Qiong-zhong Chen*, Michel Defourny # , Olivier Brüls*

1. Control and simulation of doubly-fed induction generator for variable-speed wind turbine systems based on an integrated Finite Element approach Qiong-zhong Chen*, Michel Defourny#, Olivier Brüls* *Department of Aerospace and Mechanical Engineering (LTAS), University of Liège, Belgium # SAMTECH Headquarters, Liège, Belgium EWEA 2011, Brussels, Belgium

2. Outline • Background • Control of DFIG • Integrated simulation approach • Examples & validation • Conclusions

3. Background • Wind turbine concepts • Evolution of WT size: • Increased flexibility • Increased coupling effects • Equipped gen. types (Data source: A. Perdala, dynamic models of wind turbines, PhD thesis, 2008) (Figure from EWEA factsheets)

4. Background • Computer-aided analysis for WT systems • Software specialized in a certain field • Aerodynamics: AeroDyn etc. • Structure: ADAMS/WT etc. • Electrics: DIgSILENT etc. ?Different systems on different simulation platforms ?? No detailed coupling analysis • Integrated simulation packages: • GH Bladed, Simpack Wind, HAWC2, FAST etc. ? Weak coupling (DLLs or co-simulation) ?? Numerical stability? • Need for integrated optimization tools (Bottasso, 2010)

5. Background • Samcef for Wind Turbine (S4WT) • Nonlinear FE flexible multibody solver: SAMCEF/MECANO • One single platform: • Aeroelastics, multibody, control, electrodynamics etc. • Flexibility in blades, shafts, tower etc. • Simulation approaches: Weak & strong coupling … An integrated model on S4WT (Courtesy: Samtech)

6. Highlights of the paper • Improved control strategies of DFIG WTs • Grid-synchronization • Power optimization • Strongly-coupled approach for mechatronic systems [B. & Golinval 2006] • Integrated structure-control-generator analysis on S4WT • Brüls, O. and Golinval, J. C. The generalized-α method in mechatronic applications. Zeitschrift für angewandte mathematik und mechanik (ZAMM) 86, 10 (2006), 748-758.

7. Control of DFIG • Working process of WT systems • Control of DFIG: • soft grid connection • power optimization A schematic configuration of a DFIG wind turbine

8. Grid synchronization control • Objective: • Regulate stator voltage, frequency, phase angle  grid before connection • Method: • Grid-voltage-oriented reference frame • Vector control • PI Controller designed based on internal model control (IMC) method D,q-axis rotor current control loops

9. Power control • Objective: • Follow a pre-defined power-speed characteristics profile  speed regulation • Method • Stator-flux-oriented reference frame • Vector control • q-axis rotor current  active power • d-axis rotor current  reactive power • IMC or pole placement method for design of controllers

10. Power control • Power control scheme • Controllers: PI or IP regulators • Design of controllers • PI : IMC method (current loop) • IP : pole placement method (speed loop) Decoupled speed and reactive power control of DFIG

11. Design of controllers • PI controller for q-axis rotor current • i-v transfer function • PI controller on IMC • IMC parameter: • For electrical dynamics, the rise time is set to 10ms current control block

12. Design of controllers • IP controller for speed control • Close-loop transfer function • Pole placement method • For over-damped systems: • For mechanical dynamics, the settling time is set to • 1s, DFIG alone • 2.5s, with WT system Speedcontrol block

13. Integrated simulation approach • Strongly-coupled representation for mechatronic systems • Extended generalized-α solver • Coupled 1st / 2ndorder systems • Second order accuracy • Unconditional stability • More details can be referred to [B. & Golinval 2006] Coupling in a mechatronic system

14. Mechatronic Modelling on SAMCEF • Considerations for the Mechatronic modelling: • Functional system decomposition • Modularized, parameterized components • E.g. DFIG, PI, PID modules etc. • Nodes are introduced for • Mechanical DOFs • State variables • Outputs • On a general-purpose use • User-friendly • Reusable A uniform tangent matrix for Newton iteration

15. Examples & validation • 2MW DFIG parameters: • WT parameters: Base voltage (line-to-line): Vbase= 690 V; Base power: Pbase= 2 MW; Grid frequency: fs= 50 Hz; Number of poles: np= 4; Stator resistance: Rs= 0.00488 p.u.; Rotor resistance : Rr= 0.00549 p.u.; Stator Leakage inductance: Lsl= 0.09241 p.u.; Rotor leakage inductance: Lrl= 0.09955 p.u.; Mutual inductance: Lm= 3.95279 p.u.. Inertia of the generator rotor: 100kg·m2 Blade length: 41m; Tower height: 75m; Gearbox ratio: 106 Etc.

16. Ex. 1:DFIG with defined input torque • Simulation situation • Synchronization process starts at 0.8 p.u. of the rotating speed • Reactive power reference: 0 p.u. • Speed (active power) control situation: • Reference speed: • Input torque:

17. Results • Grid synchronization Synchronization finishes Synchronization starts A-phase grid voltage A-phase stator voltage Grid synchronization process

18. Results • Power control Speed response iqr idr Rotor current response Reactive power response

19. Ex. 2: DFIG with WT structure model • Integration of DFIGwith WT structure modelon S4WT • Simulation situation: • Initial WT speed: 1.1rad/s (0.74p.u.) • Grid synchronization starts at 0.8p.u. of generator speed • Reactive power reference: 0 • Active power control according to wind speed: WT models on S4WT

20. Results • Grid synchronization Synchronization starts Synchronization finishes A-phase grid voltage A-phase stator voltage Grid synchronization process

21. Results • Power control Schematic power-speed characteristics Speed response Reactive power Active power Power response

22. Results • Influence of structural flexibility Generator torque • Otherappliedelements: • Flexible tower • Simple gearbox, bedplateelements etc. Speed response

23. Conclusions • Improved control strategies for DFIG • Grid synchronization & power control • Solution to the difficulty in the configuration of the controllers’ coefficients • Integrated FE approach with strong coupling instead of weak coupling • Unconditional stability, less intricacy • Could be less efficient • Modular models of the generator/control systems for S4WT package (on a general purpose) • Integrated variable-speed DFIG WT system model analysis and validation

24. In acknowledgement of DYNAWIND (grant number: 850533) funded by Wallonia government, Belgium Thank you for your Attention!