Modeling and Comparison of Power
Converters for Doubly Fed Induction
Generators in Wind Turbines
Ph.D. thesis by Lars Helle
1
Agenda
1.
2.
3.
4.
5.
6.
7.
Introduction
Wind turbine comparison issues
Models of surrounding components
Converter modeling
Design and maturing efforts
System comparison – an example
Conclusion and perspectives
2
Introduction
140
120
¾ Dramatic increase in the use of
windpower
¾ Requirements for active and reactive
power control.
¾ Increasing turbine size
¾ Need for active load reduction control
¾ Demand for reduced energy price
¾ Optimized technical solution
ªThese requirements seems to call
for a variable speed wind turbine
Accumulated worldwide
installed wind power [GW]
100
80
60
40
20
1980
2000
1990
2010
Year
1500 Average turbin e p ow er of new
installations[kW]
1000
500
0
1980 1985 1990 1995 2000 2005
Year
Wind energy price [DK K/kWh]
1
0.8
0.6
0.4
0.2
0
1980 1985 1990 1995 2000 2005
Year
3
Introduction
1. Several variable speed topologies
are potential candidates:
1.
2.
3.
4.
Converter fed IG
DFIG
SG with external magnetization
SG with PM magnetization
2. All these topologies need a
frequency converter
3. Good models are required to
select the most appropriate
solution.
Converter
IG
Converter
DFIG
Converter
Converter
Converter
SG
PMSG
4
Wind turbine comparison
1.
2.
3.
4.
5.
6.
Cost
Efficiency
Reliability
Serviceability
Weight
Volume
η
Two-level
Matrix
? ?
? ?
Multi-level
5
Wind turbine comparison
1. Cost (Qualitative
meas.)
¾ Cooling needs (loss)
¾ Component count
¾ Component util.
η
Two-level
Matrix
2. Efficiency
(Quantitative meas.)
¾ Converter losses
¾ Annual energy
production
? ?
? ?
Multi-level
3. Reliability
(Qualitative meas.)
¾ Component count
¾ Maturity
6
Wind turbine comparison
¾ Four converters are evaluated for a DFIG system:
¾
¾
¾
¾
Back-to-back two-level converter
Matrix converter
Diode clamped three-level converter
Transistor clamped three-level converter
¾ Common basis for evaluation
¾ Harmonic distortion (HDF)
¾ Equal turbine power capability (in terms of operating
temperature)
¾ Maturity of the converters
7
Wind turbine comparison
¾ All modeling and design guidelines are based on
analytical approaches in order to obtain:
¾ Very fast evaluation for the entire operating area
¾ Easy comparison and design evaluation based on information
from standard datasheets.
¾ Easy access to the modeling approaches.
¾ “Transparent” documentation for the obtained results
8
Models of surrounding components
¾ Establish analytical models of the wind turbine components
in order to:
¾ Calculate the correct loading of the converter in arbitrary working
conditions
¾ Determine the effect of changing surrounding component properties.
Wind
power
Aerodyn.
loss
Gear
loss
Turbine blades
Cp
Gear
Fric.
loss
Elec. Switch Cond.
loss
loss
loss
Gener ator
Converter
Elec.
