1. Hazim Namik Department of Mechanical Engineering
Deepwater Floating Offshore Wind Turbine Control Methods
Hazim Namik
Department of Mechanical Engineering

2. Outline Introduction to wind turbines Offshore wind turbines
Wind resource
Floating wind turbines
Control Methods
Summary

3. Introduction Wind energy is the fastest growing renewable energy
Wind energy is a form of solar energy
Only 2% of received solar energy is converted to wind
Wind turbines convert some of the wind energy to useful mechanical energy

4. Types of Wind Turbines Two main types of wind turbines (WTs)
Vertical axis (VAWT)
Horizontal axis (HAWT)
HAWT are generally more efficient, hence used for power generation

5. Major Components Blades Hub Nacelle High speed and low speed shafts
Gearbox
Generator
Yaw drive system
Source: US Dept. of Energy

6. Offshore vs. Onshore Winds
Advantages:
Stronger and steadier winds
Have less turbulence
Have less vertical shear
Winds are more spatially consistent
Disadvantages
The winds Interact with waves
Offshore winds are harder to measure

7. Vertical Shear
Surface roughness at sea is lower; therefore, higher wind speeds at lower heights.
Source: Wind Turbines, Erich Hau

8. Offshore Resource Availability
Source: Goldman, P., Offshore Wind Energy, in Workshop on Deep Water Offshore Wind Energy Systems. 2003, Department of Energy.

9. Going Further Offshore
Source: OCS Alternative Energy and Alternate Use Programmatic EIS

10. Deepwater Floating Wind Turbines
Source: Jonkman, J. Development and Verification of a Fully Coupled Simulator for Offshore Wind Turbines

11. NREL 5MW Wind Turbine Barge floating platform 5MW power rating
Source:
Barge floating platform
5MW power rating
126m diameter rotor (3 Blades)
90m hub height
153m tall
RePower 5MW, 126m diameter rotor
Source: RE UK
Source: Jonkman, J.M., Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine, in Department of Aerospace Engineering Sciences. 2007, University of Colorado: Boulder, Colorado, USA (to be published).

12. General Turbine Control Methods
Control Options
Blade Pitch
Generator Torque
Collective Pitch
Individual Pitch
Pitch to Feather
Pitch to Stall
Mode of Operation
Principle of Operation

13. Floating Turbine Control Methodology
Simple Onshore
Baseline controller
Complex Onshore
Special Offshore

14. Baseline Controller Overview
Generator torque controller
Maximum power below rated wind speed
Regulate power above rated
Collective pitch controller
Regulate generator speed above rated wind speed

15. Platform Pitching – Problem
Factors affecting platform pitching
Ocean waves
Aerodynamic thrust
Mooring lines

16. Modifications to the Baseline Controller
Tower feedback loop
Additional blade pitch controller
Tower top acceleration feedback
Active pitch to stall
Extra thrust force when blade is stalled may reduce platform pitching
Detuned controller gains
Reduced pitch to feather controller gains
Source: Jonkman, J.M., Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine, in Department of Aerospace Engineering Sciences. 2007, University of Colorado: Boulder, Colorado, USA (to be published).

17. Results – Tower Feedback
Poor power regulation
Marginally reduced platform pitching
Source: Jonkman, J.M., Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine, in Department of Aerospace Engineering Sciences. 2007, University of Colorado: Boulder, Colorado, USA (to be published).

18. Results – Pitch to Stall
Excellent power regulation
Large platform oscillations
Source: Jonkman, J.M., Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine, in Department of Aerospace Engineering Sciences. 2007, University of Colorado: Boulder, Colorado, USA (to be published).

19. Results – Detuned Gains
Reasonable power regulation
Reduced platform pitching – but not enough
Source: Jonkman, J.M., Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine, in Department of Aerospace Engineering Sciences. 2007, University of Colorado: Boulder, Colorado, USA (to be published).

20. Floating Turbine Control Methodology
Current State of Research Worldwide
Simple Onshore
Baseline controller
Classical Control
Complex Onshore
State space with individual blade pitch
Modern Control
Nonlinear with individual blade pitch
Special Offshore
Without adding any actuators
Adding necessary actuators

21. Summary Offshore winds are stronger and steadier than onshore winds
Floating turbines are economically feasible for deep waters
Classical control was not successful at controlling a floating wind turbine
Modern control with state space or nonlinear control is the way to go

22. Thank you

23. % of Water Depths in Different Regions up to 100km Offshore
North Europe 21 26 32 20
South Europe 16 11 23 49
Japan 22 9 18 51
USA 50 13
Source: Henderson, A.R., Support Structures for Floating Offshore Windfarms, in Workshop on Deep Water Offshore Wind Energy Systems. 2003, Department of Energy.

24. Floating Wind Turbines
Reduce the cost of construction for deep waters
Can be located close to major demand centres
Could interfere with aerial and naval navigation
Harder to control as added dynamics of platform motion affect performance

25. Power Regions Region 1 Region 2 Region 3
No power is generated below the cut in speed
Region 2
Maximise power capture
Region 3
Regulate to the rated power

26. Torque Controller Region 1 Region 2 Region 3
Regions 1.5 and 2.5 are linear transitions between the regions

27. Torque Controller Region 2.5 Region 1.0 Region 1.5 Region 2.0

28. Collective Pitch Controller
PI Controller to regulate generator speed
Controller gains calculated according to the design parameters
ωn = 0.7 rad/s and ζ = 0.7
Simple DOF model with PI controller gives

29. Pitch Sensitivity
Power sensitivity to blade pitch is found through linearization of the turbine model
Pitch sensitivity varies almost linearly with blade pitch
Gain Scheduled PI gains are calculated based on blade pitch through a gain correction factor GK(θ)

30. Gain Scheduled PI Gains

31. Baseline Controller in SIMULINK
Data Extraction and Plotting
Controllers
FAST Engine

32. Region 2 Torque Gain Derivation

33. R2 Torque Gain Derivation Cont.
Changing to generator torque and HSS speed in rpm and taking pre-cone into account
In Region 2, CP = CP,Max and λ = λo
For this Turbine:
CP,Max= 0.482, λo= 7.55, R= 63m, α=2.5°, and NGear= 97

34. Why N3
At steady state TGen = THSS

35. Pitch Controller Derivation
Single DOF model of the turbine drivetrain gives
Taylor approximation of aerodynamic and generator torques gives

36. Pitch Controller Derivation (Contd.)
Pitch commands (Δθ) comes from the PID controller equation
By making the following substitution and replacing everything in the equation of motion, we get

37. Pitch Controller Derivation (Contd.)
This is a 2nd order differential equation
Expanding ωn and ζ and solving for a PI controller (KD = 0) gives