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1002 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Power-Electronic Systems for the Grid Integration
of Renewable Energy Sources: A Survey
Juan Manuel Carrasco, Member, IEEE, Leopoldo Garcia Franquelo, Fellow, IEEE,
Jan T. Bialasiewicz, Senior Member, IEEE, Eduardo Galván, Member, IEEE,
Ramón C. Portillo Guisado, Student Member, IEEE, Ma. Ángeles Martín Prats, Member, IEEE,
José Ignacio León, Student Member, IEEE, and Narciso Moreno-Alfonso, Member, IEEE
Abstract—The use of distributed energy resources is increas-
ingly being pursued as a supplement and an alternative to large
conventional central power stations. The specification of a power-
electronic interface is subject to requirements related not only to
the renewable energy source itself but also to its effects on the
power-system operation, especially where the intermittent energy
source constitutes a significant part of the total system capacity.
In this paper, new trends in power electronics for the integration
of wind and photovoltaic (PV) power generators are presented.
A review of the appropriate storage-system technology used for
the integration of intermittent renewable energy sources is also
introduced. Discussions about common and future trends in re-
newable energy systems based on reliability and maturity of each
technology are presented.
Index Terms—Direct drives, doubly fed induction generator
(DFIG), flywheel, hydrogen, multilevel converter topologies, su-
percapacitors, superconducting magnetic energy storage (SMES),
wind diesel.
I. INTRODUCTION
T
HE INCREASING number of renewable energy sources
and distributed generators requires new strategies for the
operation and management of the electricity grid in order to
maintain or even to improve the power-supply reliability and
quality. In addition, liberalization of the grids leads to new
management structures, in which trading of energy and power
is becoming increasingly important. The power-electronic tech-
nology plays an important role in distributed generation and in
integration of renewable energy sources into the electrical grid,
and it is widely used and rapidly expanding as these applica-
tions become more integrated with the grid-based systems.
During the last few years, power electronics has undergone
a fast evolution, which is mainly due to two factors. The first
one is the development of fast semiconductor switches that
are capable of switching quickly and handling high powers.
The second factor is the introduction of real-time computer
controllers that can implement advanced and complex control
Manuscript received March 2, 2006; revised May 17, 2006. Abstract pub-
lished on the Internet May 18, 2006.
J. M. Carrasco, L. G. Franquelo, E. Galván, R. C. P. Guisado, Ma. Á. M.
Prats, J. I. León, and N. Moreno-Alfonso are with the Department of Elec-
tronics Engineering, University of Seville, 41092 Seville, Spain (e-mail:
leopoldo@gte.esi.us.es; ramonpg@gte.esi.us.es).
J. T. Bialasiewicz is with the Department of Electrical Engineering, Uni-
versity of Colorado, and the Health Sciences Center, Denver, CO 80217 USA
(e-mail: jan.bialasiewicz@cudenver.edu).
Digital Object Identifier 10.1109/TIE.2006.878356
algorithms. These factors together have led to the development
of cost-effective and grid-friendly converters.
In this paper, new trends in power-electronic technology for
the integration of renewable energy sources and energy-storage
systems are presented. This paper is organized as follows.
In Section II, we describe the current technology and future
trends in variable-speed wind turbines. Wind energy has been
demonstrated to be both technically and economically viable.
It is expected that current developments in gearless energy
transmission with power-electronic grid interface will lead to
a new generation of quiet, efficient, and economical wind
turbines. In Section III, we present power-conditioning systems
used in grid-connected photovoltaic (PV) generation plants.
The continuously decreasing prices for the PV modules lead
to the increasing importance of cost reduction of the specific
PV converters.
Energy storage in an electricity generation and supply system
enables the decoupling of electricity generation from demand.
In other words, the electricity that can be produced at times
of either low-demand low-generation cost or from intermittent
renewable energy sources is shifted in time for release at
times of high-demand high-generation cost or when no other
generation is available. Appropriate integration of renewable
energy sources with storage systems allows for a greater market
penetration and results in primary energy and emission savings.
In Section IV, we present research and development trends in
energy-storage systems used for the grid integration of intermit-
tent renewable energy sources.
II. W
IND-TURBINE TECHNOLOGY
A. Variable-Speed Wind Turbines
Wind energy has matured to a level of development where
it is ready to become a generally accepted utility generation
technology. Wind-turbine technology has undergone a dramatic
transformation during the last 15 years, developing from a
fringe science in the 1970s to the wind turbine of the 2000s
using the latest in power electronics, aerodynamics, and me-
chanical drive train designs [1], [2]. In the last five years,
the world wind-turbine market has been growing at over 30%
a year, and wind power is playing an increasingly important
role in electricity generation, especially in countries such as
Germany and Spain. The legislation in both countries favors
the continuing growth of installed capacity. Wind power is quite
0278-0046/$20.00 © 2006 IEEE
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1003
Fig. 1. Single doubly fed induction machine with two fully controlled ac–dc power converters.
different from the conventional electricity generation with syn-
chronous generators. Further, there are differences between the
different wind-turbine designs available on the market. These
differences are reflected in the interaction of wind turbines
with the electrical power system. An understanding of this is,
therefore, essential for anyone involved in the integration of
wind power into the power system.
Moreover, a new technology has been developed in the wind-
power market introducing variable-speed working conditions
depending on the wind speed in order to optimize the energy
captured from the wind. The advantages of variable-speed
turbines are that their annual energy capture is about 5%
greater than the fixed-speed technology, and that the active and
reactive powers generated can be easily controlled. There is also
less mechanical stress, and rapid power fluctuations are scarce
because the rotor acts as a flywheel (storing energy in kinetic
form). In general, no flicker problems occur with variable-speed
turbines. Variable-speed turbines also allow the grid voltage to
be controlled, as the reactive-power generation can be varied.
As disadvantages, variable-speed wind turbines need a power
converter that increases the component count and make the
control more complex. The overall cost of the power electronics
is about 7% of the whole wind turbine.
B. Current Wind-Power Technology
Variable-speed wind turbines have progressed dramatically
in recent years. Variable-speed operation can only be achieved
by decoupling the electrical grid frequency and mechanical
rotor frequency. To this end, power-electronic converters are
used, such as an ac–dc–ac converter combined with advanced
control systems.
1) Variable-Speed Concept Utilizing Doubly Fed Induction
Generator (DFIG): In a variable-speed turbine with DFIG [3],
[4], the converter feeds the rotor winding, while the stator
winding is connected directly to the grid. This converter, thus
decoupling mechanical and electrical frequencies and making
variable-speed operation possible, can vary the electrical rotor
frequency. This turbine cannot operate in the full range from
zero to the rated speed, but the speed range is quite sufficient.
