Future Directions in Wind Power
Conversion Electronics
Bob Erickson
Colorado Power Electronics Center
University of Colorado, Boulder
ece.colorado.edu/~pwrelect
rwe@colorado.edu
(303) 492-7003
ECE Department
University of Colorado, Boulder
80309-0425
Power Conversion in Variable-Speed
Wind Power Systems
Wind turbine
Ac-ac
converter
Generator
Critical issues:
• Maintaining high
efficiency over a wide
range of voltages and
wind speeds
• Reduction of capital
cost
• Quality of electrical
waveforms injected
into utility and
generator
Variable-voltage
variable-frequency
three-phase ac
AC power
to utility
480 V
three-phase
60 Hz
Wind turbine
Doubly-fed
Generator
Stator
480 V
three-phase
60 Hz
AC power
to utility
Rotor
Variable-voltage
variable-frequency
three-phase ac
Ac-ac
converter
About Power Electronics Technology
• Evolution of magnetics and capacitor technology is slow
• Evolution of microprocessor/microcontroller technology is rapid
• Evolution of power semiconductor technology is rapid
o Low voltage (< 1kV) power semiconductors are inexpensive and
exhibit high performance
o Progress in high voltage controlled devices such as HVIGBT’s
• Major gains in packaging technology
Conclusion— where to focus research thrusts:
Use of silicon to make significant gains in converter
performance, size, and/or cost
• Use silicon to improve performance
• Increased intelligence and complexity; finer structure
• Improve efficiency, reduce capital cost, improve waveform quality,
improve reliability
Conventional
converters are
not optimized
for variablespeed wind
power
applications
• Poor efficiency in
Region II reduces
energy captured
• A smaller converter
could attain higher
efficiency at low
wind?
Generator
power or
voltage
Region I
Region II
Region III
Speed
Converter
efficiency
The Problem of Poor Converter
Efficiency at Low Wind Speed
• We showed that the
origin of this problem is
the reduction of
converter efficiency that
occurs when the
generator voltage is
reduced
• Other mechanisms, such
as circulating currents in
resonant converters or
in doubly-fed systems,
can also contribute to
this phenomenon
100%
Typical efficiency vs. power throughput
Two-level dc link system in wind power environment
Inverter
Rectifier
Efficiency
• Typically observed in
variable-speed wind
generator systems
95%
Composite
• Includes semiconductor conduction
and switching losses
90%
• PWM rectifier and inverter
• Rectifier losses dominate at light load
85%
0%
20%
40%
60%
P/Pmax
80%
100%
Indirect Power in PWM Boost Rectifier
Input of conventional DC link
system reduces to boost rectifier:
–
vin
(vout – vin)
+
–
DTs
Ts
+
+
–
iout
+
4vin
vout
3vin
–
When the converter is required
to process substantial indirect
power, efficiency is degraded.
This mechanism explains the
observed problems in variablespeed wind power applications
vout
2vin
Indirect power
= iout (vout – vin)
vin
Direct power = ioutvin
0
0.2
0.4
0.6
Transistor duty cycle
0.8
1
Reconfigurable AC-DC converters
+
L
DC
output
H
L
AC input
–
Single-phase PWM boost converter example
Reconfigure converter to improve efficiency at low input
voltage, while maintaining high output voltage
Measured Efficiencies of
Single-Phase PWM Boost Rectifiers
Two-Level vs. Reconfigurable Three-Level PWM
0.96
Three level (doubler mode)
Efficiency
0.94
Two level
0.92
0.9
• DC link voltage = 385 V
• Constant power
0.88
0.86
80
100
120
140
160
180
200
AC input voltage (Vrms)
220
240
260
Multi-Level Switching
Two-level
switching
t
Three-level
switching
t
2-Level Converters
100%
Improvement
of Converter
Efficiency
via ThreeLevel
Switching
Inverter
Rectifier
95%
Composite
90%
85%
0%
20%
40%
60%
80%
100%
60%
80%
100%
P/Pmax
3-Level Converters
100%
Predicted by
experimentallyverified model of
semiconductor
conduction and
switching loss
Inverter
Rectifier
Composite
95%
90%
85%
0%
20%
40%
P/Pmax
Discussion
• Switching loss can be modeled by equations of the form
Psw = (∆v) Q fsw
•
•
•
•
Multilevel switching reduces the voltage step (∆v), and
hence improves efficiency at full load
Multilevel switching reduces the indirect power at low
input voltage
Efficiency at light load is improved, and the knee of the
efficiency curve is shifted to the left
Resonant conversion and/or soft switching techniques may
be unnecessary
How to realize multilevel switching?
