:

one machine modelling approach equipped with active power reduction control for fault ride-through

LL

L

400 kV

Lin

e 1

Lin

e 2

Lin

e 4

Lin

e 3

Off

shor

e lin

e

Active stall wind farm

Localwind turbines

400 kV

135 kV

135 kV

135 kV

135 kV

WFT

SG SG

SG

SG

Simulated fault event

Extended for the case study with:

• 160 MW offshore DFIG wind farm: connected to 135kV busbar modelled by one machine approach equipped with fault ride-through and voltage grid support controller

Damping controller RSC voltage controller GSC reactive power boosting controller

WFT

Off

shor

e lin

e

DFIG wind farm

New added wind farmfor the case study

11

LL

L

400 kV

Lin

e 1

Lin

e 2

Lin

e 4

Lin

e 3

Off

shor

e lin

e

Active stallw ind farm

Localwind turbines

400 kV

135 kV

135 kV

135 kV

135 kV

WFT

SG SG

SG

SG

Off

shor

e lin

e

DFIGwind farm

WFT

New added wind farmfor the case study

LLLLLL

LLL

400 kV

Lin

e 1

Lin

e 2

Lin

e 4

Lin

e 3

Off

shor

e lin

e

Active stallw ind farm

Localwind turbines

400 kV

135 kV

135 kV

135 kV

135 kV

WFT

SGSG SGSGSG

SGSG

SGSG

Off

shor

e lin

e

DFIGwind farm

WFT

New added wind farmfor the case study

Case study - simulation results

2 sets of simulations:• First set of simulations:

DFIG voltage grid support capability

• Second set of simulations: illustrates DFIG voltage grid support influence on the performance of a

nearby active stall wind farm

Simulated grid fault:• 3-phase short circuit grid fault on Line 4

• Grid fault lasts for 100ms and gets cleared by

permanent isolation

• DFIG wind farm operates at its rated capacity

at the fault instant

• On-land local wind turbines are disconneted

during grid faults, as they are not

equipped with any fault ride-through control

Simulated fault event

12

5.003.752.501.250.00 [s]

190.0

150.0

110.0

70.00

30.00

-10.00

5.003.752.501.250.00 [s]

1.500

1.200

0.90

0.60

0.30

-0.000

5.003.752.501.250.00 [s]

150.0

100.0

50.00

0.00

-50.00

-100.0

DIg

SIL

EN

T

Vo

ltage

WF

T [

pu]

Act

ive

pow

er

WF

T [M

W]

Rea

ctiv

epo

wer

WF

T [M

var]

[sec]

1 2

1

1

2

2

1 - DFIG wind farm without voltage grid support - DFIG wind farm with voltage grid support2

5.003.752.501.250.00 [s]

190.0

150.0

110.0

70.00

30.00

-10.00

5.003.752.501.250.00 [s]

1.500

1.200

0.90

0.60

0.30

-0.000

5.003.752.501.250.00 [s]

150.0

100.0

50.00

0.00

-50.00

-100.0

DIg

SIL

EN

T

Vo

ltage

WF

T [

pu]

Act

ive

pow

er

WF

T [M

W]

Rea

ctiv

epo

wer

WF

T [M

var]

[sec]

1 2

1

1

2

2

1 - DFIG wind farm without voltage grid support - DFIG wind farm with voltage grid support21 - DFIG wind farm without voltage grid support - DFIG wind farm with voltage grid support2

DFIG voltage grid support capability

First set of simulations:• Focus on the DFIG wind farm performance and its interaction with the power system• It is assumed the worst case for the voltage stability:

165MW offshore active stall wind farm is not equipped with power reduction control

5.003.752.501.250.00 [s]

190.0

150.0

110.0

70.00

30.00

-10.00

5.003.752.501.250.00 [s]

1.500

1.200

0.90

0.60

0.30

-0.000

5.003.752.501.250.00 [s]

150.0

100.0

50.00

0.00

-50.00

-100.0

DIg

SIL

EN

T

Vo

ltag

e

WF

T [

pu

]A

ctiv

e po

wer

WF

T [M

W]

Rea

ctiv

e po

wer

WF

T [M

var]

[sec]

1 2

1

1

2

2

1 - DFIG wind farm without voltage grid support - DFIG wind farm with voltage grid support2

13

Second set of simulations

Focus on:How DFIG voltage grid support control influences the performance of a nearby active stall wind farm during grid faults

Four control sceneries are illustrated:

DFIG WF without voltage grid support

DFIG WF with voltage grid support

AS WF without power reduction control

AS WF with power reduction control

Scenario a Scenario b

Scenario cScenario d

14a - DFIG-WF without / AS-WF without b - DFIG-WF with /AS-WF without

10.007.505.002.500.00 [s]

300.00

200.00

100.00

0.00

-100.00

10.007.505.002.500.00 [s]

300.00

200.00

100.00

0.00

-100.00

-200.00

DIg

SIL

EN

T

Act

ive

po

we

r W

FT

[M

W]

Re

act

ive

po

we

r W

FT

[M

var]

[sec]

DFIG voltage grid support – effect on a nearby wind farm

c - DFIG-WF with /AS-WF with d - DFIG-WF without / AS-WF with

ba

c d

a b

c d

15

DFIG voltage grid support – effect on a nearby wind farm

10.007.505.002.500.00 [s]

1.100

1.070

1.040

1.010

0.98

0.95

10.007.505.002.500.00 [s]

2.00

1.50

1.00

0.50

0.00

-0.50

-1.00

DIg

SIL

EN

T

Ge

ne

rato

r sp

ee

d [

pu

]M

ech

an

ica

l po

we

r [p

u]

[sec]

a - DFIG-WF without /AS-WF without b - DFIG-WF with /AS-WF without

c - DFIG-WF with /AS-WF with d - DFIG-WF without /AS-WF with

a

b

c

a

b

cd

d

16

Remarks:

• DFIG voltage grid support control has a damping effect on the active stall wind farm, no matter whether this has or has not power reduction control (case (b) and (c))

• Worst case for the active stall wind farm (case a): DFIG wind farm has no voltage grid support control Active stall wind farm has no power reduction control

• Best case for the active stall wind farm (case b):

• DFIG wind farm is equipped with voltage grid support control

• Active stall wind farm has no power reduction control

Note that AS-WF is not subjected to torsional oscillations and there is no loss in the active power production

DFIG wind farm equipped with voltage grid support control can improve the performance

of a nearby active stall wind farm during a grid fault, without any need to implement

an additional ride-through control strategy in the active stall wind farm !!!

17

Conclusions

• DFIG controllability during grid faults is enhanced by a proper coordination design between three controllers: Damping controller - tuned to damp actively drive train torsional oscillations

excited in the drive train system during grid faults RSC voltage controller - controls grid voltage as long as RSC is not blocked by

the protection system GSC reactive power boosting controller – contributes with its maximum reactive

power capacity in case of severe grid fault

• Case study: Large DFIG wind farm - placed nearby large active stall wind farm Power transmission system generic model – delivered by Danish Transmission

System Operator Energinet.dk

• DFIG wind farm equipped with voltage grid support control participates to reestablish properly the grid voltage during grid fault can help a nearby active stall wind farm to ride-through a grid fault, without any

additional fault-ride through control setup inside the nearby active stall wind farm