This limited speed range is caused by the fact that a converter
that is considerably smaller than the rated power of the machine
is used. In principle, one can say that the ratio between the
size of the converter and the wind-turbine rating is half of the
rotor-speed span. In addition to the fact that the converter is
smaller, the losses are also lower. The control possibilities of the
reactive power are similar to the full power-converter system.
For instance, the Spanish company Gamesa supplies this kind
of variable-speed wind turbines to the market.
The forced switched power-converter scheme is shown in
Fig. 1. The converter includes two three-phase ac–dc converters
linked by a dc capacitor battery. This scheme allows, on one
hand, a vector control of the active and reactive powers of the
machine, and on the other hand, a decrease by a high percentage
of the harmonic content injected into the grid by the power
converter.
Vestas and Nordic Windpower supply a variation of this de-
sign, which is the semivariable-speed turbine, in which the rotor
resistance of the squirrel cage generator can be varied instantly
using fast power electronics. So far, Vestas alone has succeeded
in commercializing this system under the trade name OptiSlip.
A number of turbines, ranging from 600 kW to 2.75 MW,
have now been equipped with this system, which allows tran-
sient rotor speed increases of up to 10% of the nominal value. In
that case, the variable-speed conditions are achieved dissipating
the energy within a resistor placed in the rotor, as shown in
Fig. 2. Using that technology, the efficiency of the system
decreases when the slip increases, and the speed control is
limited to a narrow margin. This scheme includes the power
converter and the resistors in the rotor. Trigger signals to the
power switches are accomplished by optical coupling.
2) Variable-Speed Concept Utilizing Full-Power Converter:
In this concept, the generator is completely decoupled from the
grid [5]. The energy from the generator is rectified to a dc link
and after is converted to a suitable ac energy for the grid. The
majority of these wind turbines are equipped with a multipole
synchronous generator, although it is quite possible (but rather
rare) to use an induction generator and a gearbox. There are
several benefits of removing the gearbox: reduced losses, lower
costs due to the elimination of this expensive component, and
increased reliability due to the elimination of rotating mechan-
ical components. Enercon supplies such technology.
1004 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Fig. 2. Single doubly fed induction machine controlled with slip power
dissipation in an internal resistor.
Fig. 3 shows the scheme of a full power converter for a wind
turbine. The machine-side three-phase converter works as a
driver controlling the torque generator, using a vector control
strategy. The grid-side three-phase converter permits wind-
energy transfer into the grid and enables to control the amount
of the active and reactive powers delivered to the grid. It also
keeps the total-harmonic-distortion (THD) coefficient as low
as possible, improving the quality of the energy injected into
the public grid. The objective of the dc link is to act as energy
storage, so that the captured energy from the wind is stored as
a charge in the capacitors and may be instantaneously injected
into the grid. The control signal is set to maintain a constant
reference to the voltage of the dc link V
dc
. An alternative
to the power-conditioning system of a wind turbine is to use
a synchronous generator instead of an induction one and to
replace a three-phase converter (connected to the generator)
by a three-phase diode rectifier and a chopper, as shown in
Fig. 4. Such choice is based on the low cost as compared to
an induction generator connected to a voltage-source inverter
(VSI) used as a rectifier. When the speed of the synchronous
generator alters, the voltage value on the dc side of the diode
rectifier will change. A step-up chopper is used to adapt the
rectifier voltage to the dc-link voltage of the inverter. When the
inverter system is analyzed, the generator/rectifier system can
be modeled as an ideal current source. The step-up chopper
used as a rectifier utilizes a high switching frequency, so the
bandwidth of these components is much higher than the band-
width of the generator. Controlling the inductance current in the
step-up converter can control the machine torque and, therefore,
its speed. The Spanish Company MADE has marketed that
design.
3) Semiconductor-Device Technology: Improvements in the
performance and reliability of power-electronic variable fre-
quency drives for wind-turbine applications have been directly
related to the availability of power semiconductor devices with
better electrical characteristics and lower prices because the
device performance determines the size, weight, and cost of the
entire power electronics used as interfaces in wind turbines.
The insulated gate bipolar transistor (IGBT) is now the main
component for power electronics and also for wind-turbine
applications. They are now mature technology turn-on compo-
nents adapted to a very high power (6 kV–1.2 kA), and they are
in competition with gate turn-off thyristors (GTOs) for high-
power applications [6].
Recently, the integrated gated control thyristor (IGCT) has
been developed as a mechanical integration of a GTO plus a
delicate hard drive circuit that transforms the GTO into a mod-
ern high-performance component with a large safe operation
area (SOA), lower switching losses, and a short storage time
[7]. The comparison between IGCT and IGBT for frequency
converters that are used, especially in wind turbines, is ex-
plained below.
1) IGBTs have higher switching frequency than IGCTs, so
they introduce less distortion in the grid.
2) IGCTs are made like disk devices. They have to be
cooled with a cooling plate by electrical contact on
the high-voltage side. This is a problem because high
electromagnetic emission will occur. Another point of
view is the number of allowed load cycles. Heating and
cooling the device will always bring mechanical stress
to the silicon chip, and it can be destroyed. This is a
serious problem, especially in wind-turbine applications.
On the other hand, IGBTs are built like modular devices.
The silicon is isolated to the cooling plate and can be
connected to ground for low electromagnetic emission
even with higher switching frequency. The base plate of
this module is made of a special material that has exactly
the same thermal behavior as silicon, so nearly no thermal
stress occurs. This increases the lifetime of the device by
ten folds approximately.
3) The main advantage of IGCTs versus IGBTs is that they
have a lower
ON-state voltage drop, which is about 3.0 V
for a 4500-V device. In this case, the power dissipation
due to a voltage drop for a 1500-kW converter will be
2400 W per phase. On the other hand, in the case of IGBT,
the voltage drop is higher than IGCTs. For a 1700-V
device having a drop of 5 V, the power dissipation due
to the voltage drop for a 1500-kW condition will be 5 kW
per phase.
In conclusion, with the present semiconductor technology,
IGBTs present better characteristics for frequency converters in
general and especially for wind-turbine applications.
C. Grid-Connection Standards for Wind Farms
1) Voltage Fault Ride-Through Capability of Wind Turbines:
As the wind capacity increases, network operators have to
ensure that consumer power quality is not compromised. To
enable a large-scale application of the wind energy without
compromising the power-system stability, the turbines should
stay connected and contribute to the grid in case of a dis-
turbance such as a voltage dip. Wind farms should generate
like conventional power plants, supplying active and reactive
powers for frequency and voltage recovery, immediately after
the fault occurred.
Thus, several utilities have introduced special grid-
connection codes for wind-farm developers, covering reactive-
power control, frequency response, and fault ride through,
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1005
Fig. 3. Double three-phase VSI.
