1

Control of offshore windmills

Rambabu Kandepu

2

Contents

• Introduction• Physics• Classification of wind turbines• Offshore wind turbines• Control of wind turbines

3

Introduction

• Source of wind energy– The sun is source of all renewable energy (except

tidal and geothermal power)

– The earth receives kWH energy per hour from sun

– 1-2% of this energy converted to wind energy

– Wind energy > 50 to 100 times biomass energy

141.74 10×

4

Introduction

• Advantages of wind energy– Clean energy

– Widely distributed

– Generation at local level

– Low risk

– Energy diversity

5

IntroductionHistory

– Wind power to Mechanical power (Persia, Tibet and China, 1000 AD)

– Wind power to Electrical power (Dane PoulLaCour, 1891)

6

IntroductionWind energy conversion systems

– Aerodynamic drag– Aerodynamic lift

• Vertical axis (VAWT) (ex., Darrieus trubine)• Horizontal axis (HAWT)

7

Introduction - components

• Rotor blades • The hub • Gearbox• Generator• Nacelle.• Tower.

8

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

9

• Power– Wind force to torque

• Wind energy transferred– density, – wind velocity – rotor area

Introduction

10

Introduction – power curveThe Physics

3

-3

-1

2

1Power in wind (watts),2

where air density (kg m ) wind speed (m s ) = the intercepting area (m )

AV

VA

ρ

ρ

=

=

=

Maximum power extraction is 59% (Betz)

11

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

12

Introduction – power curve

• Power produced

• Tip speed ratio

312 PP AC Vρ=

RVωλ =

13

Wind turbine topologies

Losses in power electronics, more components, increased cost

Mechanical stress, limited power quality control

Increased energy capture, improved power quality, reduced mechanical stress

Simple, robust, reliable and well proven

Max. efficiency over a wide range of wind speeds

Max. efficiency at one particular wind speed

Variable speedFixed speed

14

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

15

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

16

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

17

Offshore wind turbines

Technology progression of 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 2010

18

Offshore wind turbines

Typical cost breakdown of an offshore wind plant in shallow water

19

Offshore substructures

Cost of Offshore Wind Turbine Substructures with Water Depth

20

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

21

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.

22

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

23

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

24

Floating technology challenges

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

25

Floating technology – Hywind

Hydro currently has a license to place a demonstration turbine offshore near Karmøy

• Power > 5MW• Height above sea 80m• Rotor diameter 120m• Water depth – 200-700m

26

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

27

Control of wind turbine• Classical control

– PI controllers– Multiple control loops, dynamics unknown

• Modern control– Pole placement– Linear Quadratic Control– control– Adaptive control– Disturbance Accommodating Control (DAC) – Model Predictive Control

H∞

28

Control of offshore wind turbine

312

is the tip-speed ratio

PPCAvρ

λ

=⎛ ⎞⎜ ⎟⎝ ⎠

29

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

30

Aerodynamics of blade section

2

2

1212

, - lift and drag coeff. depend on (AoA)

L L

D D

L D

F AW C

F AW C

C C

ρ

ρ

α

⎛ ⎞= ⎜ ⎟⎝ ⎠⎛ ⎞= ⎜ ⎟⎝ ⎠

31

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

32

Control of offshore wind turbine

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

33

Control of offshore wind turbine

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

34

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

35

Control of offshore wind turbine

• Drive train damping– Serious impact on gear box

( ) ( )

- angular displacement- aerodynamic torque

- generator torque

- lumped inertia - lumped stiffness

aero gen

aero

gen

J t K t T T

TT

JK

θ θ

θ

+ = −

36

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

37

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

38

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, control, adaptive control, Disturbance Accommodating Control (DAC), MPC

– State estimator

H∞

39

Control of offshore wind turbine• DAC control

– Augment states with disturbances• Control

– Include uncertinitoes• MPC

– Include constraints (generator power, shaft speed, limits on DOFs)

• Adaptive control– Updating model parameters

• Controller complexities Vs implementation

H∞

40

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

41

Conclusions

• Classical control– Decentralized control

• Present research– State feed back control– Linearized models

• Challenges– Complex dynamics– Unknown disturbances– Design issues

42

Thank youfor

yourattention ☺

43

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