A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions

1. A Novel Control Scheme for a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions Ted Brekken, Ph.D. Assistant Professor in Energy Systems Oregon State University

2. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

3. Global Wind Energy • Almost 12 GW added between 2004 and 2005. Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

4. New Installations - 2005 • Most of new installations continue to be in US and Europe. Source: Global Wind Energy Outlook 2006, Global Wind Energy Council

5. Wind Energy Overview • GermanyUSSpainDenmarkIndia

6. US Installed Projects • Because of slow Midwest growth, the US still has huge potential. Source: American Wind Energy Association, www.awea.org/projects

7. Wind Energy Overview • Wind generators and farms are getting larger. • 5 MW wind generators are now available with 7 MW in the works. (graphic from Vestas.com)

8. Wind Generator Topologies • Direct connected. • Simplest. • Requires switch to prevent motoring. • Draws reactive power with no reactive control.

9. Wind Generator Topologies • Doubly-fed. • The doubly-fed topology is the most common for high power. • Rotor control allows for speed control of around 25% of synchronous. • Rotor converter rating is only around 25% of total generator rating. • Reactive power control.

10. Wind Generator Topologies • Full-rated converter connected. • Lower cost generator than DFIG. Lower maintenance. • Converter must be full-rated. • Full-rated converter allows for complete speed and reactive power control. • Could also be used with a synchronous generator.

11. Wind Generator Topologies • Direct-drive. • Eliminate the gearbox by using a very-high pole synchronous generator. • Resulting generator design is relatively wide and flat. • No gearbox issues. • Full-rated converter is required. • Full speed and reactive power control.

12. Wind Energy Issues • Wind is intermittent • Limits wind’s percentage of the energy mix • Wind energy is often located in rural areas • Rural grids are often weak and unstable, and prone to voltage sags, faults, and unbalances • Unbalanced grid voltages cause many problems for induction generators • Torque pulsations • Reactive power pulsations • Unbalanced currents

13. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

14. Research Objectives • Research was carried out from 2002 to 2005 at the U of M and at NTNU in Trondheim, Norway on a Fulbright scholarship • Doubly-fed induction generators are the machines of choice for large wind turbines • The objective is to develop a control methodology for a DFIG that can achieve: • Variable speed and reactive power control • Compensation of problems caused by an unbalanced grid • Reduce torque pulsations • Reduce reactive power pulsations • Balance stator currents

15. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

16. DFIG Overview - Topology • Rotor control allows for speed and reactive power control. (Cage IG are fixed.)

17. DFIG Overview – Variable Speed Control • Higher Cp means more energy captured • Maintain tip-speed ratio at nominal value (graphic from Mathworks)

18. DFIG Overview – Reactive Power Control

19. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Simulation Results • Hardware Results

20. DFIG Control • Control is done by transforming three-phase to two-phase

21. DFIG Control – Machine Flux Oriented • q-axis controls reactive power (flux) • d-axis controls torque

22. DFIG Control – Grid Flux Oriented • Align d-axis with voltage, instead of flux • Easier, more stable • d-axis -> torque • q-axis -> reactive power (Qs)

23. DFIG Control • d-axis controls torque, hence speed

24. DFIG Control • q-axis controls reactive power (Qs)

25. DFIG Control – Stability • DFIGs naturally have complex poles near the RHP, near the grid frequency (ird/vrd transfer function)

26. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

27. 3 Phase Voltage Unbalance • Causes torque puslations, reactive power pulsations, unbalanced currents, possible over heating • Unbalance can be seen as the addition of a negative sequence • Unbalance factor (VUF, IUF) is the magnitude of the negative sequence over the magnitude of the positive sequence

28. Unbalance – Second Harmonic balanced unbalanced • Therefore, compensate for the second harmonic in the dq system

29. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

30. Unbalance Compensation • Intentionally injecting a disturbance with an auxiliary controller to drive the disturbance to zero

31. d-axis Inner Loop • Compensation controller looks like a bandpass and lead-lag filter

32. Compensation Controller Design (Cd,comp) (d-axis loop gain)

33. Outline • Wind Energy Overview • Research Objectives • DFIG Overview • DFIG Control • Unbalance and Induction Machines • DFIG Unbalance Compensation • Hardware Results

34. Hardware Pictures

35. Hardware Results (15 kW) • Transient activation of compensation • VUF = 0.04

36. Hardware Results (15 kW)

37. Reduction, Simulation:Torque -> 11.5Qs -> 17.7IUF -> 7.4 Reduction, Hardware: Torque -> 29.1Qs -> 22.8IUF -> 5.5 Hardware Results (15 kW) • Steady state

38. Thank You! Questions?