11

th

INTERNATIONAL SYMPOSIUM on

POWER ELECTRONICS - Ee 2001

XI Me

NOVI SAD, YUGOSLAVIA, OCT. 31 – NOV. 2, 2001

1

Abstract: Variable speed wind turbines are a growing,
dominant principle of design for power converters
applied in wind power turbines today. Up to 60% of all
wind turbines built in the near past are variable speed
wind turbines, surely 70% of the ones built in 2001, and
up to 80% of these that will be built in 2002. The recent
plans for large, high-power, offshore Wind Parks with
power of 0.2-2 GW all include variable speed wind
turbines.
Variable speed wind turbines can use more wind, due to
the fact that they adapt to the particularity of the wind
power itself - the changeable force of the wind. They
start at lower wind speeds, and increase the power with
speed. The design itself may be more demanding than
classic, constant-speed turbine, but the reported energy
increase of up to 10% is rewarding.

SEMIKRON has delivered power converters for

variable speed wind turbines to various suppliers and
wind turbine manufacturers, with different circuit
configurations and designs. Most solutions are the so-
called IGBT STACKs configurations, IGBT together with
heat sink, DC link capacitors, built in protection
features, insulation and auxiliary power supply. Power
STACKs are delivered for up to 480V grid voltage, with
a 1200V IGBT. For the grid voltages of 690V, IGBTs of
1700V are used. All such delivered power systems are
100% tested in application conditions, and ready to use.
Proper sizing of converters in the whole operation cycle
is possible using SEMIKRON calculation software,
SEMISEL, available on Internet, on the
www.semikron.com page.
The applied circuit designs, with their benefits and
disadvantages, are shown and explained. The existing
solutions include solutions for Asynchronous Induction
Generator and for Synchronous Generator. All these
circuits have been built and delivered, in power ranges
from 500 kW up to 2.5 MW. Typical construction designs
is shown.
For higher wind turbine power ranges, spatially for the
offshore applications, recommended solution would be

medium and high voltage drive. That are the wind
turbines with the generator voltage in the range of 3.3kV,
as well as 4.2kV or 6.3kV, and more. The corresponding
semiconductor power converters are completely new
constructed.
Key Words: Power Electronics, wind power, IGBT,
variable drive

1.

INTRODUCTION

In general, there are two wind power generator

principles. An old and very simple one, is with constant,
fixed rotor speed. Generator is a simple ac induction
motor, with squirrel cage rotor, connected direct, (or via
transformer), to the utility grid. Generator mode of
operation started when the rotor speed is higher than
synchronous speed.

Fig.1 Torque vs. Speed, (slip)

The advantage of such construction is that it is very

simple. Power electronics part is only W3C, Thyristor
circuit, called soft starter, used for starting-up procedure.
Wind turbine can produce the power, only if the wind is
strong enough, and the rotor speed is higher than the
synchronous speed. Control of the produced power,
when the wind is stronger, is only possible due to
mechanical pitch control of the blades. (In case of weak
wind, the rotor blades are in the “full open” position,
“catching” the whole wind energy. When the wind is

State of the Art of Variable Speed Wind

Turbines

Dejan Schreiber

Application Manager, SEMIKRON International

Sigmundstrasse 200, 90431 Nuremberg, Germany

Phone: [phone] fax: [phone]

e-mail: [e-mail]

2

stronger than rated value, (approximately 12m/s), pitch
control will take the rotor blades in the position that only
a part of wind energy is “caught”. Such mechanical
control is slow, and the transient overload conditions are
very often and large. That produce the mechanical stress
and the vibrations of the tower. For the larger wind
turbines, 1.5 MW or more, with rotor diameter over 70m,
the investment for the stronger towers are very high,
comparing to the other wind turbine principles, such as
the principle with variable rotor speed.

Variable speed wind turbines are a growing,

dominant principle of design for power converters
applied in wind power turbines today. Up to 60% of all
wind turbines built in the near past are variable speed
wind turbines, surely 70% of the ones built in 2000, and
up to 75% of these that will be built in 2001. The recent
plans for large, high-power, offshore Wind Parks with
power of 0.2-2 GW all include variable speed wind
turbines.

Variable speed wind turbines can use more wind, due

to the fact that they adapt to the particularity of the wind
power itself - the changeable force of the wind. They
start at lower wind speeds, and increase the power with
speed. The design itself may be more demanding than
classic, constant-speed turbine, but the reported energy
increase of up to 10%, or 15% is rewarding.