Fig. 4. Step-up converter in the rectifier circuit and full power inverter topology used in wind-turbine applications.
Fig. 5. E.ON Netz requirements for fault ride-through capability of wind
turbines connected to the grid.
especially in places where wind turbines provide for a signif-
icant part of the total power. Examples are Spain, Denmark,
and part of Northern Germany.
The correct interpretation of these codes is crucial for wind-
farm developers, manufacturers, and network operators. They
define the operational boundary of a wind turbine connected
to the network in terms of frequency range, voltage tolerance,
power factor, and fault ride through. Among all these require-
ments, fault ride through is regarded as the main challenge to
the wind-turbine manufacturers. Although the definition of fault
ride through varies, the German Transmission and Distribution
Utility (E.ON) regulation is likely to set the standard [8]. This
stipulates that a wind turbine should remain stable and con-
nected during the fault while voltage at the point of connection
drops to 15% of the nominal value (i.e., a drop of 85%) for a
period of 150 ms (see Fig. 5).
Only when the grid voltage drops below the curve, the turbine
is allowed to disconnect from the grid. When the voltage is in
the shaded area, the turbine should also supply a reactive power
to the grid in order to support the grid-voltage restoration.
2) Power-Quality Requirements for Grid-Connected Wind
Turbines: The grid interaction and grid impact of wind turbines
have been focused on during the past few years. The reason
behind this interest is that wind turbines are among the utilities
considered to be potential sources of bad power quality. Mea-
surements show that the power-quality impact of wind turbines
has been improved in recent years. Especially, variable-speed
wind turbines have some advantages concerning flicker. But, a
new problem arose with variable-speed wind turbines. Modern
forced-commutated inverters used in variable-speed wind tur-
bines produce not only harmonics but also interharmonics.
The International Electrotechnical Commission (IEC) initi-
ated the standardization on the power quality for wind tur-
bines in 1995 as part of the wind-turbine standardization in
TC88, and ultimately 1998 IEC issued a draft IEC-61400-21
standard for “power-quality requirements for Grid Connected
Wind Turbines” [9]. The methodology of that IEC standard
consists of three analyses. The first one is the flicker analysis.
IEC-61400-21 specifies a method that uses current and voltage
time series measured at the wind-turbine terminals to simulate
the voltage fluctuations on a fictitious grid with no source
of voltage fluctuations other than the wind-turbine switching
operation. The second one regards switching operations. Volt-
age and current transients are measured during the switching
operations of the wind turbine (startup at cut wind speed and
startup at rated wind speed). The last one is the harmonic analy-
sis, which is carried out by the fast Fourier transform (FFT)
algorithm. Rectangular windows of eight cycles of fundamental
frequency width, with no gap and no overlapping between
1006 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Fig. 6. Two HVDC transmission solutions. (a) Classical LCC-based system with STATCOM. (b) VSC-based system.
successive windows, are applied. Furthermore, the current total
THD is calculated up to 50th harmonic order.
Recently, high-frequency (HF) harmonics and interharmon-
ics are treated in the IEC 61000-4-7 and IEC 61000-3-6 [10],
[11]. The methods for summing harmonics and interharmonics
in the IEC 61000-3-6 are applicable to wind turbines. In order
to obtain a correct magnitude of the frequency components,
the use of a well-defined window width, according to the
IEC 61000-4-7, Amendment 1, is of a great importance, as
has been reported in [12]. Wind turbines not only produce
harmonics; they also produce interharmonics, i.e., harmonics
that are not a multiple of 50 Hz. Since the switching frequency
of the inverter is not constant but varies, the harmonics will also
vary. Consequently, since the switching frequency is arbitrary,
the harmonics are also arbitrary. Sometimes they are a multiple
of 50 Hz, and sometimes they are not.
D. Trends in Wind-Power Technology
1) Transmission Technology for the Future—Connecting
Wind Generation to the Grid: One of the main trends in wind-
turbine technology is offshore installation. There are great wind
resources at sea for installing wind turbines in many areas
where the sea is relatively shallow. Offshore wind turbines
may have slightly more favorable energy balance than onshore
turbines, depending on the local wind conditions. In places
where onshore wind turbines are typically placed on flat terrain,
offshore wind turbines will generally yield some 50% more
energy than a turbine placed on a nearby onshore site. The
reason is that there is less friction on the sea surface. On the
other hand, the construction and installation of a foundation
requires 50% more energy than onshore turbines. It should
be remembered, however, that offshore wind turbines have a
longer life expectancy than onshore turbines, which is around
25–30 years. The reason is that the low turbulence at sea gives
lower fatigue loads on the wind turbine.
Conventional heating–ventilation–airconditioning (HVAC)
transmission systems are a simple and cost-efficient solution for
the grid connection of wind farms. Unfortunately, for offshore
wind parks, the distributed capacitance of undersea cables is
much higher than that of overhead power lines. This implies
that the maximum feasible length and power-transmission ca-
pacity of HVAC cables is limited. Grid access technology in
the form of high-voltage dc (HVDC) can connect the wind-farm
parks to the grid and transmit the power securely and efficiently
to the load centers. Looking at the overall system economics,
HVDC transmission systems are most competitive at trans-
mission distances over 100 km or power levels of between
approximately 200 and 900 MW. The HVDC transmission
offers many advantages over HVAC [13].
1) Sending and receiving end frequencies are independent.
2) Transmission distance using dc is not affected by cable
charging current.
3) Offshore installation is isolated from mainland distur-
bances and vice versa.
4) Power flow is fully defined and controllable.
5) Cable power losses are low.
6) Power-transmission capability per cable is higher.
Classical HVDC transmission systems [as shown in
Fig. 6(a)] are based on the current source converters with
naturally commutated thyristors, which are the so-called line-
commutated converters (LCCs). This name originates from the
fact that the applied thyristors need an ac voltage source in order
to commutate and thus only can transfer power between two
active ac networks. They are, therefore, less useful in connec-
tion with the wind farms as the offshore ac grid needs to be
powered up prior to a possible startup. A further disadvantage
of LCC-based HVDC transmission systems is the lack of the
possibility to provide an independent control of the active and
reactive powers. Furthermore, they produce large amounts of
harmonics, which make the use of large filters inevitable.
Voltage-source-converter (VSC)-based HVDC transmission
systems are gaining more and more attention not only for the
grid connection of large offshore wind farms. Nowadays, VSC-
based solutions are marketed by ABB under the name “HVDC
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1007
Fig. 7. Multilevel back-to-back converter for a direct connection of a wind turbine to the utility grid.
Light” [14] and by Siemens under the name “HVDC Plus.”
Fig. 6(b) shows the schematic of a VSC-based HVDC trans-
mission system. This comparatively new technology (with first
commercial installation in 1999) has only become possible by
the development of the IGBTs, which can switch off currents.
