Voltage grid support of DFIG wind turbines during grid faults

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. 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

6. 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

7. 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

8. 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 - +

9. 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)

10. 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

11. 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. 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

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 WFwithvoltage grid support DFIG WFwithoutvoltage grid support AS WFwithoutpower reduction control Scenario b Scenario a AS WFwithpower reduction control Scenario c Scenario d

14. 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

15. 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