loss
Produced
power
Transformer
Grid
Stator power
9
Converter modeling
¾ Model of the four converters:
Net filter
Grid filter
Generator filter
Rotor filter
A
1
sw
2
sw
3
sw
Net filter
Net filter
B
Generator filter
Generator filter
C
Matrix Converter
10
Converter modeling
- Model of converter losses
¾ Model of the converter conducting
losses:
iC
it
id
ir
¾ Model of the converter switching
losses:
11
Converter modeling
- Model of converter temperature
¾ Calculation of peak semiconductor temperatures for the
different converter topologies and modulation method:
k·Tda
k·Ttb Zthjc,d
Zthjc,d k·T ta
k·Tdb Zthjc,t
Zthjc,t
Z thx
T tb
Td b
Ptb
Pdb
Zthch
Zthh a
Tam b
Tta
C th x1
Rthx1
Cth x2
Rthx 2
Cth xy
Rthxy
Tda
Pta
Pda
12
Converter modeling
- Model of converter temperature
13
Converter modeling
- Model of converter temperature
14
Design and maturing efforts
¾ Design for equal harmonic performance
¾ New modulation methods for three-level converters
¾ New modulation methods for matrix converters
¾ Filter design guide lines
15
Design and maturing efforts
50
0
200
0
-200
1.1
1.2
1.3
-400
Resistor current [A]
-50
DC-link voltage [V]
Output voltage [V]
Load current [A]
-Modulation strategy for three-level inverters
1.1
1.2
1.3
1.4
1.2 1.3
Time [s]
1.4
20
350
300
10
250
200
1.1
1.2 1.3
Time [s]
0
1.1
16
Design and maturing efforts
-Modulation strategy for three-level inverters
Load current [A]
After imbalance is
applied
50
0
-50
Output voltage [V]
1.06
1.08
0
1.1
200
0
-200
-400
50
-50
Output voltage [V]
Load current [A]
Before imbalance is
applied
1.36
1.38
1.4
1.36
1.38
Time [s]
1.4
200
0
-200
1.06
1.08
Time [s]
1.1
-400
17
Design and maturing efforts
-Modulation strategy for three-level inverters
18
System comparison – an example
1. All the developed models have been
implemented in a simulation tool “D’rives”
2. D’rives calculates all component loadings,
component temperatures, losses etc. in a few
seconds for the entire operating area. -> initial
design validation
3. D’rives calculates the power capability
determined by the individual components – also
within a few seconds.
19
System comparison – an example
20
System comparison – an example
21
System comparison – an example
22
System comparison – an example
23
System comparison – an example
24
System comparison – an example
25
System comparison – an example
¾ 2MW turbine
¾ Diameter of 80 m
¾ Gear ratio of 1:100
¾ Max tip speed of 72 m/s (steady
state)
¾ Max tip speed of 80 m/s (transient)
26
System comparison – an example
¾ 2MW turbine
¾ Diameter of 80 m
¾ Gear ratio of 1:100
¾ Max tip speed of 72 m/s (steady
state)
¾ Max tip speed of 80 m/s (transient)
¾ DFIG generator
¾ Nom. speed 1719 RPM
¾ Winding ratio 1.5 (Y-Y connection)
¾ Nom. stator voltage 690V (p-p)
27
System comparison – an example
¾ Two-level converter design (as
an example):
Gen. inverter
Grid
inverter diodes
IGBT
diodes
IGBT
28
System comparison – an example
¾ Power losses of the two-level
converter designs
Grid inverter losses
Rotor inverter losses
29
System comparison – an example
¾ Power losses of the two-level
converter designs
30
System comparison – an example
¾ The power capability of the designed
converters is determined by operating
each design until it reached maximum
temperature.
(TC)
¾ The switch utilization factor:
31
System comparison – an example
¾ The power losses of the considered
converter designs become:
¾ The normalized annual energy
production:
(TC)
¾ The component counts of the
considered converter designs
32
Conclusion
Tools and methods could
¾ Comparison of different converter
wind turbine based on a DFIG.
be used for any drive train
configuration
topologies
for use in a
¾ Analytical expressions for individual component currents and
losses.
¾ Methods for calculating peak temperatures in the semiconductors.
¾ A comprehensive tool D’rives for evaluating a certain turbine
layout under various loading conditions.
¾ Maturing efforts.
¾ Design guidelines for the different converter topologies in order to
obtain a fair comparison.
¾ New modulation methods for the matrix converter.
¾ New modulation methods for the three-level converters.
¾ An example of a design and comparison of 4 different
converter topologies.
33
Perspectives
¾ Semiconductor lifetime estimation
¾ Converter design margins due to speed variations
¾ Including an estimation of current harmonics
34
Thank You For Your Attention
35