This means that there is no need for an active commutation
voltage. Therefore, VSC-based HVDC transmission does not
require a strong offshore or onshore ac network and can even
start up against a dead network (black-start capability). But,
VSC-based systems have several other advantages. The active
and reactive powers can be controlled independently, which
may reduce the need for reactive-power compensation and
can contribute to the stabilization of the ac network at their
connection points [15].
2) High-Power Medium-Voltage Converter Topologies: In
order to decrease the cost per megawatt and to increase the effi-
ciency of the wind-energy conversion, nominal power of wind
turbines has been continuously growing in the last years [16].
The different proposed multilevel-converter topologies can
be classified into the following five categories [17]:
1) multilevel configurations with diode clamps;
2) multilevel configurations with bidirectional switch inter-
connection;
3) multilevel configurations with flying capacitors;
4) multilevel configurations with multiple three-phase
inverters;
5) multilevel configurations with cascaded single-phase
H-bridge inverters.
A common feature of the five different topologies of mul-
tilevel converters is that, in theory, all the topologies may be
constructed to have an arbitrary number of levels, although in
practice, some topologies are easier to realize than others.
As the ratings of the components increase and the switching
and conducting properties improve, the advantages of applying
multilevel converters become more and more evident. In recent
papers, the reduced content of harmonics in the input and output
voltages is highlighted together with the reduced electromag-
netic interference (EMI) [18]. Moreover, the multilevel convert-
ers have the lowest demands for the input filters or alternatively
reduced number of commutations [19]. For the same harmonic
performance as a two-level converter, the switching frequency
of a multilevel converter can be reduced to 25% that results
in the reduction of the switching losses [20]. Even though the
conducting losses are higher in the multilevel converter, the
overall efficiency depends on the ratio between the switching
and the conducting losses.
The most commonly reported disadvantage of the multilevel
converters with split dc link is the voltage unbalance between
the capacitors that integrate it. Numerous hardware and soft-
ware solutions are reported: the first one needs additional
components that increase the cost of the converter and reduce
its reliability; the second one needs enough computational
capacity to carry out the modulation signals. Recent papers
illustrate that the balance problem can be formulated in terms of
the model of the converter, and this formulation permits solving
the balancing problem directly modifying the reference voltage
with a relatively low computational burden [21], [22].
Trends on wind-turbine market are to increase the nominal
power (some megawatts) and due to the voltage and current
ratings. This makes the multilevel converter suitable for modern
high-power wind-turbine applications. The increase of voltage
rating allows for connection of the converter of the wind turbine
directly to the wind-farm distribution network, avoiding the use
of a bulky transformer [23] (see Fig. 7). The main drawback of
some multilevel topologies is the necessity to obtain different
dc-voltage independent sources needed for the multilevel mod-
ulation. The use of low-speed permanent-magnet generators
that have a large number of poles allows obtaining the dc
sources from the multiple wounds of this electrical machine, as
can be seen in Fig. 8. In this case, the power-electronic building
block (PEBB) can be composed of a rectifier, a dc link, and an
H-bridge. Another possibility is to replace the rectifier by an
additional H-bridge. The continuous reduction of the cost per
kilowatt of PEBBs is making the multilevel cascaded topologies
to be the most commonly used by the industrial solutions.
3) Direct-Drive Technology for Wind Turbines: Direct-drive
applications are on increase because the gearbox can be elimi-
nated. As compared to a conventional gearbox-coupled wind-
turbine generator, a direct-drive generator has reduced the
overall size, has lower installation and maintenance cost, has
a flexible control method and quick response to wind fluctua-
tions, and load variation. For small wind turbine, permanent-
magnet synchronous machines are more popular because of
their higher efficiency, high-power density, and robust rotor
structure as compared to induction and synchronous machines.
A number of alternative concepts have been proposed for direct-
drive electrical generators for use in grid-connected or stand-
alone wind turbines. In [24], the problem to adapt a standard
permanent-magnet synchronous machine to a direct-drive ap-
plication is presented. A complete design of a low-speed direct-
drive permanent-magnet generator for wind application is
depicted in [25] and [26].
A new trend that is very popular for propulsion systems
applications is to use an axial flux machine [27]. These new
1008 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Fig. 8. Five-level cascaded multilevel converter connected to a multipole low-speed wind-turbine generator.
machines are applied in small-scale wind and water-turbine
direct-drive generators because higher torque density can be
obtained in a more simple and easy way.
4) Future Energy-Storage Technologies Applied in Wind
Farms: Energy-storage systems can potentially improve the
technical and economic attractiveness of wind power, partic-
ularly when it exceeds about 10% of the total system energy
(about 20%–25% of the system capacity). The storage system
in a wind farm will be used to have a bulk power storage from
wind during the time-averaged 15-min periods of high avail-
ability and to absorb or to inject energy over shorter time peri-
ods in order to contribute to the grid-frequency stabilization.
Several kinds of energy-storage technologies are being ap-
plied in wind farms. For wind-power application, the flow (zinc
bromine) battery system offers the lowest cost per energy stored
and delivered. The zinc–bromine battery is very different in
concept and design from the more traditional batteries such
as the lead–acid battery. The battery is based on the reaction
between two commonly available chemicals: zinc and bromine.
The zinc–bromine battery offers two to three times higher
energy density (75–85 W · h per kilogram) along with the size
and weight savings over the present lead/acid batteries. The
power characteristics of the battery can be modified for selected
applications. Moreover, zinc–bromine battery suffers no loss of
performance after repeated cycling. It has a great potential for
renewable energy applications [28].
As the wind penetration increases, the hydrogen options
become most economical. Also, sales of hydrogen as a vehicle
fuel are more lucrative than reconverting the hydrogen back
into electricity. Industry is developing low-maintenance elec-
trolysers to produce hydrogen fuel. Because these electrolysers
require a constant minimum load, wind turbines must be in-
tegrated with grid or energy systems to provide power in the
absence of wind [28].
Electrical energy could be produced and delivered to the grid
from hydrogen by a fuel cell or a hydrogen combustion gener-
ator. The fuel cell produces power through a chemical reaction,
and energy is released from the hydrogen when it reacts with the
oxygen in the air. Also, wind electrolysis promises to establish
new synergies in energy networks. It will be possible to grad-
ually supply domestic-natural-gas infrastructures, as reserves
diminish, by feeding hydrogen from grid-remote wind farms
into natural-gas pipelines. Fig. 9 shows a variable-speed wind
turbine with a hydrogen storage system and a fuel-cell system
to reconvert the hydrogen to the electrical grid.
