Transcript Slide 1

Voltage grid support of
DFIG wind turbines during grid faults
Gabriele Michalke
University of Technology Darmstadt, Germany
Anca D. Hansen
Risø National Laboratory, Denmark
EWEC Milan
7-10 May 2007
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
2
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
3
DFIG wind turbine – modelling, control issues in case of grid faults:
DFIG system – control and protection
k
DFIG
c
Aerodynamics
Drive train
with gearbox
RSC
GSC
~
=
=
~

~
~
~
Pitch angle
control
Crowbar
Power converter
control
Pref
Wind turbine
Fault
detection
Qref
Control mode :
• normal operation
• fault operation
4
Drive train and pitch control system
• 2 mass mechanical model
Jrot
 Free – free frequency:
f osc 
n gear
Jgen
k
1

2
k
J eq
 Equivalent inertia:
Trot
Tgen
c
J eq 
2
J rot  n gear
 J gen
2
J rot  n gear
 J gen
• Pitch control system
 Pitch angle controls the speed

+

ref
PI
+
-
-
1
Tservo
d
dt
 Prevent over-speed both in:
- normal operations
- grid faults operations
1
s
ref
KPI
Gain schedulling

 Rate of change limitation
important during grid faults
5
DFIG system control (normal operation)
Power converter control
• RSC controls Pgrid and Qgrid
independently!
• GSC controls UDC and QGSC=0 !
Power converter
RSC
AC
GSC
DC
DC
I qRSC
AC
Reference signals:
• Active power for RSC is defined
by MPT:
I qGSC
I dRSC
PI PIPrefgrid
I dGSC
PI PI
Maximum power tracking point
RSC
I qref
RSC
I dref
GSC
I qref
GSC
I dref
MPT

Fast control (current)
P grid
Q GSC
PI PI
PI PI
Q grid
Prefgrid
Slow control (power)
Qrefgrid
GSC
Qref
U dc
U refDC
Prefgrid
P

• Reactive power for RSC - certain
value or zero
• GSC is reactive neutral
• DC voltage is set to constant value
6
DFIG system control and protection during grid faults
New grid codes require:
• Fault ride-through capability:
wind turbine has to remain connected
to the grid during grid faults
Power converter is very sensitive to grid faults !!!
• Protection system monitors DFIG signals
• Crowbar protection:
 external rotor impedance
Electromagnetic torque [p.u.]
3
R1crowbar  R2crowbar  R3crowbar
2
1
-1
-2
-3
-1
Severe grid faults triggers crowbar:
• RSC disabled
• DFIG behaves as SCIG
• GSC can be used as a STATCOM
-0.5
0
0.5
1
1.5
2
2.5
3
Speed [p.u.]
0
Reactive power [Mvar]
• Increased crowbar:
 improved dynamic stability of the
generator
 reduces reactive power demand
Damping controller
0
.
.
.
.
-5
-10
-15
-20
R1crowbar  R2crowbar  R3crowbar
-25
-1
-0.5
0
0.5
1
1.5
Speed [p.u.]
2
2.5
3
7
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
D IgSILEN T
Fault Ride Through – Damping of Torsional oscillations during grid faults
1.150
Generator speed [pu]
1.125
1.100
1.075
1.050
Damping controller:
• designed and tuned to damp torsional
oscillations
• provides active power reference for
RSC control
Optimal
speed
ref
PI
+

0.00
2.50
5.00
7.50
[s]
10.00
7.50
[s]
10.00
3.0E+4
Mechanical torque [Nm]
2.0E+4
1.0E+4
Damping controller
Wind
speed
1.025
Prefgrid
0.0E+0
-1.0E+4
0.00
2.50
Without damping controller
[sec]
5.00
With damping controller
8
DFIG wind turbine – voltage grid support control
DFIG control structure – normal operation
Prefgrid
GSC
Qref
Qrefgrid
Damping
Controller
RSC
GSC
co-ordination
Voltage
Reactive Power
Controller
Boosting
Third stage (voltage grid support)
RSC voltage controller
Uacgrid,ref +
Qrefgrid
-
U acgrid
PI
• During grid faults DFIG controllability is
enhanced by a proper co-ordination of three
controllers:
 Damping controller
 RSC voltage controller
 GSC reactive power boosting
• Damping controller
 damps actively the torsional
oscillations of the drive train system
during grid faults
• 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
9
Power transmission system test model
Power transmission system model:
• delivered by the Danish Transmission
SG
SG
400 kV
400 kV
135 kV
Line 2
Line 1
135 kV
L
135 kV
L

135 kV
Offshore line
SG

Simulated
fault
event
Offshore line
L
Line 4
Line 3
SG
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:
Extended for the case study with:
Local
wind turbines
WFT
one machine modelling approach
equipped with active power reduction
control for fault ride-through
WFT
• 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

Active stall
wind farm
DFIG
wind farm
New added wind farm
for the case study
10
Case study - simulation results
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
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
11
DIgSILENT
Voltage WFT [pu]
DFIG voltage grid support capability
1.500
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:
1.200
0.90
0.60
0.30
2
1 stall wind
 165MW offshore active
farm is not equipped with
power reduction control
-0.000
0.00
1.25
2.50
110.0
70.00
[s]
5.00
3.75
[s]
5.00
3.75
[s]
5.00
D IgSILEN T
Voltage WFT [pu]
1.500
150.0
1.200
0.90
0.60
2
0.30
2
1
-0.000
-10.00
150.0
Active power WFT [MW]
0.00
30.00
190.0
150.0
50.00
0.00
-50.00
-100.0
1.25
2.50
3.75
[s]
5.00
1
110.0
0.00
21.25
2.50
70.00
30.00
1
-10.00
0.00
100.0
Reactivepower WFT M
[ var]
Reactive power WFT [Mvar]
Active power WFT [MW]
190.0
3.75
1.25
2.50
150.0
3.75
[s]
1
100.0
1
50.00
5.00
2
2
0.00
-50.00
-100.0
0.00
1.25
[sec]
1 - DFIG wind farm without voltage grid support
1.25
0.00
2.50
3.75
[s]
5.00
2 - DFIG wind farm with voltage grid support
2.50
[sec]
1 - DFIG wind farm without voltage grid support
2 - DFIG wind farm with voltage grid support
12
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
AS WF without power
reduction control
AS WF with power
reduction control
DFIG WF with
voltage grid support
Scenario a
Scenario b
Scenario d
Scenario c
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DIgSILENT
DFIG voltage grid support – effect on a nearby wind farm
Active power WFT [MW]
300.00
a
b
200.00
100.00
0.00
c
d
-100.00
0.00
2.50
5.00
7.50
[s]
10.00
5.00
7.50
[s]
10.00
Reactive power WFT [Mvar]
300.00
200.00
d
c
100.00
0.00
-100.00
a
b
-200.00
0.00
2.50
a - DFIG-WF without / AS-WF without
b - DFIG-WF with /AS-WF without
[sec]
c
d
- DFIG-WF with /AS-WF with
- DFIG-WF without / AS-WF with
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DIgSILENT
DFIG voltage grid support – effect on a nearby wind farm
Generator speed [pu]
1.100
a
1.070
b
1.040
1.010
c
0.98
d
0.95
0.00
2.50
5.00
7.50
[s]
10.00
5.00
7.50
[s]
10.00
2.00
Mechanical power [pu]
a
b
1.50
1.00
0.50
c
0.00
d
-0.50
-1.00
0.00
2.50
[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
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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 !!!
16
Conclusions
•
DFIG controllability during grid faults is enhanced by a proper
coordination design between three controllers:



•
Case study:


•
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
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
17