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Future Directions in Wind Power Conversion Electronics
Bob EricksonColorado Power Electronics Center
University of Colorado, Boulder
ece.colorado.edu/[email protected](303) 492-7003ECE DepartmentUniversity of Colorado, Boulder80309-0425
Power Conversion in Variable-Speed Wind Power Systems
AC powerto utility
480 Vthree-phase
60 Hz
Generator
Wind turbine
Ac-acconverter
Variable-voltagevariable-frequency
three-phase ac
AC powerto utility
480 Vthree-phase
60 Hz
Doubly-fedGenerator
Wind turbine
Ac-acconverter
Variable-voltagevariable-frequency
three-phase ac
Rotor
Stator
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
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 variable-speed wind power applications
Generatorpower orvoltage
Speed
Region I Region II Region III
Converterefficiency
• Poor efficiency in Region II reduces energy captured
• A smaller converter could attain higher efficiency at low wind?
The Problem of Poor Converter Efficiency at Low Wind Speed
85%
90%
95%
100%
0% 20% 40% 60% 80% 100%
P/Pmax
Composite
Rectifier
Inverter
Effi
cien
cy
Typical efficiency vs. power throughputTwo-level dc link system in wind power environment
• Includes semiconductor conduction and switching losses
• PWM rectifier and inverter
• Rectifier losses dominate at light load
• Typically observed in variable-speed wind generator systems
• 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
Indirect Power in PWM Boost Rectifier
Transistor duty cycle
0 0.2 0.4 0.6 0.8 1
vin
2vin
3vin
4vin
Indirect power = iout (vout – vin)
vout
Direct power = ioutvin
+–
+
vout
–
vin
DTs Ts
+–
iout– (vout – vin) +
When the converter is required to process substantial indirect power, efficiency is degraded. This mechanism explains the observed problems in variable-speed wind power applications
Input of conventional DC link system reduces to boost rectifier:
Reconfigurable AC-DC converters
HL
L
+
DCoutput
–
AC input
Single-phase PWM boost converter example
Reconfigure converter to improve efficiency at low input voltage, while maintaining high output voltage
Measured Efficiencies ofSingle-Phase PWM Boost Rectifiers
Two-Level vs. Reconfigurable Three-Level PWM
0.86
0.88
0.9
0.92
0.94
0.96
80 100 120 140 160 180 200 220 240 260
AC input voltage (Vrms)
Two level
Three level (doubler mode)Efficiency
• DC link voltage = 385 V• Constant power
Multi-Level Switching
t
t
Two-level switching
Three-level switching
Improvement of Converter Efficiency via Three-Level Switching
Predicted by experimentally-verified model of semiconductor conduction and switching loss
2-Level Converters
85%
90%
95%
100%
0% 20% 40% 60% 80% 100%P/Pmax
Composite
Rectifier
Inverter
3-Level Converters
85%
90%
95%
100%
0% 20% 40% 60% 80% 100%P/Pmax
Composite
RectifierInverter
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 converterscapable of multilevel switching
+–
+–
+–
+ –+ –
+ –
a b cA
B
C
n
N
ia ib ic
iA
iB
iC
Three-phaseac system 1
Three-phaseac system 2
Basic converter
Q1 Q2
Q3 Q4
D1 D2
D3 D4
a A
a A
Modular switch cellSymbol
Realization
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 Converterin a Wind Power Application
SAa
SBa
SCa
SAa
SBa
SCa
SAa
SBa
SCa
AC powerto utility60 Hz
Generator
Wind turbine Proposednew matrixconverter
Variable-voltagevariable-frequency
three-phase ac
Q1 Q2
Q3 Q4
D1 D2
D3 D4
a A
Switch cell:
Converter contains a matrix of switch cell modules
AC powerto utility60 HzGenerator
Wind turbine Proposednew matrixconverter
Variable-voltagevariable-frequency
three-phase ac
AC powerto utility60 Hz
Doubly-fedGenerator
Wind turbine
Variable-voltagevariable-frequency
three-phase ac
Rotor
Stator
Proposednew matrixconverter
Increasing the number
of levels
Doubly fed system
Experimental DataUtility-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 voltageUpper 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
ConclusionsContinued
• 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 low-
voltage silicon in megawatt applications– Need additional work in decentralized control and
modular topologies