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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

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Voltage grid support of dfig wind turbines during grid faults

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

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


  • Background

    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


    Dfig wind turbine modelling control issues in case of grid faults

    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


    Drive train and pitch control system

    • Free – free frequency:

    n

    T

    T

    gen

    gear

    rot

    • Equivalent inertia:

    J

    J

    rot

    gen

    k

    ref

    

    +

    +

    c

    PI

    -

    -

    KPI

    Gain schedulling

    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


    Dfig system control normal operation

    • Reference signals:

      • Active power for RSC is defined by MPT:

    PI

    PI

    PI

    PI

    Maximum power tracking point

    MPT

    Fast control (current)

    P

    PI

    PI

    PI

    PI

    Slow control (power)

    DFIG system control (normal operation)

    • Power converter control

      • RSC controls Pgridand Qgrid

      • independently!

      • GSC controls UDC and QGSC=0 !

    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


    Dfig system control and protection during grid faults

    0

    3

    .

    .

    .

    .

    2

    -5

    1

    -10

    Damping controller

    Reactive power [Mvar]

    Electromagnetic torque [p.u.]

    0

    -15

    -1

    -20

    -2

    -25

    -3

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    2.5

    3

    -1

    -0.5

    0

    0.5

    1

    1.5

    2

    2.5

    3

    Speed [p.u.]

    Speed [p.u.]

    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


    Fault ride through damping of torsional oscillations during grid faults

    Generator speed [pu]

    Mechanical torque [Nm]

    [sec]

    Without damping controller

    With damping controller

    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

    -

    +


    Dfig wind turbine voltage grid support control

    • Damping controller

    • RSC voltage controller

    • GSC reactive power boosting

    RSC

    Voltage

    Controller

    GSC

    Reactive Power

    Boosting

    co-ordination

    Damping

    Controller

    • Damping controller

      • damps actively the torsional oscillations of the drive train system

      • during grid faults

    RSC voltage controller

    • RSC voltage controller

      • controls grid voltage as long as the

      • protection device is not triggered

    +

    PI

    -

    • GSC reactive power boosting

      • controls grid voltage when RSC is

        blocked by the protection device

    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)


    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

    L

    L

    L

    SG

    SG

    SG

    SG

    Extended for the case study with:

    Offshore line

    • 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

    DFIG

    wind farm

    New added wind farm

    for the case study

    Power transmission system test model

    400 kV

    400 kV

    135 kV

    135 kV

    Line 1

    Line 2

    135 kV

    Simulated

    fault event

    Line 3

    Line 4

    135 kV

    Offshore line

    Local

    wind turbines

    WFT

    Active stall

    wind farm


    Case study simulation results

    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


    Dfig voltage grid support capability

    Voltage WFT [pu]

    2

    1

    2

    Active power WFT [MW]

    1

    1

    2

    Reactive power WFT [Mvar]

    [sec]

    2

    1

    - DFIG wind farm without voltage grid support

    - DFIG wind farm with voltage grid support

    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


    Second set of simulations

    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 WFwithvoltage grid support

    DFIG WFwithoutvoltage grid support

    AS WFwithoutpower reduction control

    Scenario b

    Scenario a

    AS WFwithpower reduction control

    Scenario c

    Scenario d


    Dfig voltage grid support effect on a nearby wind farm

    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


    Dfig voltage grid support effect on a nearby wind farm1

    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


    Remarks

    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 !!!


    Conclusions

    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


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