European Wind Energy Conference & Exhibition. February-March 2006, Athens.
INNOVATIVE SOLUTIONS IN POWER ELECTRONICS FOR VARIABLE SPEED WIND TURBINES
J.L. VILLATE, E. ROBLES, P. IBÁŃEZ, I. GABIOLA, S. CEBALLOS
ROBOTIKER , Parque Tecnológico Edif. 202 48170 Zamudio (Bizkaia), Spain.
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Tél : + 34 94 600 22 66, Fax : + 34 94 600 22 99
ABSTRACT: Wind energy has experienced a dramatic development over the last decade. Offshore wind farms with multi-
megawatt machines and variable speed solutions are gaining ground. The power converter is a key component in modern
wind turbines with higher power, higher efficiency and lower costs. In addition, the utilities demand increasingly exacting
power quality requirements, which can only be met if research on power converters is done. This paper presents a survey of
existing power electronics solutions for wind turbines, covering trends and new studies in this field.
Keywords: Power electronics, variable speed wind turbines, multilevel converters, matrix converters.
1 Introduction In this context, grid codes which define the new
requirements for grid connection of wind farms have
Wind energy has developed considerably over the either been developed or are in process in those
last decade. Its implementation in the electrical countries with a higher installed power. There are
system is already widespread in countries such as basically three main aspects covered in the grid codes
Germany, Denmark or Spain and an increasingly (1): voltage and reactive power control, frequency
greater impact is predicted. The implementation of control, and fault ride-through capabilities.
these technologies brings a set of advantages
foremost amongst which is a reduction in CO2 There are three main types of wind turbine at the
emissions and access to clean energy sources in those present time (2). On the one hand we have fixed-
countries without fossil energy resources. speed turbines based on asynchronous generators
with a squirrel-cage rotor, and on the other the
Established trends point to an increase in unitary variable speed turbines which use doubly-fed
power, with ever greater machines being asynchronous generators or synchronous generators
manufactured such as those shown in Fig. 1. This in a full converter configuration. (Fig. 2).
trend has led to the development of offshore wind
farms, where large multimegawatt turbines may be Fig. 3 shows the evolution in the use of these
installed. generators over the last few years. It can be observed
that variable speed configurations are gaining ground
Nevertheless, important problems and challenges over the asynchronous squirrel-cage generator.
remain to be tackled if we are not to endanger the
stability of the electrical system. The aim of this paper is to review the power
electronics solutions applicable to variable speed
turbines, with a view to meeting the challenges
5000
facing wind generation (higher powered turbines and
quality requirements and ever more demanding grid
4000
connections).
The article begins with a review of the commutation
3000
devices which are currently available on the market.
kW Three topologies are subsequently presented whose
features make them attractive for use in wind
2000
applications. Finally, an analysis of a multilevel
converter is carried out with the aim of validating the
most advantageous features within the field of wind
1000
generation.
0
1986198819901992199419961998200020022004
Fig. 1. Evolution of unitary power.
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European Wind Energy Conference & Exhibition. February-March 2006, Athens.
converter depends on the characteristics of these
devices.
They may be classified into three groups depending
GRID
AC-AC
on their degree of controllability.
1. Diodes. These are controlled by the
(a)
currents and voltages of the power circuit.
2. Thyristors (SCR). These devices are
activated through a control signal although
GRID
GEAR
they are then deactivated through the
power circuit.
3. Controllable switches. Activated and
deactivated through a control signal.
Amongst the latter group the MOSFET, IGBTs and
AC-AC
IGCTs should be stressed.
MOSFET devices are used in lower power
(b)
applications with high commutation frequencies.
Their main disadvantage is a high resistance to
conduction, though fortunately there are new
technologies available on the market which have
GRID
GEAR
resulted in the appearance of 'CoolMos', in which
conduction resistance has been reduced, thereby
improving behaviour.
The most used semiconductor devices are currently
the IGBT's. There are IGBTs from 600V to 3300V,
capable of withstanding currents up to 3600A
(depending on the voltage, see Fig. 4 [2]). The IGBT
(c)
has been improving its features over a period of time.
Nevertheless, due to its particular structure, an
Fig. 2. Wind turbine configurations. (a) Synchronous
ongoing improvement to an equal degree across all
generator. (b) Doubly fed asynchronous generator.
its features is not possible, which has led to
(c) Squirrel Cage Generator.
specialised devices being developed to perfect
particular features. We may thus find ultrafast
IGBTs possessing excellent speed features with
acceptable losses, together with generic IGBTs and
HVIGBTs (high voltage IGBTs) which allow for
high working voltages permitting their use in
medium voltage application.
