1. Power Electronics and Control in Wind Energy Conversion Systems
Final Project
Simon P. Teeuwsen
Class 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
Hence, the power of the air stream is
and the power absorbed by the turbine is

3. According to Betz [2], the maximum wind turbine output is
for and
The ratio of the power absorbed by the turbine to that of the
moving air mass is the so called performance coefficient:
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

5. Conclusion The performance coefficient is to maximize!
Differentiation between:
slow running multi blade turbines with a large torque
(i.e. for pumping purposes) and
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
Most often employed are three phase generators of the following type:
Asynchronous Generator (Induction Engine)
Synchronous Generator
There are a plenty of different ways to connect these generators to the grid.

7. Asynchronous Generator (Short-Circuit Rotor) with Gear System
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
AC-DC plus DC-AC Converter as Gearless Unit
Synchronous Generator (Permanently Excited) as Gearless Units

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
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?
Pitch Control (smaller systems)
Stall Control (rated outputs of 30 kW and over)

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. Stall Control Aerodynamic design of the rotor blades:
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
Rectifier
Converter
Generator + R S T SG _
Advantages:
use of standard components instead of a
complicated electrical system
wide range of speed and torque
Excitation

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:
any operating point above the
synchronous speed can be reached
by controlling the rectified rotor
current with the inverter
Advantages:

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

22. There are plenty of different control strategies:
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

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

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

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