Future Directions in Wind Power Conversion Electronics

Bob EricksonColorado Power Electronics Center

University of Colorado, Boulder

ece.colorado.edu/~pwrelectrwe@colorado.edu(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