A trend towards the use of IGBTs with NPT (non
punch through) structures can be seen in the market
in order to facilitate parallel connection.
Furthermore, we must not forget that this technology
is both more robust in the presence of short circuits
as well as cheaper to manufacture.
Finally, the appearance of IGCTs represents a
substantial improvement on GTOs since they
combine the high voltages and low losses of the latter
Fig. 3. Evolution of wind generator technology. with the high frequencies and small commutation
losses of the IGBTs.
2 Semiconductor devices Developments within the last two components
(IGBTs and IGCTs) together with a reduction in their
This section will make a brief analysis of the current cost have both been fundamental factors giving rise
state of semiconductor power devices. These devices to the implementation of PWM VSC (voltage source
represent the basic element in a converter converter) over the rectifiers and cicloconverters
commutation process, since the efficiency of the previously used.
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European Wind Energy Conference & Exhibition. February-March 2006, Athens.
Fig. 4 [3] shows the power range of some
semiconductor devices currently available on the
market.
C
All these devices are based on silicon, though the
material best placed for using in the near future will
be silicon carbide, due to its physical properties.
Amongst other advantages from using this material
we may stress its greater energy efficiency, improved
reliability and reduced maintenance costs, together
with greater operation frequencies, integration Fig. 5. Back to Back PWM two level converter.
density and high operation temperatures.
Multilevel Converters
Multilevel converters are based on connecting
together a set of various semiconductor devices,
thereby allowing greater working voltages to be
reached, which in turn increases the power they are
able to handle. This feature can be useful in current
wind applications, since the unitary power of each
wind turbine has increased exponentially over the
past few years as we have seen.
Its working principle is based on the generation of a
staircase waveform formed by more than two levels
of voltage. The waveforms thus generated present a
more sinusoidal nature than those generated by two-
level converters thereby allowing a greater quality in
generated energy.
The main advantages deriving from the use of this
type of topology are the following:
Fig. 4. Semiconductor devices currently available on
" Possibility of reaching high output voltages
the market.
without submitting the semiconductors to
high voltages.
3 Power Converters
" Better efficiency across the whole power
In this section we will analyse the most attractive
range, relatively more stressed when
power converters for use in wind applications. The
working with low input powers [4].
first to be analysed will be the two-level back to back
connection converter which is practically the only
" Low harmonic content in the voltages and
one which is currently used in this type of
currents generated, or in other words the
application. We will then go on to propose three
possibility of reducing the commutation
topologies (multilevel, matrix and AC link
frequency of the semiconductor devices
converters) whose features may prove useful in near
and size of the grid connection inductances
future applications.
while obtaining a similar quality as with a
two-level converter.
PWM back to back converter
All these advantages make the multilevel converter a
Fig. 5 shows a diagram of this converter. As shown
good alternative in wind energy applications. It helps
it is made up of two 2-level converters linked by
to fulfil the objectives of improved quality in energy
means of a DC bus. This bus allows us to uncouple
generated, and allows greater working powers to be
the two converters, whereby one does not influence
reached whilst minimising losses. This in turn results
the other so that they may be controlled separately.
in a simpler design of the dissipating elements.
However, the size and weight of the DC link can be
high, making a reduction desirable as far as possible.
These days a large number of topologies of
This objective brings us on to some of the following
multilevel converters are being proposed, with the
topologies, which are not exempt from problems
three main ones being  diode clamped topology",
themselves as shall be seen.
"flying capacitor topology" and "cascaded connection
of H bridges" [5]. Of these the most frequently used
at the present time is the three level diode clamped
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European Wind Energy Conference & Exhibition. February-March 2006, Athens.
converter since it requires a lesser number of Its main disadvantages include the fact that if
capacitive components and voltage sources than the overmodulation is not produced, the maximum level
other two. This topology is shown in Fig. 6. produced at output is 0.866 times that of the input.
As such, in order to obtain the same power as in a
back to back converter, it will be necessary to
oversize the semiconductor devices besides
increasing the conduction losses.
Another aspect which is necessary to improve in this
type of converter is its Ride-Through capability,
since due to the absence of the continuous bus there
are no energy storage elements.
