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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. - PowerPoint PPT Presentation
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1
Hazim Namik
Department of Mechanical Engineering
Deepwater Floating Offshore Wind Turbine Control Methods
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
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Vertical Shear
• Surface roughness at sea is lower; therefore, higher wind speeds at lower heights.
Source: Wind Turbines, Erich Hau
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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/wind/index.cfm
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Deepwater Floating Wind Turbines
Source: Jonkman, J. Development and Verification of a Fully Coupled Simulator for Offshore Wind Turbines
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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).
Source: http://www.mlg.org.au/visual.htm
RePower 5MW, 126m diameter rotorSource: RE UK
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General Turbine Control Methods
Collective Pitch
Individual Pitch
Pitch to Feather
Pitch to Stall
Control Options
Blade Pitch Generator Torque
Mode of Operation
Principle of Operation
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Floating Turbine Control Methodology
Simple Onshore Baseline controller
Complex Onshore
Special Offshore
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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
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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).
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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).
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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).
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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).
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Floating Turbine Control Methodology
Simple Onshore Baseline controller
Complex Onshore
Special Offshore
State space with individual blade pitch
Nonlinear with individual blade pitch
Without adding any actuators
Adding necessary actuators
Current State of Research Worldwide
Cla
ssic
al
Co
ntr
ol
Mo
de
rn
Co
ntr
ol
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
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Thank you
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Water Depths
% of Water Depths in Different Regions up to 100km Offshore
RegionWater Depth
<25m 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.
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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
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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
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Torque Controller
• Region 1
• Region 2
• Region 3
• Regions 1.5 and 2.5 are linear transitions between the regions
2HSSGen KT
0GenT
HSSGen
RatedGen
PT
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Applied Generator Torque
0
5
10
15
20
25
30
35
40
45
0 500 1000 1500
Th
ou
san
ds
High Speed Shaft Speed (rpm)
Gen
erat
or
To
rqu
e (N
m) Tg_rated (Nm)
Tg_r1 (Nm)
Tg_r1.5 (Nm)
Tg_r2 (Nm)
Tg_r2.5 (Nm)
Tg_r3 (Nm)
T=Kw 2̂
Torque Controller
Reg
ion
1.0
Reg
ion
1.5
Reg
ion
2.0
Reg
ion
3.0
Region 2.5
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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
P
N
IKand
PN
IK
Gear
nratedRotorDrivetrainI
Gear
nratedRotorDrivetrainP
2,,2
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Pitch Sensitivity
• Power sensitivity to blade pitch is found through linearization of the turbine model
DxCP
BxAx Model Space State Linearized
• 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)
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Gain Scheduled PI GainsGain Scheduled PID Controller
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0 5 10 15 20 25
Pitch (deg)
Co
rrec
tio
n F
acto
r
KP(θ)
KI(θ)
31
Baseline Controller in SIMULINK
FAST Engine
Data Extraction and Plotting
Controllers
32
Region 2 Torque Gain Derivation
RotRotP TPV
RAVCP and,
2
1 3
23
5
23
223
2
1
2
1
2
1
2
1
RotP
Rot
RotPRot
PPRot
RCT
RRCT
RVRC
V
RAVCT
33
33
532
3
22
3
5
1800
cos
1
302
cos
Gear
PHSSGen
GearHSS
PGen
N
RCKandKT
N
RCT
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
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23
53
1800
cosRot
PRot
RCT
Why N3
• At steady state TGen = THSS
2233
53
2
3
53
1800
cos
1800
cos
HSSGenHSSGear
PGen
Gear
HSSPGen
Gear
HSSRot
Gear
RotGen
KTN
RCT
NN
RCT
NAnd
N
TT
35
Pitch Controller Derivation
• Single DOF model of the turbine drivetrain gives
DrivetrainGenGearRotorGenGearAero Idt
dINITNT 0
2
• Taylor approximation of aerodynamic and generator torques gives
PPT
N
P
N
PT
Aero
GearGearGen
00
0
20
0
0
0
1
36
Pitch Controller Derivation (Contd.)
• Pitch commands (Δθ) comes from the PID controller equation
GearD
t
GearIGearP NKdtNKNK0
• By making the following substitution and replacing everything in the equation of motion, we get
0111
000
K
IGear
C
PGear
M
DGearDrivetrain KNP
KNP
KNP
I
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Pitch Controller Derivation (Contd.)
• This is a 2nd order differential equation
• Expanding ωn and ζ and solving for a PI controller (KD = 0) gives
nn
K
IGear
C
PGear
M
DGearDrivetrain
M
Cand
M
K
KNP
KNP
KNP
I
2
0111
000
PN
IKand
PN
IK
Gear
nDrivetrainI
Gear
nDrivetrainP
2002