Hazim Namik

1
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 EIShttp//ocsenergy.anl.gov/guide/wi
nd/index.cfm
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
• 126m diameter rotor (3 Blades)
• 90m hub height
• 153m tall

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
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
Classical Control
Simple Onshore
Baseline controller
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
Water Depths
of Water Depths in Different Regions up to 100km Offshore of Water Depths in Different Regions up to 100km Offshore of Water Depths in Different Regions up to 100km Offshore of Water Depths in Different Regions up to 100km Offshore of Water Depths in Different Regions up to 100km Offshore
Region Water Depth Water Depth Water Depth Water Depth
Region lt25m 25-50m 50-100m 100-300m
North Europe 21 26 32 20
South Europe 16 11 23 49
Japan 22 9 18 51
USA 50 26 13 11
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
• 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 1.0
Region 1.5
Region 2.0
Region 3.0
28
Collective Pitch Controller

• PI Controller to regulate generator speed
• Controller gains calculated according to the
  design parameters
• ?n 0.7 rad/s and ? 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(?)

30
Gain Scheduled PI Gains
31
Baseline Controller in SIMULINK
Data Extraction and Plotting
FAST Engine
Controllers
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Region 2 Torque Gain Derivation
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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 ? ?o
• For this Turbine
• CP,Max 0.482, ?o 7.55, R 63m, a2.5, 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 (??) 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 ?n and ? and solving for a PI
  controller (KD 0) gives