In conclusion, we may say that despite the very
useful possibilities offered by this technology, it is
still not sufficiently mature to apply in real-life
situations, with several aspects such as the one above
still needing a solution. Despite of this, there are
Fig 6. Three level diode clamped converter. some works that incorporate this technology in wind
turbines [6].
Matrix Converters AC-link converter
Fig. 7 shows the scheme of a three-phase matrix Fig. 8 shows the scheme of this converter [7]. Its way
converter. As may be observed, it is generally made of working is completely different to the converters
up of an array of n x m bidirectional switches. commented on up to now. In this case it is based on
the transfer of load packets between input and output
Its working principle is based on connecting the (using the central capacitor), depending on the power
output phases to the input ones for as long as required for the application and the value of the
necessary to be able to obtain the desired average currents needed to generate at both input and output
voltage from the output and current from the input. at any given time.
The main characteristic of the matrix converter lies in Amongst its advantages we can stress a reduction in
its being a solution based wholly on silicon, without the size of the central capacitor and filters in
reactive elements to augment the weight, volume and comparison with the back to back converter, high
cost of the converter. Although it is necessary to performance, use of slow commutation devices such
improve its working by introducing an input filter to as SCRs and the low dv/dt it generates.
improve the currents generated and clamping circuit,
the size of these elements will always be smaller than Another important feature is its low cost making it
those used in other topologies. generally attractive for installation in turbines located
in areas lacking in wind.
Another important feature which should be stressed
is that this converter is totally bi-directional and also
very suitable to work in environments where the
power generated is continuously changing. This
makes it very attractive for use in wind systems.
a
b
Fig. 8. AC-link converter.
c
4 Analysis of a multilevel converter
In the previous section we have described three of the
B C
A
emerging topologies for use in wind applications. Of
these the most mature technology is that of multilevel
Fig. 7. 3x3 Matrix converter. converters, since despite being barely used in this
field it is very tried and tested in other applications.
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European Wind Energy Conference & Exhibition. February-March 2006, Athens.
Two of the main benefits presented by the topology 7 References
described above will be analysed in this section,
namely an increase in efficiency and quality [1] I. Martínez de Alegría, J. Andreu, J.L. Martín, P.
improvement in the currents generated. IbáÅ„ez, J.L. Villate, H. Camblong,  Connection
requirements for wind farm: A survey on
Fig. 9 [8] shows the results in performance obtained technical requirements and regulation,
by carrying out simulations of a two and three level Renewable and Sustainable Energy Review.
clamped converter by levels of 2MW commuting at
5KHz. It may be observed that the multilevel [2] L.H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie,
converter presents a greater efficiency across the S. Munk-Nielsen, H. Bindner, P. Sorensen and B.
whole working power range. Bak-Jensen,  Conceptual survey of Generators
and Power Electronics for Wind Turbines, RISØ
Experimental quality measurements of current National Laboratory, Roskilde, Denmark,
injected into the grid at different percentages of December 2001.
nominal power have also been taken. This
measurements show that the three level diode [3] S. Bernet,  Recent Developments of High Power
clamped converter allows to reduce the THD in a Converters for Industry and Traction
20% compared with those generated by a two level Applicatios, IEEE Trans. on Power Electronics,
converter. Vol. 15, nº6, pp. 1102-1117, November 2000.
[4] L.M. Tolbert, F.Z. Peng and T.G. Habetler
 Multilevel Inverters for Electric Vehicle
Applications, WPET 98. pp. 79-84, Dearborn,
Michigan 22-23 October 1998.
[5] J. Rodríguez, J-S Lai and F.Z. Peng,  Multilevel
Inverters: A Survey of Topologies, Controls and
Aplications, IEEE Trans. Indus. Electron., vol.
49, no. 4, pp. 724-738, August 2002.
[6] Patent. US 6856038, feb. 15 2005 Rebsdorf,
A.V, Helle, L,  Variable speed wind turbine
having a matrix converter . (Industrial property:
Vestas).
Fig. 9. Efficiency curves for two and three level
[7]
converters.
http://www.princetonpower.com/tech/ACLink_Tech
_Operation.pdf (Last access February 2006)
5 Conclusions
[8] J.L. Villate, S. Ceballos, E. Robles, P. IbáÅ„ez, I.