The principle is similar to the variable speed as motor

4-qudrant drive. Fig.1 shows torque vs. speed
characteristics for the constant motor voltage supply
frequency. If the supply frequency is lower, or higher,
we will have the family of the characteristics, shown on
Fig.2. We can see that the generator mode of operation is
possible to achieve at the different rotor speeds, if the
supply frequency is changed.

Fig.2 Torque vs. speed with the full line-side

converter and motor side inverter

With advancements in power electronics components,

especially IGBT-a, such complex solutions became
reality, with efficient cost, exploitation, and reliability.

2.

PRINCIPLES OF VARIABLE SPEED WIND

TURBINES

For the variable speed wind turbines, several power

electronics circuit are in use, with induction or
synchronous motor, used as a generator.

The commonly used circuits are:

1. Simple induction, squirrel cage motor

Rare, but using a synchronous motor, with line side

converter and motor side inverter, both for full generated
power, shown on Fig. 3.

Fig 3. 4-Q AC Drive

Induction motor / generator with the full line-side

converter and motor side inverter, both for full generated

power

Advantages

- Simple AC Induction motor;
- No minimum and maximum turbine speed limits;
- Generated power and voltage increase with the speed;
- Possible VAR-reactive power control;

Disadvantages

- Two full-power converters in series;
- High dv/dt applied at generator windings;
- Power loss of up to 3% of generated power;
- Big DC-link capacitors;
- Line side inductance of 12-15% of generated power

2. Speed control by slip-power recovery

Induction motor with slip rings and wound rotor, as

well as the converter and inverter for rotor-power
recovery. For that principle circuit, three different mode
of operation are possible.

2.1 Stator winding has to be connected to the grid,

only when the rotor speed is near synchronous speed.
Rotor circuit is always connected to the grid, over
inverter and converter. For the lower generator speed
(80%), 20% of the generator power will be supplied from
the grid to the rotor. For the higher rotor speed (120%),
20% of the generator power will be supplied to the grid
via rotor circuit. That circuit is known also as Cascade
circuit, and it is shown on Fig. 4.

2.2 Stator and rotor circuit with inverter and

converter, are always connected together, and at the
beginning disconnected from the grid. Wind start
rotation, and for the lower speed than the rated one, the
rotor inverter produce the frequency (Frotor), so that
frequency corresponding to the speed Frotation + Frotor
= 50Hz, (line frequency). For the stronger wind and
higher speed, the output frequency is Frotation - Frotor=
50Hz. Stator winding and rotor converter, will be

3

connected to the grid, when the generator voltage is
synchronized with grid.

That principle of operation is used for the generator

which has to supply constant frequency, and are driven
with variable shaft speed.

For both solutions, rotor side inverter operates at

lower output frequency, between 10 Hz and near to zero
Hz. The connection between rotated rotor and inverter is
made via slip rings.

Fig 4.

Speed control by slip-power recovery

Induction motor / generator with slip rings, wound

rotor, converter and inverter for rotor-power recovery

Advantages

- Two power converters in series for rotor-power
exchange (usually 20-30% of the generated power) only
- Maximum power is 120-130% of the motor power
- Semiconductor power losses up to 0,6-0,9% of the
generated power
- Line-side inductance is only 3-4.5% (12-15% of the
rotor power)
- VAr -reactive power control

Disadvantages

- AC Induction motor with slip rings and rotor windings,
non-standard design
- Maintenance problem
- Minimum and maximum turbine-speed limits, (75% -
125%) corresponding to the rotor-power exchange
(usually 20-30% of generated power)
- Rotor-side converter operates at low frequency,
therefore double size semiconductors needed
- High dv/dt applied at rotor windings
- High frequency current through the rotor bearings
- Non-standard start-up and protection procedure

2.3 Solution with similar circuit but without slip rings

is with stator with two three-phase windings. One
winding is connected to the grid, and the other is
connected to the converter and inverter for rotor-power
recovery. Energy transfer from the rotor to the additional
stator windings is achieved in inductive way, as in a
simple transformer. Rotor power can be taken or given
using different directions and frequency of the inverter.

Additional advantages of that circuit are: no slip rings,
and no need for lower inverter frequency. Disadvantage:
additional stator winding,

3.

VARIABLE SPEED WIND TURBINES WITH

SYNCHRONOUS MOTOR

When the synchronous motor is in use, there is no

need for inverter on the generator side; simple rectifier
can be used. Synchronous generator has separate
excitation and no reactive power supply, as by induction
motor is needed. The output voltage of synchronous
generator is lower at lower speed, therefor is one boost
chopper built-in, between the rectifier and the DC link
capacitors. At the lower speed, boost chopper pump the
rectified generator voltage, up to the DC link value
necessary for the line side converter operation,
(Vdc>Vline-peak).