III. PV T
ECHNOLOGY
This section focuses on the review of the recent develop-
ments of power-electronic converters and the state of the art
of the implemented PV systems. PV systems as an alternative
energy resource or an energy-resource complementary in hy-
brid systems have been becoming feasible due to the increase
of research and development work in this area. In order to
maximize the success of the PV systems, a high reliability, a
reasonable cost, and a user-friendly design must be achieved
in the proposed PV topologies. Several standards given by the
utility companies must be obeyed in the PV-module connection.
Nowadays, the standards EN61000-3-2 [29], IEEE1547 [30],
and the U.S. National Electrical Code (NEC) 690 [31], and the
future international standard (still a Committee Draft for Vote-
CDV) IEC61727 [32] are being considered. These standards
deal with issues like power quality, detection of islanding
operation, grounding, etc. They define the structure and the
features of the present and future PV modules.
A. Market Considerations
Solar-electric-energy demand has grown consistently by
20%–25% per annum over the past 20 years, which is mainly
due to the decreasing costs and prices. This decline has
been driven by 1) an increasing efficiency of solar cells;
2) manufacturing-technology improvements; and 3) economies
of scale. In 2001, 350 MW of solar equipment was sold to add
to the solar equipment already generating a clean energy. In
2003, 574 MW of PV was installed. This increased to 927 MW
in 2004. The European Union is on track to fulfilling its own
target of 3 GW of renewable electricity from PV sources for
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1009
Fig. 9. Variable-speed wind turbine with a hydrogen storage system and a fuel-cell system that reconverts hydrogen to electrical grid.
Fig. 10. PV energy applications. (a) Grid-connection application. (b) Power-supply application.
2010, and in Japan, the target is 4.8 GW. If the growth rates of
the installation of PV systems between 2001 and 2003 could
be maintained in the next years, the target of the European
Commission’s White Paper for a Community Strategy and
Action Plan on Renewable Sources of Energy would already
be achieved in 2008. It is important to notice that the PV
installation growth-rate curve in the European Union exactly
mirrors that of wind power, with a delay of approximately
12 years. This fact predicts a great future for PV systems in
the coming years.
B. Design of PV-Converter Families
An overview of some existing power inverter topologies for
interfacing PV modules to the grid is presented. The approaches
are further discussed and evaluated in order to recognize the
most suitable topologies for future PV converters, and, finally,
a conclusion is given.
Due to advances in transistor technology, the inverter
topologies have changed from large thyristor-equipped grid-
connected inverters to smaller IGBT-equipped ones. These
transistors permit to increase the power switching frequency
in order to extract more energy and fulfill the connecting stan-
dards. One requirement of standards is that the inverters must
also be able to detect an islanding situation and take appropriate
measures in order to protect persons and equipment [33]. In
this situation, the grid has been removed from the inverter,
which then only supplies local loads. This can be troublesome
for many high-power transformerless systems, since a single-
phase inverter with a neutral-to-line grid connection is a system
grounded on the grid side.
In general, PV cells can be connected to the grid (grid-
connection application), or they can be used as isolated power
supplies. These two different applications of PV systems are
shown in Fig. 10.
Several classifications of converter topologies can be done
with respect to the number of power processing stages, location
of power-decoupling capacitors, use of transformers, and types
of grid interface. However, before discussing PV converter
topologies, three designs of inverter families are defined: central
inverters, module-oriented or module-integrated inverters, and
string inverters [34], [35]. The central converters connect in
parallel and/or in series on the dc side. One converter is used for
the entire PV plant (often divided into several units organized
in master–slave mode). The nominal power of this topology is
up to several megawatts. The module-oriented converters with
several modules usually connect in series on the dc side and
in parallel on the ac side. The nominal power ratings of such
PV power plants are up to several megawatts. In addition, in
the module-integrated converter topology, one converter per PV
module and a parallel connection on the ac side are used. In this
topology, a central measure for main supervision is necessary.
1010 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Fig. 11. (a) One-phase multistring converter. (b) Three-phase multistring converter.
Fig. 12. Detail of a multistring converter with a single-phase inverter stage.
Although this topology optimizes the energy yield, it has a
lower efficiency than the string inverter. This concept can be
implemented for PV plants of about 50–100 W.
In Fig. 11, a one-phase multistring converter [Fig. 11(a)] and
a three-phase multistring converter [Fig. 11(b)] are shown. A
detail of a multistring converter with a single-phase inverter
stage is illustrated in Fig. 12.
The multistring topology permits the integration of PV
strings of different technologies and orientations (north, south,
east, and west).
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1011
C. PV Topologies
Conventionally, a classification of PV topologies is divided
into two major categories: PV inverters with dc/dc converter
(with or without isolation) and PV inverters without dc/dc
converter (with or without isolation) [34], [36].
The isolation used in both categories is acquired using a
transformer that can be placed on either the grid or low-
frequency (LF) side or on the HF side. The line-frequency
transformer is an important component in the system due to
its size, weight, and price. The HF transformer is more com-
pact, but special attention must be paid to reduce losses [34],
[37]. The use of a transformer leads to the necessary isolation
(requirement in U.S.), and modern inverters tend to use an
HF transformer. However, PV inverters with a dc/dc converter
without isolation are usually implemented in some countries
where grid-isolation is not mandatory.
Basic designs focused on solutions for HF dc/dc converter
topologies with isolation such as full-bridge or single-inductor
push–pull permit to reduce the transformer ratio providing
a higher efficiency together with a smoother input current.
However, a transformer with tap point is required. In addition,
a double-inductor push–pull is implemented in other kind of
applications (equivalent with two interleaved boost converters
leading to a lower ripple in the input current), but extra in-
ductor is needed [38]. A full-bridge converter is usually used
at power levels above 750 W due to its good transformer
utilization [34].
Another possible classification of PV inverter topologies
can be based on the number of cascade power processing
stages. The single-stage inverter must handle all tasks such as
maximum-power-point-tracking (MPPT) control, grid-current
control, and voltage amplification. This configuration, which is
useful for a centralized inverter, has some drawbacks because it
must be designed to achieve a peak power of twice the nominal
power. Another possibility is to use a dual-stage inverter. In this
case, the dc/dc converter performs the MPPT (and perhaps volt-
age amplification), and the dc/ac inverter is dedicated to control
the grid current by means of pulsewidth modulation (PWM),
space vector modulation (SVM), or bang–bang operation. Fi-
nally, multistage inverters can be used, as mentioned above.
In this case, the task for each dc/dc converter is MPPT and,
normally, the increase of the dc voltage. The dc/dc converters
are connected to the dc link of a common dc/ac inverter, which
takes care for the grid-current control. This is beneficial since
a better control of each PV module/string is achieved, and that
common dc/ac inverter may be based on a standard variable-
speed-drive (VSD) technology.
