This document discusses various aspects of wind turbine power electronics and control systems. It begins with basics on how wind turbines convert kinetic wind energy into rotational energy. It then covers different types of generators that can be used, such as asynchronous and synchronous, and various ways to connect them to the electrical grid directly or via power converters. The document also discusses methods for controlling the turbine speed, such as pitch control and stall control, as well as optimizing the turbine's performance through control of the generator excitation or frequency. Finally, it provides two examples of control strategies - one for a fixed speed grid-connected system and one for a variable speed system with pitch control.

1. Power Electronics and Control
in Wind Energy Conversion Systems
Final Project
Simon P. Teeuwsen
Class 453 Electric Drives
March 15, 2001

2. Basics about Wind Turbines
• Flowing air represents a moving mass that contains kinetic energy.
• The windturbine converts a part of this energy into rotation energy
by decelerating the wind velocity from to .
1
V 3
V
Hence, the power of the air stream is
3
1
0
2
V
A
P R 



and the power absorbed by the turbine is
2
2
3
2
1 )
(
2
V
V
V
A
P R
W 





density
air
:
area
rotor
the
in
speed
air
:
V
area
section
cross
effective
:
2

R
A

3. 3
1
max
2
27
16
V
A
P R
W 




According to Betz [2], the maximum wind turbine output is
for and
1
2
3
2
V
V 
 1
3
3
1
V
V 

0
P
P
c W
P 
The ratio of the power absorbed by the turbine to that of the
moving air mass is the so called performance coefficient:
max
W
W P
P 
593
.
0
27
16
2
2
27
16
3
1
3
1
0
max
max 








V
A
V
A
P
P
c
R
R
W
P


The maximum performance coefficient is given for

4. To get the maximum efficiency, the performance coefficient
should be near its maximal value of 0.593
upstream
speed
wind
speed
tip
blade
V
V
ratio
speed
tip u


1
: 

5. Conclusion
Differentiation between:
P
c
The performance coefficient is to maximize!
 slow running multi blade turbines with a large torque
(i.e. for pumping purposes) and
s
W
W T
P 


 fast running little blade turbines with smaller torque,
but a lot bigger efficiency (high performance coefficient)
i.e. for generation of electric power

6. Energy Converter Systems
There are a plenty of different ways to connect these generators to the grid.
Most often employed are three phase generators of the following type:
• Asynchronous Generator (Induction Engine)
• Synchronous Generator

7. • Direct Grid Connection
• AC-DC plus DC-AC Converter
• AC-AC Converter
Asynchronous Generator (Slip Ring Rotor) with Gear System
• Rotor Voltage Injection by 2nd Generator on the Shaft
• RVI by the Grid with AC-DC plus DC-AC Converter
• RVI by the Grid with AC-AC Converter
Synchronous Generator (Separated Excited) with Gear System
• Direct Grid Connection
• AC-DC plus DC-AC Converter
• AC-DC plus DC-AC Converter as Gearless Unit
Synchronous Generator (Permanently Excited) as Gearless Units
• AC-DC plus DC-AC Converter
• AC-AC Converter
Asynchronous Generator (Short-Circuit Rotor) with Gear System

10. • a) and g) show extremely rigid grid coupling
• h) for DC supply
• i), j) and k) must draw their reactive power from the grid
• f) and g) allow control of reactive power, are also able to
provide the reactive power necessary themselves and can control
the voltage in grid branches

11. Speed Control of the Wind Turbine
• Pitch Control (smaller systems)
• Stall Control (rated outputs of 30 kW and over)
Why speed control?
• Adjust speed to control the power flow
• Drive the system at its optimal performance
• Protection from over-revving
How to control the speed?

12. Pitch Control
Variation of the yaw angle between rotor blade and the direction
of wind pressure changes the effective flow rotor cross section

13. Reduction of the effective flow rotor cross section leads to a
drastic drop of the performance coefficient:

14. Aerodynamic design of the rotor blades:
Stall Control
• Low wind speed: Laminar flow obtains the rotor blades
• High wind speed Further torque development at the
near operating point: rotor will be inhibited
• Wind speed beyond Rotor torque and performance
rated range: coefficient decrease (!)

15. Generator and Turbine Torque
The speed torque characteristic for the wind turbine
depends on the wind speed !
The speed torque characteristic for the generator depends
on the generator type and the grid connection !

16. Direct Grid Coupling
The Grid sets a Constant Frequency:
• Synchronous generators are constraint by the grid frequency
• Asynchronous generators vary for increasing wind speed from
this frequency because of the increasing slip
When the wind speed lies below
nominal levels, the machines
act as motors and drive the turbine !

17. Indirect Grid Coupling using Converter
Wind Turbine Frequency is Independent on the Grid Frequency !
Synchronous Generator:
The optimal turbine performance
can be found by adjusting the
excitation of the generator

18. Optimal Performance Control
Performance Control by
• Controlling the rotation speed
• Adjusting the excitation for
synchronous generators
• Variation of the stator frequency
for asynchronous generators

19. Example for a Synchronous Generator
with Frequency Converter
SG
Excitation
R
S
T
+
_
Rectifier Converter
Generator
Advantages:
• use of standard components instead of a
complicated electrical system
• wide range of speed and torque

20. Example for a Rotor Cascade Induction
Generator System
The rectified slip power can be recovered by feeding it to the net
via an inverter and a transformer:
Advantages:
any operating point above the
synchronous speed can be reached
by controlling the rectified rotor
current with the inverter

21. Power Control and Grid Connection
Generator
Rectifier
~
=
=
~
Inverter
Intermediate-Circuit Grid
generator
variables
rectifier
variables
intermediate-circuit
variables
inverter
variables
grid
variables
control, plant management and monitoring

22. • Constant or not constant grid frequency
• Controlled or uncontrolled wind energy supply
• Isolated or grid operation
• Wind turbine with and without blade adjustment
• Fixed or variable turbine speed
... or combinations depending on the system and the desired operation
There are plenty of different control strategies:

23. Control Strategy Example 1
Control and management of a fixed speed grid
connected wind power plant with blade pitch adjustment:
Generator
voltage
frequency
Management System Remote Monitoring
desired
values
state interrogation
parameter input
V f
Grid
el
P
Energy
Feed
(wind)
Plant
State
External
Influences
W
P
0
P
Regulation
actual
values
V
n, f
f
V
n  constant


24. Control and management of a wind power plant
operated at variable speed with blade pitch adjustment:
Control Strategy Example 2
Generator
voltage
frequency
Grid
Energy
Feed
(wind)
Plant
State
External
Influences
W
P el
P
0
P
Management System Remote Monitoring
desired
values
state interrogation
parameter input
1
V 1
f
~
~ 1
f
G
f
n
Regulation
actual
values
V
n, f

1
V

25. References
[1] Grid Integration of Wind Energy Conversion Systems,
Siegfried Heier
John Wiley & Sons, 1998
[2] Wind-Energie und ihre Ausnutzung durch Windmühlen,
A. Betz
Vandenhoeck und Ruprecht, 1926
[3] Variable Speed AC-Generators in Wind Energy Convertors
O. Carlson, J. Hylander
Chalmers University of Technology, Sweden
[4] Enercon Homepage