Transcript Wind Energy

Power Electronics and Control
in Wind Energy Conversion Systems
Final Project
Simon P. Teeuwsen
Class 453 Electric Drives
March 15, 2001
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 V1 to V3 .
Hence, the power of the air stream is
P0  AR 

2
 V13
and the power absorbed by the turbine is
PW  AR 

2
 (V12  V32 ) V2
AR : effective cross section area
V2 : air speed in the rotor area
:
air density
According to Betz [2], the maximum wind turbine output is
PW max 
for
2
V2  V1
3
16

 AR  V13
27
2
and
1
V3   V1
3
The ratio of the power absorbed by the turbine to that of the
moving air mass is the so called performance coefficient:
PW
cP 
P0
The maximum performance coefficient is given for
cP max 
PW max
P0
PW  PW max
16

 AR  V13
16
2
 27

 0.593
 3
27
AR  V1
2
To get the maximum efficiency, the performance coefficient
should be near its maximal value of 0.593
tip speed ratio :  
Vu
blade tip speed

V1 wind speed upstream
Conclusion
The performance coefficient
cP 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
PW  TW   s
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.
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
• 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
• 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
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)
Pitch Control
Variation of the yaw angle between rotor blade and the direction
of wind pressure changes the effective flow rotor cross section
Reduction of the effective flow rotor cross section leads to a
drastic drop of the performance coefficient:
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
rotor will be inhibited
near operating point:
• Wind speed beyond
rated range:
Rotor torque and performance
coefficient decrease (!)
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 !
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 !
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
Optimal Performance Control
Performance Control by
• Controlling the rotation speed
• Adjusting the excitation for
synchronous generators
• Variation of the stator frequency
for asynchronous generators
Example for a Synchronous Generator
with Frequency Converter
Generator
Rectifier
Converter
+
SG
_
Advantages:
Excitation
• use of standard components instead of a
complicated electrical system
• wide range of speed and torque
R
S
T
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
Power Control and Grid Connection
Rectifier
Generator
generator
variables
Intermediate-Circuit
~
Inverter
Grid
=
=
rectifier
variables
~
intermediate-circuit
variables
inverter
variables
control, plant management and monitoring
grid
variables
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
Control Strategy Example 1
Control and management of a fixed speed grid
connected wind power plant with blade pitch adjustment:
state interrogation
Remote Monitoring
Management System
parameter input
desired
values
n, f

External
Influences
V
Regulation
V
actual
values
V
voltage
Energy
Feed
(wind)
P0
PW
Pel
Generator
Grid
frequency
Plant
State
f
n  constant
f
Control Strategy Example 2
Control and management of a wind power plant
operated at variable speed with blade pitch adjustment:
state interrogation
Remote Monitoring
Management System
parameter input
desired
values
n, f

External
Influences
V1
V
Regulation
actual
values
V1
voltage
Energy
Feed
(wind)
P0
PW
Pel
Generator
Grid
frequency
Plant
State
n
fG
~
~
f1
f1
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