There is no any standard PV inverter topology. Several
useful proposed topologies have been presented, and some
good studies regarding current PV inverters have been done
[39], [40]. The current control scheme is mainly used in PV
inverter applications [41]. In these converters, the current into
the stage is modulated/controlled to follow a rectified sinusoidal
waveform, and the task for the circuit is simply to recreate the
sine wave and inject it into the grid. The circuits apply zero-
voltage switching (ZVS) and zero-current switching (ZCS).
Thus, only conduction losses of the semiconductors remain.
If the converter has several stages, power decoupling must be
achieved with a capacitor in parallel with the PV module(s).
The current control scheme is employed more frequently be-
cause a high-power factor can be obtained with simple control
circuits, and transient current suppression is possible when
disturbances such as voltage changes occur in the utility power
system. In the current control scheme, operation as an isolated
power source is difficult, but there are no problems with grid
interconnection operation.
PV automatic-control (AC) module inverters used to be dual-
stage inverters with an embedded HF transformer. Classical
solutions can be applied to develop these converters: flyback
converters (single or two transistors), flyback with a buck–boost
converter, resonant converters, etc. For string or multistring
systems, the inverters used to be single or dual-stage inverters
with an embedded HF transformer. However, new solutions try
to eliminate the transformer using multilevel topologies.
A very common ac/dc topology is the half-bridge two-level
VSI, which can create two different voltage levels and requires
double dc-link voltage and double switching frequency in order
to obtain the same performance as the full bridge. In this
inverter, the switching frequency must be double the previous
one in order to obtain the same size of the grid inductor. A
variant of this topology is the standard full-bridge three-level
VSI, which can create a sinusoidal grid current by applying the
positive/negative dc-link or zero voltage, to the grid plus grid
inductor [42]. This inverter can create three different voltages
across the grid and inductor, the switching frequency of each
transistor is reduced, and good power quality is ensured. The
voltage across the grid and inductor is usually pulsewidth
modulated but hysteresis (bang-bang) current control can also
be applied.
Other multilevel topologies can be taken into account and
in [43] cascade multilevel inverters are studied. Seven basic
three-level cells can be used to achieve fifteen levels in the
output signals without using an output transformer. This is
beneficial for the power system and results in an improve-
ment in the THD performance of the output signals. However,
other problems such as commutation and conduction losses
appear [34].
D. Future Trends
The increasing interest and steadily growing number of
investors in solar energy stimulated research that resulted in
the development of very efficient PV cells, leading to uni-
versal implementations in isolated locations [44]. Due to the
improvement of roofing PV systems, residential neighborhoods
are becoming a target of solar panels, and some current projects
involve installation and setup of PV modules in high building
structures [45].
PV systems without transformers would be the most suitable
option in order to minimize the cost of the total system. On the
other hand, the cost of the grid-connected inverter is becoming
more visible in the total system price. A cost reduction per
inverter watt is, therefore, important to make PV-generated
power more attractive. Therefore, it seems that centralized
converters would be a good option for PV systems. However,
1012 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
Fig. 13. Typical compensation system for renewable energy applications based on flywheel energy storage.
problems associated with the centralized control appear, and it
can be difficult to use this type of systems.
An increasing interest is being focused on ac modules that
implement MPPT for PV modules improving the total sys-
tem efficiency. The future of this type of topologies is to
develop “plug and play systems” that are easy to install for
nonexpert users. This means that new ac modules may see
the light in the future, and they would be the future trend
in this type of technology. The inverters must guarantee that
the PV module is operated at the maximum power point
(MPP) owing to use MPPT control increasing the PV sys-
tems efficiency. The operation around the MPP without too
much fluctuation will reduce the ripple at the terminals of the
PV module.
Therefore, the control topics such as improvements of
MPPT control, THD improvements, and reduction of current
or voltage ripples will be the focus of researchers in the
years to come [46]. These topics have been deeply studied
during the last years, but some improvements still can be
done using new topologies such as multilevel converters. In
particular, multilevel cascade converters seem to be a good
solution to increase the voltage in the converter in order
to eliminate the HF transformer. A possible drawback of
this topology is control complexity and increased number of
solid-state devices (transistors and diodes). It should be no-
ticed that the increase of commutation and conduction losses
has to be taken into account while selecting PWM or SVM
algorithms.
Finally, it is important to remember that standards, regarding
the connection of PV systems to the grid, are actually becoming
more and more strict. Therefore, the future PV technology will
have to fulfil them, minimizing simultaneously the cost of the
system as much as possible. In addition, the incorporation of
new technologies, packaging techniques, control schemes, and
an extensive testing regimen must be developed. Testing is not
only the part of each phase of development but also the part of
validation of the final product [44].
IV. S
TORAGE SYSTEMS
A. Flywheels
In order to improve the quality of the generated power,
as well as to support critical loads during mains’ power in-
terruption, several energy-storage technologies have been in-
vestigated, developed, proved, and implemented in renewable
energy systems. However, flywheels are very commonly used
due to the simplicity of storing kinetic energy in a spinning
mass. For approximately 20 years, it has been a primary tech-
nology used to limit power interruptions in motor/generator
sets where steel wheels increase the rotating inertia providing
short power interruptions protection and smoothing of delivered
power. One of the first commercial uses of flywheels in con-
junction with active filtering to improve frequency distortion
on a high-voltage power-system line is described in [47].
There are two broad classes of flywheel-energy-storage tech-
nologies. One is a technology based on low-speed flywheels (up
to 6000 r/min) with steel rotors and conventional bearings. The
other one involves modern high-speed flywheel systems (up to
60 000 r/min) that are just becoming commercial and make use
of advanced composite wheels that have much higher energy
and power density than steel wheels. This technology requires
ultralow friction bearing assemblies, such as magnetic bearings,
and stimulates a research trend [48].
Most applications of flywheels in the area of renewable
energy delivery are based on a typical configuration where
an electrical machine (i.e., high-speed synchronous machine
or induction machine) drives a flywheel, and its electrical
part is connected to the grid via a back-to-back converter, as
shown in Fig. 13. Such configuration requires an adequate
control strategy to improve power smoothing [49]–[52]. The
basic operation could be summarized as follows. When there
is excess in the generated power with respect to the demanded
power, the difference is stored in the flywheel that is driven
by the electrical machine operating as a motor. On the other
hand, when a perturbation or a fluctuation in delivered power
CARRASCO et al.: SYSTEMS FOR THE GRID INTEGRATION OF RENEWABLE ENERGY SOURCES 1013
is detected in the loads, the electrical machine is driven by
the flywheel and operates as a generator supplying needed
extra energy. A typical control algorithm is a direct vector
control with rotor-flux orientation and sensorless control using
a model-reference-adaptive-system (MRAS) observer.
