wind energy i. lesson 9. control strategies

Wind Energy I

Michael Hölling, WS 2010/2011 slide 1

Control strategies

Wind Energy I

slideMichael Hölling, WS 2010/2011 2

Class content

4 Wind power

5 Wind turbines in general 6/7 Wind - blades

interaction

9 Control strategies

8 Power losses at the rotor blade

10 Generator

11 Electrics / grid

3 Wind field characterization

2 Wind measurements

Wind Energy I


Control objectives and strategies

Development of a wind turbine control system can be divided into four major steps:

define clearly control objectives

selection of suitable control strategies which determines the operation point of the wind turbine for each wind speed

decide how the control strategy will be realized --> selection of the control schemes, the controlled variables, the reference signals, the switching procedure between different controllers, etc.

design of the input-output map, meaning the characteristics of the controller according to the specifications

Wind Energy I


Control objectives for wind turbines

Control objectives

Energy capture: Maximization of energy capture taking into account safe operation restrictions such as rated power, rated speed and cut-out wind speed, etc.

Mechanical loads: Preventing WECS from excessive dynamic mechanical loads. This general goal includes reduction of transient loads, reduction of high frequency loads and resonance avoidance.

Power quality: Conditioning the generated power to comply with interconnection standards.

Wind Energy I


Operation point

Where / what is the steady-state of operation ?

the steady-state of operation is reached when the aerodynamic torque developed by the rotor equals the reaction torque of the generator

net torque applied to the system is zero

At the steady-state operation point the aerodynamical power equals the converted power (minus losses at the generator):

Pae = Pgen

Tae · ! = Tgen · !

! · (Tae ! Tgen) = 0" Tae ! Tgen = 0

Wind Energy I


Torque and power coefficient

Aerodynamic torque Tae:

Tae =12

· ! · " · R2 · u21 · R · cT

Tae = Fae · R · cT

The power converted by the WEC is given by:

PWEC = Tae · ! = Pair · cp

! cT = cp · 1!

How does the aerodynamic torque change with u1 ?

Wind Energy I


Aerodynamic torque

The torque coefficient can be determined from the power coefficient. Until now we determined the maximum power coefficient by taking into consideration:

Betz limit with the expansion by Schmitz

losses at the rotor blades (drag losses and tip losses)

0 5 10 15 200.0

0.2

0.4

0.6

!

cp

r(!

)

cpSchmitz

cpSchmitz, z=3,"(#)=60

These curve represents the maximum power coefficient for each tip speed ratio BUT design is only possible for one tip speed ratio λ0!

λ0

Wind Energy I


Aerodynamic torque

0 5 10 15 200.0

0.2

0.4

0.6

!

cpr(!)

λ0

Without control system the WEC is designed and optimized for one u1 and one ω.

β

α

u2= 2/3.u1

ures

urot

α*

β*

u2= 2/3.u*1

u*res

urotby changing u1 to u*1 and with

it λ το λ*

the angle of attack changes

and cl(α) to cl(α*) and cd(α) to cd(α*)

as well

Wind Energy I


Aerodynamic torque

0 5 10 15 200.0

0.2

0.4

0.6

!

cpr(!)

λ0

0.0 0.5 1.00.0

0.2

0.4

0.6

u3/u1

cp

cp

λ0

0 5 10 15 200.0

0.2

0.4

0.6

!

cp

r(!

)cpSchmitz

cpSchmitz,

z=3,"(#)=60

cp!0(!,#)

λ0

From Betz we know that there is one optimum ratio between u3 and u1. The WEC meets this at the design for λ0.

The cp coming from Betz in combination with the changing angle of attack for different u1, we get a power coefficient that depends on λ and α - cp(λ,α)

Wind Energy I


Aerodynamic torque

Note: In reality λmax and λ0 must NOT necessarily coincide !!

cp(λ) and cT(λ) can be plotted:! cT = cp · 1!

With

0 4 8 120.0

0.2

0.4

0.6

0.00

0.05

0.10

0.15

!

cp(!)

cp(!)

cT(!)

cT(!)

Wind Energy I


Aerodynamic torque

How does the torque change with changing ω for different but fixed u1?

0 4 8 120.0

0.2

0.4

0.6

!

cp(!)

cp(!)

Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

!

cp(!)

Wind Energy I


Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

!

cp(!)

!

torq

ue

[N

m]

u1 = 25m/s

Wind Energy I

slideMichael Hölling, WS 2010/2011

!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

!

cp(!)

