PhD trail lecture

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Control of offshore windmills
Rambabu Kandepu

2. 2
Contents
• Introduction
• Physics
• Classification of wind turbines
• Offshore wind turbines
• Control of wind turbines

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Introduction
• Source of wind energy
– The sun is source of all renewable energy (except
tidal and geothermal power)
– The earth receives 1.74 ×1014 kWH energy per hour
from sun
– 1-2% of this energy converted to wind energy
– Wind energy > 50 to 100 times biomass energy

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Introduction
• Advantages of wind energy
– Clean energy
– Widely distributed
– Generation at local level
– Low risk
– Energy diversity

5. 5
Introduction
History
– Wind power to Mechanical power (Persia,
Tibet and China, 1000 AD)
– Wind power to Electrical power (Dane Poul
LaCour, 1891)

6. 6
Introduction
Wind energy conversion systems
– Aerodynamic drag
– Aerodynamic lift
• Vertical axis (VAWT) (ex., Darrieus trubine)
• Horizontal axis (HAWT)

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Introduction - components
• Rotor blades
• The hub
• Gearbox
• Generator
• Nacelle.
• Tower.

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Introduction
• Wind turbines
– Downwind
• The wind passes the tower first before
the rotor
• “free-yaw” wind turbine
– Upwind
• the wind impinges upon the rotor first
• “yaw-driven” machine

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Introduction
• Power
– Wind force to torque
• Wind energy transferred
– density,
– wind velocity
– rotor area

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Introduction – power curve
The Physics
1
Power in wind = ρ AV 3 (watts),
2
where ρ = air density (kg m -3 )
V = wind speed (m s -1 )
A = the intercepting area (m 2 )
Maximum power extraction is 59% (Betz)

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Introduction – power curve
• Cut-in wind speed
• Rated wind speed (12-16 m/s)
• Cut-out wind speed (20-25 m/s)
• Depends on air pressure, aerodynamic shape of rotor,
placing of turbine

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Introduction – power curve
• Power produced
1
P = ρ ACPV 3
2
• Tip speed ratio
ωR
λ=
V

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Wind turbine topologies
Fixed speed Variable speed
Max. efficiency at one Max. efficiency over a wide
particular wind speed range of wind speeds
Simple, robust, reliable and Increased energy capture,
well proven improved power quality,
reduced mechanical stress
Mechanical stress, limited Losses in power electronics,
power quality control more components, increased
cost

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Types of power control
• Stall control (passive control)
– The blades are bolted to the hub at a fixed
angle
– Design of rotor aerodynamics causes the
rotor to stall
– Simplest, robust and cheapest
– Lower efficiency at low wind speeds, no
assisted start up

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Types of power control
• Pitch control (active control)
– Blades can be turned out or into the wind
– Extra complexity and higher fluctuations at
high wind speeds
– Good power control, assisted startup and
emergency stop
– Requires fast response control loop –
additional costs and fatigue loading increases

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Types of power control
• Active stall control
– The stall of the blade is actively controlled
by pitching the blades
– Smoother power control without high
power fluctuations
– Easier to carry out emergency stops and
to start up

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Offshore wind turbines
• More faster and uniform
wind
•The most critical aspect
is the substructures.
• Over 11 GW of new
offshore wind projects are
planned before the year
Technology progression of offshore wind turbines 2010

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Offshore wind turbines
Typical cost breakdown of an offshore wind plant in shallow water

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Offshore substructures
Cost of Offshore Wind Turbine Substructures with Water Depth

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Shallow water foundations
• 5-18m deep
• Monoplies – 160 MW wind farm
at Horns Rev (Denmark)
– Simple and minimal design
– Depth limited due inherent
flexibility
• Gravity based – 160 MW Nysted
project (Denmark)
– Overcome flexibility
– Increase of cost with water
depth
• Suction bucket foundations
– Not yet used

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Transitional technology
1) tripod tower, 2) guyed monopole, 3) full-height jacket
(truss), 4) submerged jacket with transition to tube tower,
5) enhanced suction bucket or gravity base.

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Transitional technology
• Two wind turbine generators
near the Beatrice Oil Field,
North sea.
• 22km offshore
• Water depth off approximately
45 meters
• 87 meters above sea level
• 5 MW for each turbine

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Floating technology
• The Spar-buoy
concept, stability by
ballast
• The Tension Leg
Platform (TLP), stability
by mooring line tension
• The barge concept,
stability through its
waterplane area

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Floating technology challenges
• Turbulent winds
• Irregular waves
• Gravity / inertia
• Aerodynamics
• Hydrodynamics
• Elasticity
• Mooring dynamics
• Control system
• Fully coupled

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Floating technology – Hywind
• Power > 5MW Hydro currently has a license to
• Height above sea 80m place a demonstration turbine
• Rotor diameter 120m offshore near Karmøy
• Water depth – 200-700m

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Control of wind turbine
• Wind turbines are relatively simple compared with
complex electrical power plants
• The stochastic nature of the wind introduces
complexity
• Control system objectives
– Improve energy capture
– Keep the power output and rotor speed with in design limits
– Reduce structural dynamic loading
• Offshore wind turbine
– More dynamics
– Vertical stability in case of floating turbines

