Power Point Template

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

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
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

• Pitch control system

• 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 QGSC0 !

Power converter
RSC
GSC
AC
DC
AC
DC

• 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

• 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

• 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

• 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

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

Damping controller
Optimal speed
Wind 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
Third stage (voltage grid support)
10
Power transmission system test model
400 kV
400 kV
135 kV
135 kV
Line 1
Line 2
135 kV
Simulated fault event
Line 4
Line 3
135 kV
Offshore line
Local wind turbines
WFT
Active stall wind farm
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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

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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

13
Second set of simulations

• Focus onHow 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 b
Scenario a
AS WF with power reduction control
Scenario c
Scenario d
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DFIG voltage grid support effect on a nearby
wind farm
a
b
Active power WFT MW
c
d
d
c
Reactive power WFT Mvar
a
b
sec
c - DFIG-WF with /AS-WF with d - DFIG-WF
without / AS-WF with
a - DFIG-WF without / AS-WF without b -
DFIG-WF with /AS-WF without
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DFIG voltage grid support effect on a nearby
wind farm
a
b
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
• 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