Experimental alternatives for wind farms include flywheel
compensation systems connected to the dc link, which are the
same as the systems used for power smoothing for a single
or a group of wind turbines [53]. Usually, a control strategy
is applied to regulate the dc voltage against the input power
surges/sags or sudden changes in the load demand. A simi-
lar configuration can be applied to solar cells [54]. Another
renewable energy resource where power oscillations need to
be smoothed is wave energy. In [55], a D-static synchronous
compensator (STATCOM) is proposed, as an alternative to fly-
wheels, to accomplish the output power smoothing on a wave-
energy converter where several operating conditions should be
taken into account. Recent proposals on using flywheels to
regulate the system frequency include the disposal of a matrix
of several flywheels to compensate the difference between the
network’s load and the power generated [56].
Recently, there has been research where integrated flywheel
systems can be encountered. Those systems use the same steel
rotor of the electrical machine as energy-storage element [57].
Two of the main advantages of a system like that are its high-
power density and its similarity with a standard electrical ma-
chine. It seems that a new trend for energy storage in renewable
energy systems is to combine several storing technologies (as
what occurs in uninterruptible power system (UPS) applica-
tion), where a storage system integrates compressed-air system,
thermal storage unit, and flywheel energy storage [58].
B. Hydrogen
This section aims to analyze new trends in hydrogen-storage
systems for high-quality back-up power. The hydrogen-fuel
economy has been rapidly increasing in industrial application
due to the advantages of the hydrogen of being storable, trans-
portable, highly versatile, efficient, and clean energy carrier
to supplement or replace many of the current fuel options. It
can be used in fuel cells to produce electricity in a versatile
way, for example, in portable applications, stationary use of
energy, transportation, or high-power generation. The use of
fuel cells in such applications is justified since they are a very
important alternative power source due to their well-known spe-
cific characteristics such as very low toxic emissions, low noise
and vibrations, modular design, high efficiency (especially with
partial load), easy installation, compatibility with a lot of types
of fuels, and low maintenance cost.
The increase of the penetration of renewable energies world-
wide makes the storage issue critical both in stand-alone [59]
and grid-connected application. An example of the hydrogen-
storage application to improve the grid power quality through
smoothing large and quick fluctuations of wind energy is re-
ported in [60].
Hydrogen could be stored as compressed or liquefied gas [61]
or by using metal hydrides or carbon nanotubes [62]. For a par-
ticular application, the choice of a storage technology implies a
tradeoff between the characteristics of available technologies in
terms of technical, economical, or environmental performance
[63]. Applications must also include a discussion of the life-
cycle efficiency and cost of the proposed storage system. This
analysis should consider the total life of the proposed hydrogen-
storage system including raw-material requirements, manufac-
turing and fabrication processes, integration of the system into
the vehicle or off-board configuration, useful service life, and
removal and disposal processes including recycling. Recently,
research and development are focused on new materials or tech-
nologies for hydrogen storage: metal hydrides (reduce the volu-
metric and pressure requirements for storage, but they are more
complex than other solutions), chemical hydrides, carbon-based
hydrogen-storage materials, compressed- and liquid-hydrogen-
tank technologies, off-board hydrogen-storage systems (a typ-
ical refueling station will be delivering 200–1500 kg/day
of hydrogen), and new materials and approaches for storing
hydrogen on board a vehicle. Applications to identify and
investigate advanced concepts for material storage that have the
potential to achieve 2010 targets of 2 kWh/kg and 1.5 kWh/L.
C. Compressed-Air Energy Storage (CAES)
Energy storage in compressed air is made using a compressor
that stores it in an air reservoir (i.e., an aquifer like the ones used
for natural-gas storage, natural caverns, or mechanically formed
caverns, etc.). When a grid is operating off peak, the compressor
stores air in the air reservoir. During discharge at peak loads,
the compressed air is released to a combustor where it is mixed
with oil or gas driving a gas turbine. Such systems are available
for 100–300 MW and burn about one-third of the premium fuel
of a conventional simple cycle combustion turbine.
An alternative to CAES is the use of compressed air in
vessels (called CAS), which operates exactly in the same way
as CAES except that the air is stored in pressure vessels rather
than underground reservoirs. Such difference makes possible
variations consisting of the use of pneumatic motor acting as
compressors or driving a dc motor/generator according to the
operation required by the system, i.e., storing energy when
there is no extra demand of energy or delivering extra power at
peak loads.
Recent research is devoted to the maximum-efficiency point-
tracking control [64] or integrated technologies for power-
supply applications [58].
D. Supercapacitors
Supercapacitors, which are also known as ultracapacitors or
electric double layer capacitors (EDLC), are built up with mod-
ules of single cells connected in series and packed with adjacent
modules connected in parallel. Single cells are available with
capacitance values from 350 to 2700 F and operate in the range
of 2 V. The module voltage is usually in the range from 200
to 400 V. They have a long life cycle and are suitable for short
discharge applications and are less than 100 kW. New trends
focused on using ultracapacitors to cover temporary high peak-
power demands [65], integration with other energy-storage
technologies, and development of high-voltage applications.
1014 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006
E. Superconducting Magnetic Energy Storage (SMES)
In an SMES, a coil of superconducting wire stores electrical
energy in a magnetic field without resistive losses. Also, there is
no need for conversion between chemical or mechanical forms
of energy.
Recent systems are based on both general configurations of
the coil: solenoidal or toroidal. The second topology has a
minimal external magnetic field but the cost of superconductor
and coil components is higher than the first topology. Such
devices require cryogenic refrigerators (to operate in liquid
helium at −269
◦
C) besides the solid-state power electronics.
The system operates by injecting a dc current into the su-
perconducting coil, which stores the energy in magnetic field.
When a load must be fed, the current is generated using the en-
ergy stored in the magnetic field. One of the major advantages
of SMES is the ability to release large quantities of power dur-
ing a fraction of a cycle. Typical applications of SMES are cor-
rections of voltage sags and dips at industrial facilities (1-MW
units) and stabilization of ring networks (2-MW units).
New trends in SMES are related to the use of low-
temperature superconductors (liquid-nitrogen temperature), the
use of secondary batteries, and the integration of STATCOM
[66] and several topologies of ac–dc–ac converters with
SMES [67].
F. Battery Storage
The use of batteries as a system to interchange energy with
the grid is well known. There are several types of batteries used
in renewable energy systems: lead acid, lithium, and nickel.
Batteries provide a rapid response for either charge or dis-
charge, although the discharge rate is limited by the chemical
reactions and the type of battery. They act as a constant voltage
source in the power systems. New trends in the use of batteries
for renewable energy systems focused on the integration with
several energy sources (wind energy, PV systems, etc.) and
also on the integration with other energy-storage systems com-
plementing them. Also, there are attempts to optimize battery
cells in order to reduce maintenance and to increment its life-
time [68].