The circuit is shown on the Fig. 5.

Fig 5. Synchronous motor / generator with the

rectifier, boost chopper, and line-side converter for the

full generated power

That circuit is often used without gear box, using low

speed synchronous generator. In use are the circuits
without boost chopper too. DC Link voltage control is
achieved due to excitation control of the synchronous
generator.

Advantages

- No minimum and maximum turbine-speed limits
- Generated power and voltage increase with speed
- VAR-reactive power control possible
- Simple generator-side converter and control
- No high dv/dt applied to the motor windings

Disadvantages

- Two (three) full-power converters in series
- Power loss of up to 2-3% of the generated power
- Large DC link capacitors
- Line-side inductance of 10-15% of the generated power

4.

POWER ELECTRONICS USED IN VARIABLE

SPEED WIND TURBINES

SEMIKRON has delivered power converters, power

part, without the controller, for variable speed wind
turbines to various suppliers and wind turbine
manufacturers, with different circuit configurations and
designs. About 70 % of variable speed wind turbines are
running using SEMIKRON components. Most solutions

4

are the so-called IGBT STACKs configurations, IGBT
switchers, together with heat sink, DC link capacitors,
built-in protection features, insulation and auxiliary
power supply. Power STACKs are delivered for up to
480V grid voltage, with a 1200V IGBT blocking voltage.
For the grid voltages of 690V, IGBTs of 1700V are used.
All such delivered power systems are 100% tested in
application conditions, and ready to use and delivered, in
power ranges from 250kW up to 2.5 MW. Typical
construction design, of 400 kVA, at 3 x 690V, is shown
on the Fig.6.

Fig. 6

IGBT STACK for 400 kVA, at 3 x 690V

5.

CALCULATION OF OPERATING JUNCTION

TEMPERATURE

Proper sizing of converters in the whole operation

cycles is simple possible using calculation software
developed for that porpoise. It is available in Internet
under http://www.semikron.com and it is simple in use.
For any common power electronic application, first the
circuit has to be selected. After that the circuit
parameters have to be defined:

input voltage
output voltage
cosinusphi
output power
output current
switching frequency
output frequency
load and overload parameter
factor
duration
user defined load cycle
min. output frequency
min. output voltage

After that, you can select your package and device:

for example SkiiP1092GB170, and on the next page is to
define ambient and heatsink parameters. After pressing
the calculate button, calculation results are available. All
temperatures vs. time are shown too, as on Fig. 7

Fig. 7

Heatsink, Diode and IGBT temperatures vs. time at

overload conditions

6.

CONCLUSION

Power electronics found new application in wind

turbines with variable rotation speed. Of all types of
motor and generator speed control, the widespread one
is the one usually rarely used in four quadrant mode of
operation. That is the asynchronous motor with wound
rotor and speed control by slip-power recovery. This
classical method, started with thyristors in use and
mostly used in the '60 and '70, is now finding fresh
application.

At the same time, use of synchronous generator is

spreading, applying diode rectifier, step-up chopper and
line side inverter. This, better, method is allowing full
regulation of generator speed, with less power
electronics components compared to the asynchronous
generator with short-circuited rotor.

It is characteristic that, in both cases, the dominant

solutions are the pioneering ones, coming out of the need
for pragmatic solutions of the problem, but without
participation of famous and successful firms - the
manufacturers of the control speed motor drives.

However, as this new industrial branch develops, and

is reaching the sum of yearly trade of around ten billion
Euro, the pioneering times of wind turbine construction
are over. New, powerful forces, equally potent in
research and development as they are in investments -
the big industrial manufacturers - are progressing into the
stage. This will certainly have an impact on the
development of power electronics in this field of use.

7.

REFERENCES

[1] SEMIKRON Data Book
[2]

www.semikron.com

[3] SEMISEL Calculation Program on

http://semisel.semikron.com/semisel/start.asp

[4] D. Srajber, W. Lukasch: “The Calculation of The

Power Dissipation for the IGBT and the Inverse Diode in
the Circuits with the Sinusoidal Output Voltage”
Electronica´92 München

[5] Messe Windtech 2001 Husum September 2001.,

Data sheets: ENERCON, VESTAS, Neg-Micon,
RePower, Enron, DeWind, FRISIA, Pfleiderer, SEG,
Weier, mTorres