Gabiola,  Experimental validation of multilevel
This paper has carried out an analysis of
Converters for Variable Speed Wind Turbines
semiconductor devices currently available on the
EPE 2005. Dresden, September 2005.
market. Three topologies of converters are proposed
which may start to be installed in variable speed wind
turbines in the foreseeable future, responding to the
ever more exacting requirements demanded of wind
generation.
Finally, the features of one of the topologies
proposed (the multilevel converter) is analysed, and
compared with those of a two-level converter,
practically the only topology in current use. The
results clearly show the advantages which would be
brought by the use of multilevel topologies.
6 Acknowledgements
This work has been developed with the support of the
Basque Government under the programme
SAIOTEK and the Education and Science Ministry
of Spain (project RECENER ENE/2004-07881-C03-
03/ALT).
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INNOVATIVE SOLUTIONS IN POWE ELECT ONICS
INNOVATIVE SOLUTIONS IN POWE ELECT ONICS
INNOVATIVE SOLUTIONS IN POWE ELECT ONICS
INNOVATIVE SOLUTIONS IN POWE ELECT ONICS
FO VA IABLE SPEED WIND TU BINES
FO VA IABLE SPEED WIND TU BINES
FO VA IABLE SPEED WIND TU BINES
FO VA IABLE SPEED WIND TU BINES
J.L. Villate, E. obles, P. IbáÅ„ez, I. Gabiola, S. Ceballos
J.L. Villate, E. obles, P. IbáÅ„ez, I. Gabiola, S. Ceballos
J.L. Villate, E. obles, P. IbáÅ„ez, I. Gabiola, S. Ceballos
J.L. Villate, E. obles, P. IbáÅ„ez, I. Gabiola, S. Ceballos
1
1
1
1
OBOTIKE ENE GY BUSINESS UNIT
OBOTIKE ENE GY BUSINESS UNIT
OBOTIKE ENE GY BUSINESS UNIT
OBOTIKE ENE GY BUSINESS UNIT
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ABST ACT:
ABST ACT
ABST ACT
ABST ACT
Wind energy has experienced a dramatic development last decade. The offshore wind farms with multi-megawatt machines and variable speed
solutions are gaining ground. The power converter is a key component in modern wind turbines with higher power, higher efficiency and lower cost
needs. In addition, utilities demand increasingly exacting power quality requirements, which can only be met if research on power converters is done.
This paper presents a survey of existing power electronics solutions for wind turbines, trends and new studies in this field
160 m
126 m
112 m
15 m
85 87 89 91 93 95 97 99 01 03 05 ?
0.05 0.3 0.5 1.3 1.6 2 4.5 5 8/10 MW
EVOLUTION IN SIZE AND CAPACITY OF WIND TURBINES (Source: Jos Beurskens - ECN)
3-PHASE MATRIX CONVERTER
3-LEVEL CONVERTER
a
b
c
POWER SEMICONDUCTOR DEVICES
A B C
VOLTAGE DIP
Voltage
POWER
ELECTRONICS
SOLUTIONS
Time (s)
Source: Recent Developments of High Power Converters for Industry and
Source: Spanish grid code
Traction Apllications. Steffen Bernet
EFFICIENCY OF A 2 MW 3-LEVEL CONVERTER
Permanent
Permanent Magnet
Permanent
Permanent
GRID
AC-AC
Generator
Generator
Generator
Generator
VESTAS
GRID
GEAR
Doubly Fed Induction AC-AC
Doubly Fed Induction
Doubly Fed Induction
Doubly Fed Induction
GRID
Generator
Generator
Generator
Generator
VARIABLE SPEED WIND TURBINES
WIND GENERATOR TECHNOLOGY
100%
Squirrel Cage Gen.
90%
80%
Synchronous Gen.
70%
Doubly fed
60%
Asynchronous Gen.
50%
40%
30%
20%
10%
0%
1997 1998 1999 2000 2001 2002 2003
CONCLUSIONS
CONCLUSIONS
CONCLUSIONS
CONCLUSIONS
Wind turbines are evolving towards higher power and variable speed, partly related to the development of offshore wind farms. This
trend and the new requirements of grid codes make power converters a key component in modern wind turbines.
This paper analyses the market solutions in power semiconductor devices and new concepts for power conversion circuits. In
particular the potential advantages of multilevel converters, related to efficiency and power quality, are studied:
" Greater efficiency across the whole power range.
" Lower harmonic distortion.