G. Pumped-Hydroelectric Storage (PHS)
As batteries, PHS is a mature technology where a swamp of
water stored at a certain high elevation is used to generate elec-
tric energy by hydroturbines, whenever there is an additional
power demand in the grid. When no extra generation is needed,
the water is pumped back up to recharge the upper reservoir.
One limitation of PHS is that they require significant land areas
with suitable topography. There are units with sizes from 30 to
350 MW, with efficiencies around 75%.
New trends in PHS are focused on the integration with
variable-speed drives (cycloconverters driven doubly fed induc-
tion machine) [69] and the use of underground PHS (UPHS),
where the lower reservoir is excavated from subterranean rock.
Such a system is more flexible and more efficient but requires a
higher capital cost.
V. C
ONCLUSION
The new power-electronic technology plays a very important
role in the integration of renewable energy sources into the grid.
It should be possible to develop the power-electronic interface
for the highest projected turbine rating, to optimize the energy
conversion and transmission and control reactive power, to
minimize harmonic distortion, to achieve at a low cost a high
efficiency over a wide power range, and to have a high reliabil-
ity and tolerance to the failure of a subsystem component.
In this paper, the common and future trends for renewable
energy systems have been described. As a current energy
source, wind energy is the most advanced technology due to
its installed power and the recent improvements of the power
electronics and control. In addition, the applicable regulations
favor the increasing number of wind farms due to the attractive
economical reliability. On the other hand, the trend of the PV
energy leads to consider that it will be an interesting alternative
in the near future when the current problems and disadvan-
tages of this technology (high cost and low efficiency) are
solved. Finally, for the energy-storage systems (flywheels, hy-
drogen, compressed air, supercapacitors, superconducting mag-
netic, and pumped hydroelectric), the future presents several
fronts, and actually, they are in the same development level.
These systems are nowadays being studied, and only research
projects have been developed focusing on the achievement of
mature technologies.
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Juan Manuel Carrasco (M’97) was born in San
Roque, Spain. He received the M. Eng. and Dr. Eng.
degrees in industrial engineering from the University
of Seville (US), Seville, Spain, in 1989 and 1992,
respectively.
He was an Assistant Professor from 1990 to 1995,
and is currently an Associate Professor with the De-
partment of Electronic Engineering, US. He has been
working for several years in the power-electronics
field where he was involved in industrial applications
for the design and development of power converters
applied to renewable-energy technologies. His current research areas are dis-
tributed power generation and the integration of renewable energy sources.
Leopoldo Garcia Franquelo (M’85–S’96–F’05) re-
ceived the Ing. and Dr. Ing. Industrial degrees from
the University of Seville (US), Seville, Spain, in
1977 and 1980, respectively.
He is currently a Professor with the Department
of Electronics Engineering, US. His current research
interest includes industrial applications of electronic
power converters.
Jan T. Bialasiewicz (M’86–SM’87) received the
M.S. degree from the Warsaw University of Tech-
nology, Warsaw, Poland, and the Ph.D. and D.Sc.
degrees from the Silesian University of Technology,
Gliwice, Poland, all in electrical engineering.
He is currently an Associate Professor with the
Electrical Engineering Department, University of
Colorado, Denver, and also with the Health Sciences
Center, Denver, CO. His research interests include
control theory, modeling and identification of dy-
namic systems, renewable energy systems, and the-
ory and applications of wavelets. He is the author of two books and numerous
research publications.
Dr. Bialasiewicz is an Associate Editor of the IEEE T
RANSACTIONS ON
INDUSTRIAL ELECTRONICS.
Eduardo Galván (M’99) was born in Aracena,
Spain, in 1964. He received the M.Sc. degree in
electrical engineering and the Ph.D. degree in indus-
trial engineering from the University of Seville (US),
Seville, Spain, in 1991 and 1994, respectively.
He is an Associate Professor of electronic en-
gineering with the Escuela Superior de Ingenieros,
US. He has been working for several years in the
power-electronics field where he was involved in in-
dustrial applications for the design and development
of power converters applied to renewable energy
technologies. His research interests include control of power converters (wind-
turbine applications, active filters, and electric machines).
Ramón C. Portillo Guisado (S’06) was born in
Seville, Spain, in 1974. He received the Ingeniero
Industrial degree from the University of Seville (US),
Seville, Spain, in 2002. He is currently working
toward the Ph.D. degree in electrical engineering in
the Power Electronics Group, US.
In 2001, he joined the Power Electronics Group,
US, working in I + D projects. Since 2002, he has
been an Associate Professor with the Department of
Electronic Engineering, US. His research interests
include electronic power systems applied to energy
conditioning and generation, power quality in renewable generation plants,
applications of fuzzy systems in industry and wind farms, and modeling and
control of power-electronic converters and industrial drives.
Ma. Ángeles Martín Prats (M’04) was born in
Seville, Spain, in 1971. She received the Licenciado
and Doctor degrees in physics from the University
of Seville (US), Seville, Spain, in 1996 and 2003,
respectively.
In 1996, she joined the Spanish Aerospatial Tech-
nical National Institute (INTA), where she worked
in the Renewable Energy Department. In 1998, she
joined the Department of Electrical Engineering,
University of Huelva, Spain. Since 2000, she has
been an Assistant Professor with the Department of
Electronics Engineering, US. Her research interests focus on multilevel con-
verters and fuel-cell power-conditioner systems. She is involved in industrial
applications for the design and development of power converters applied to
renewable-energy technologies.
José Ignacio León (S’04) was born in Cádiz, Spain,
in 1976. He received the B.S. degree in telecommu-
nications engineering from the University of Seville
(US), Seville, Spain, in 2001, where he is cur-
rently working toward the Ph.D. degree in electrical
engineering.
In 2002, he joined the Power Electronics Group,
US, working in I + D projects. Since 2002, he has
been an Associate Professor with the Department of
Electronics Engineering, US. His research interests
include electronic power systems, modeling and con-
trol of power-electronics converters and industrial drives, and power quality in
renewable generation plants.
Narciso Moreno-Alfonso (M’00) was born in Seville, Spain, in 1971. He
received the Ingeniero Técnico Industrial degree in electrical engineering and
the Ingeniero en Electrónica degree, in 1994 and 2000, respectively, all from
the University of Seville (US), Seville, Spain. His major field of study is in the
application of power electronics to distributed generation, renewable energy
systems, and drives.
He has been a Profesor Titular de Escuela Universitaria with the Department
of Electrical Engineering, US, since 1994. Since 2002, his research activity has
been developed in the Electronic Technology Group (GTE) of the Department
of Electronics Engineering, Escuela Superior de Ingenieros, US.