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Control of wind turbine
• Classical control
– PI controllers
– Multiple control loops, dynamics unknown
• Modern control
– Pole placement
– Linear Quadratic Control
– H ∞ control
– Adaptive control
– Disturbance Accommodating Control (DAC)
– Model Predictive Control

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Control of offshore wind turbine
P
CP =
⎛1 ⎞
⎜ ρ Av3 ⎟
⎝2 ⎠
λ is the tip-speed ratio

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Wind model
• Wind field varies both in space and time
– A slowly varying component with a “mean” wind speed
– A slowly varying “wind shear” component
– A rapidly varying “turbulent” wind component

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Aerodynamics of blade section
⎛1 ⎞
FL = ⎜ ρ AW 2 ⎟ CL
⎝2 ⎠
⎛1 ⎞
FD = ⎜ ρ AW 2 ⎟ CD
⎝2 ⎠
CL , CD - lift and drag coeff. depend on α (AoA)

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Control of offshore wind turbine
• Variable speed turbine with pitch control
• Four independent control inputs
– Three blade pitch inputs
– Generator torque command
• Regulated variables
– Power captured
– Mechanical loads on turbine structure

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Control of offshore wind turbine
• State-of-the-art in control
– Control below rated speed
– Control above rated speed
– Drive train damping

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Control of offshore wind turbine
• Control below rated speed
– Objective is maximize power capture
– Torque control
– Pitch angle is maintained constant

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Control of offshore wind turbine
• Control above rated wind speed
– Objective is keep power output and loads
on turbine structure within design limits
– Collective pitch control
– Keep generator torque constant

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Control of offshore wind turbine
• Drive train damping
– Serious impact on gear box
Jθ (t ) + Kθ (t ) = Taero − Tgen
θ - angular displacement
Taero - aerodynamic torque
Tgen - generator torque
J - lumped inertia
K - lumped stiffness

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Control of offshore wind turbine
• Decentralized control
– Below rated speed – torque control
– Above rated speed – pitch control
– Advantage in absence of reliable wind
speed information
– Easy to implement and tune the control
algorithms
– Simple to design for SISO case

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Control of offshore wind turbine
• Dynamics using 5 DOF
model
– Blade flap
– Blade edge
– Tower fore-aft
– Tower side-to-side
– Drive train torsion
• Coupled dynamics

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Control of offshore wind turbine
• Advanced control design is necessary
– The neglected coupled dynamics cause problems
• Present research
– State space control design
– Pole placement, LQR, H ∞control, adaptive control,
Disturbance Accommodating Control (DAC), MPC
– State estimator

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Control of offshore wind turbine
• DAC control
– Augment states with disturbances
• H ∞Control
– Include uncertinitoes
• MPC
– Include constraints (generator power, shaft speed,
limits on DOFs)
• Adaptive control
– Updating model parameters
• Controller complexities Vs implementation

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Control of offshore wind turbine
• Use of nonlinear models
– Present research focuses on linearized
models
• Vertical stability in case of floating
turbines
– Tension in mooring lines

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Conclusions
• Classical control
– Decentralized control
• Present research
– State feed back control
– Linearized models
• Challenges
– Complex dynamics
– Unknown disturbances
– Design issues

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Thank you
for
your
attention
☺

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References
• Keld Hammerum, A Fatigue Approach to Wind Turbine Control, Master Thesis, Technical
University of Denmark
• Alexandra Bech Gjørv, Hywind - Floating wind power production, Hydro
• Beatrice Wind Farm Demonstrator, Project Scoping Report, Talisman Energy
• Shashikanth Suryanarayanan and Amit Dixit, Control of Large Wind Turbines: Review and
Suggested Approach to Multivariable Design
• W. E. Leitheat and B. Connor, Control of variable speed wind turbines: design task, Int. J.
Control, 2000, VOL. 73, NO. 13, 1189-1212
• William E. Leithead and Sergio Dominguez, Coordinated Control Design for Wind Turbine
Control Systems
• E.N. Wayman, P.D. Sclavounos, S. Butterfield, J. Jonkman, and W. Musial, Coupled
Dynamic Modeling of Floating Wind Turbine Systems, Offshore Technology Conference
Houston, Texas May 1–4, 2006
• Alan D. Wright, Mark J. Balas, Design of State-Space-Based Control Algorithms for Wind
Turbine Speed Regulation, Transactions of the ASME, Vol. 125, November, 2003
• W. Musial, S. Butterfield and B. Ram, Energy from Offshore Wind, Offshore Technology
Conference, Houston, Texas, May 1–4, 2006
• S. Butterfield, W. Musial, J. Jonkman and P. Sclavounos, Engineering Challenges for
Floating Offshore Wind Turbines, Offshore Wind Conference, Copenhagen, Denmark,
October 26–28, 2005
• Alan D. Wright, Modern Control Design for Flexible Wind Turbines, Technical report, July
2004.
• Danish wind industry association, (http://www.windpower.org)
• Lars Christian Henriksen, Model Predictive Control of a Wind Turbine, Master's thesis,
Informatics and Mathematical Modelling, Technical University of Denmark, DTU