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

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

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 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

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

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

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 +

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

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

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

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

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

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

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

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 !!!

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