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 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 Overall goal: Focus in this presentation: 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:
DFIG system – control and protection k
DFIG c
Drive train
with gearbox
RSC
GSC
Aerodynamics ~ = = ~ ~ ~ ~
Power converter
control
Pitch angle
control
Crowbar
Control mode :
normal operation
fault operation
Fault
detection
Wind turbine

5. Drive train and pitch control system
2 mass mechanical model
Free – free frequency:
Equivalent inertia: n
gear J
rot T
gen c k
Pitch control system + -
ref 
Gain schedulling
KPI  PI
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)
Power converter control
RSC controls Pgrid and Qgrid
independently!
GSC controls UDC and QGSC=0 !
Power converter
RSC
GSC AC DC DC AC
Reference signals:
Active power for RSC is defined by MPT: P 
MPT
Maximum power tracking point PI
Fast control (current) PI
Slow control (power)
Reactive power for RSC - certain value or zero
GSC is reactive neutral
DC voltage is set to constant value

7. DFIG system control and protection during grid faults
Damping controller -1
-0.5
0.5 1
1.5 2
2.5 3 -3 -2
Speed [p.u.]
Electromagnetic torque [p.u.]
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
Increased crowbar:
improved dynamic stability of the generator
reduces reactive power demand -1
-0.5
0.5 1
1.5 2
2.5 3
-25
-20
-15
-10 -5
Speed [p.u.]
Reactive power [Mvar] .
Severe grid faults triggers crowbar:
RSC disabled
DFIG behaves as SCIG
GSC can be used as a STATCOM

8. Fault Ride Through – Damping of Torsional oscillations 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
Generator speed [pu]
Without damping controller
With damping controller
Mechanical torque [Nm]
[sec]
Damping controller:
designed and tuned to damp torsional
oscillations
provides active power reference for
RSC control
Damping controller
Wind
speed
Optimal
speed - PI +

9. DFIG wind turbine – voltage grid support control
During grid faults DFIG controllability is
enhanced by a proper co-ordination of three
controllers:
DFIG control structure – normal operation
Damping
Controller
Damping controller
RSC
Voltage
Controller
RSC voltage controller
co-ordination
GSC
Reactive Power
Boosting
GSC reactive power boosting
Damping controller
damps actively the torsional oscillations of the drive train system
during grid faults
Third stage (voltage grid support) + -
RSC voltage controller PI
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 SG SG
400 kV
400 kV
135 kV L L
135 kV SG
Line 1
Line 2
135 kV L SG
Line 3
Line 4
Simulated
fault event
135 kV
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
Offshore line
DFIG
wind farm
New added wind farm
for the case study
Offshore line
Local
wind turbines
WFT
Active stall
wind farm

11. 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

12. DFIG voltage grid support capability
Voltage WFT [pu]
Active power WFT [MW]
Reactive power WFT [Mvar]
[sec] 1 2
- DFIG wind farm without voltage grid support
- DFIG wind farm with voltage grid support
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

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
Scenario a
Scenario b
AS WF with power reduction control
Scenario d
Scenario c

14. DFIG voltage grid support – effect on a nearby wind farm
Active power WFT [MW] c d c d
Reactive power WFT [Mvar] a b
[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

15. DFIG voltage grid support – effect on a nearby wind farm
Generator speed [pu] c d a b
Mechanical power [pu] c d
[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

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
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