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

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

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

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

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

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

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

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

Going Further Offshore Source: OCS Alternative Energy and Alternate Use Programmatic EIS http://ocsenergy.anl.gov/guide/wind/index.cfm

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

NREL 5MW Wind Turbine Barge floating platform 5MW power rating Source: http://www.mlg.org.au/visual.htm 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).

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

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

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

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

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).

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).

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).

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).

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

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

Thank you

% 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.

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

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

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

Torque Controller Region 2.5 Region 1.0 Region 1.5 Region 2.0

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

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)

Gain Scheduled PI Gains

Baseline Controller in SIMULINK Data Extraction and Plotting Controllers FAST Engine

Region 2 Torque Gain Derivation

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

Why N3 At steady state TGen = THSS

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

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

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