The Case for Small Module Size
• Low-voltage IGBT’s have very low cost
– Less than $1 in high volume for 600V 50A 100KHz
IGBT: specific cost of $0.03/KVA
– Higher voltage IGBT modules typically have specific
costs of $0.50/KVA
• Built by machine on printed circuit boards: low
manufacturing cost
• High quality utility and machine waveforms
• Lower switching loss and better utilization of
silicon
• Improved efficiency at low wind speed
A new family of ac-ac matrix converters
capable of multilevel switching
Basic converter
Modular switch cell
n
b
Three-phase
ac system 1
a
A
Realization
ic
Three-phase
ac system 2
c
A
B
iA
iB
+
–
a
ib
–
+
N
C
iC
Q1
D1
a
Q3
+
–
ia
–
+
+
–
–
+
Symbol
D3
D2
A
D4
Q2
Q4
At rated voltage: two-level operation
At low voltage: three-level operation
Advantages of Proposed Converters
• Multilevel conversion is possible, even in the basic
version. This enables improvement of the low-wind
efficiency of the converter, without sacrificing
performance at rated power
• The converter can both step up and step down the voltage
magnitude
• Switch commutation is simple
• Modular construction allows scaling to higher voltage and
current levels, using inexpensive low-voltage silicon
• Simple bus bar structures
• High quality waveforms
New Modular Multilevel Matrix Converter
in a Wind Power Application
Variable-voltage
variable-frequency
three-phase ac
SAa
SAa
SAa
Generator
Wind turbine
Proposed
new matrix
converter
Switch cell:
Q1
Converter contains a matrix of
switch cell modules
SBa
SBa
SBa
SCa
SCa
SCa
D1
a
Q3
D3
D2
A
D4
AC power
to utility
60 Hz
Q2
Q4
Variable-voltage
variable-frequency
three-phase ac
Increasing
the number
of levels
AC power
to utility
60 Hz
Generator
Wind turbine
Proposed
new matrix
converter
Doubly-fed
Generator
Doubly fed
system
Stator
AC power
to utility
60 Hz
Rotor
Wind turbine
Variable-voltage
variable-frequency
three-phase ac
Proposed
new matrix
converter
Experimental Data
Utility-side AC
voltage and current
(60 Hz)
Machine-side AC
voltage and current
(30 Hz)
60 Hz to 30 Hz Data
Maintenance of
DC capacitor
voltage
Upper trace: capacitor
voltage of one switch
module
Lower trace: 60 Hz
current injected into
utility
Controller Block Diagram
Implemented in Verilog and downloaded
into programmable logic arrays
Issues: Multilevel Modular Converters
• Complexity of control of individual module
voltages and currents
– Centralized control algorithm not feasible as
number of modules is increased
– Requires new decentralized control approaches
• Topologies: interconnection of modules
– Other modular topologies may allow better
control
– Effect on efficiency
Conclusions
• The variable-speed wind power application
requires better ac-ac converters having
– Lower capital cost
– Improved efficiency over a wide range of wind speeds
and generator voltages
– Better terminal waveforms
• Electronic power converters having finer structure
are becoming feasible:
– Inexpensive, high performance silicon switches
– Sophisticated controllers
– High level of packaging technology
Conclusions
Continued
• Multilevel switching can address the issues of
variable speed wind power
– Reduced switching loss improves efficiency without
need for resonant techniques
– Improved efficiency over wide range of wind speeds
– Improved waveform quality
• New modular converter topologies
– Allow scaling to higher powers and higher voltages
– Could allow use of advances in packaging and lowvoltage silicon in megawatt applications
– Need additional work in decentralized control and
modular topologies