13

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

cp(!)

!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

14

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

!

cp(!)

15

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

cp(!)

!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

16

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

!

cp(!)

17

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

cp(!)

!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

18

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

!

cp(!)

19

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

cp(!)

!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

u1 = 6m/s

20

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

u1 = 6m/s

u1 = 4m/s!

cp(!)

21

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

Wind Energy I


!

torq

ue [N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

u1 = 6m/s

u1 = 4m/s

Tcpmax

22

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

!

cp(!)

Wind Energy I


!

torq

ue

[N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

u1 = 6m/s

u1 = 4m/s

Tcpmax

Trated power

23

Aerodynamic torque


Tae =12

· ! · " · R2 · u31 · cp

!# · R

u1

"

# $% &!

· 1#

!

cp(!)

Wind Energy I


Control strategies

fixed-speed, fixed-pitch (FS-FP)

variable-speed, fixed-pitch (VS-FP)

fixed-speed, variable-pitch (FS-VP)

variable speed, variable-pitch (VS-VP)

Points in this torque-rotational speed plane (Tae-ω plane) that intersect with the generator torque define the steady-state operating conditions of the WEC. Different WEC control strategies results in different power curves P(u1), power coefficients cp(u1) and dynamical behavior.

Different strategies are:

Wind Energy I


Control strategies

The interesting region for the control system is marked in the red box.

!

torq

ue [N

m]

u1 = 25m/s

u1 = 22m/s

u1 = 20m/s

u1 = 17m/s

u1 = 14m/s

u1 = 12m/s

u1 = 10m/s

u1 = 8m/s

u1 = 6m/s

u1 = 4m/s

Tcpmax

Trated power

Wind Energy I


Fixed-speed, fixed-pitch

Fixed rotational-speed ω0 is realized by coupling an asynchronous generator directly to the grid.

0 4 8 120.0

0.2

0.4

0.6

!

cp(!)

cp(!)

!

torq

ue

[N

m]

u1 = 17m/s

u1 = 8m/s

u1 = 4m/s

Trated power

Tcpmax

u1min

u1max

!0AA

B

B

CC

D

D

Wind Energy I



For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For FS-FP the P(u1) curve looks in principle like:

!

torq

ue

[N

m]

u1 = 17m/s

u1 = 8m/s

u1 = 4m/s

Trated power

Tcpmax

u1min

u1max

!00 5 10 15 20 25 30

u1 [m/s]

P(u

1)/

Pra

ted

ideal power curve

power curve

C

C

BB

A A

D

D

Wind Energy I



For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For FS-FP the cp(u1) curve looks in principle like:

!

torq

ue

[N

m]

u1 = 17m/s

u1 = 8m/s

u1 = 4m/s

Trated power

Tcpmax

u1min

u1max

!00 5 10 15 20 25 30

u1 [m/s]

cp(u

1)

ideal cp

real cp

AA

B

B

CC D

D

Wind Energy I



Power regulation by passive stall

Wind Energy I


Variable-speed, fixed-pitch

For a variable-speed, fixed.pitch machine the rotational speed ωrot can be adapted to meet the optimum tip speed ratio λ0.

!rot ="0 · u1

R

The rotational speed ωrot changes linearly with the ambient wind speed u1.This is applied in the region below rated wind speed.

Wind Energy I



Variable rotational-speed ωrot is realized by adding AC/DC-DC/AC converter before feeding into the grid.

0 4 8 120.0

0.2

0.4

0.6

!

cp(!)

cp(!)A-E

D

G

Wind Energy I


For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For VS-FP the P(u1) curve looks in principle like:


Wind Energy I


For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For VS-FP the cp(u1) curve looks in principle like:


Wind Energy I


Fixed-speed, variable-pitch

There are two different ways to adjust the pitch to keep the power above rated wind speed constant:

pitch to feather pitch to stall

Wind Energy I



By adjusting the angle of attack the cp(λ) curves are different for each pitch angle:

tip speed ratio

Wind Energy I



These modified cp(λ) curves result in modified torque above rated wind speed to meet the rated power:

Wind Energy I


For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For FS-VP the P(u1) curve looks in principle like:


Wind Energy I


For the characterization of the WEC power curve P(u1) and cp(u1) are of interest. For FS-VP the cp(u1) curve looks in principle like:


Wind Energy I


Variable-speed, variable-pitch

wind energy i. lesson 9. control strategies

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