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Journal ArticleDOI

Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

TL;DR: New trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented and a review of the appropriate storage-system technology used for the Integration of intermittent renewable energy sources is introduced.
Abstract: The use of distributed energy resources is increasingly 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 renewable energy systems based on reliability and maturity of each technology are presented

Summary (4 min read)

I. INTRODUCTION

  • 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.
  • The first one is the development of fast semiconductor switches, which are capable of switching quickly and handling high powers.
  • New trends in power electronics technology for the integration of renewable energy sources and energy storage systems are presented.
  • Appropriate integration of renewable energy sources with storage systems allows for greater market penetration and result in primary energy and emissions savings.

A. Variable Speed Wind Turbines

  • Wind turbine technology has undergone a dramatic transformation during the last 15 years, developing from a fringe science in the 1970's to the wind turbine of the 2000's using the latest in power electronics, aerodynamics and mechanical drive train designs [1] [2] .
  • The legislation in both countries favors continuing growth of installed capacity.
  • Wind power is quite different from conventional electricity generation with synchronous generators.
  • There is also less mechanical stress and rapid power fluctuations are scarce, because the rotor acts as a flywheel (storing energy in a kinetic form).
  • Variable speed turbines also allow the grid voltage to be controlled, as the reactive power generation can be varied.

B. Current Wind Power Technology

  • Variable speed wind turbines have progressed dramatically in recent years.
  • Variable speed operation can only be achieved by decoupling electrical grid frequency and mechanical rotor frequency.
  • This scheme allows, on one hand, a vector control of the active and reactive power 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.
  • The silicon is isolated to the cooling plate and can be connected to ground for low electromagnetic emission even with higher switching frequency.
  • 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:.
  • To enable large-scale application of wind energy without compromising power system stability, the turbines should stay connected and contribute to the grid in case of a disturbance such as a voltage dip.
  • Though the definition of fault ride through varies, the E.ON (German Transmission and Distribution Utility) regulation is likely to set the standard [8] .
  • Modern forced-commutated inverters used in variable-speed wind turbines produce not only harmonics but also inter-harmonics.
  • The last one is the harmonic analysis, which is carried out by the FFT algorithm.

III. PHOTOVOLTAIC TECHNOLOGY

  • This section focuses on a review of recent developments of power electronics converters and the state of art of implemented photovoltaic (PV) systems.
  • PV systems as an alternative energy resource or energy resource complementary in hybrid systems have been becoming feasible due to the increase of research and development work in this area.
  • Several standards given by the utility companies must be obeyed in the PV modules connection.

A. Market Considerations

  • Energy demand has grown consistently by 20-25% per annum over the past 20 years, mainly due to the decreasing costs and prices.
  • This decline has been driven by a) increasing efficiency of solar cells b) manufacturing technology improvements, and c) economies of scale.
  • In 2001, 350 Megawatts of solar equipment were sold to add to the solar equipment already generating clean energy.
  • If the growth rates of the installation of photovoltaic 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.

B. Design of PV Converters Families

  • An overview of some existing power inverter topologies for interfacing PV modules to the grid is presented.
  • Before discussing PV converter topologies, three designs of inverter families are defined: central inverters, module-oriented or moduleintegrated inverters, and string inverters [34][35] .
  • The use of a transformer leads to the necessary isolation (requirement in US) and modern inverters tend to use a high-frequency transformer.
  • The DC/DC converter performs the MPPT (and perhaps voltage amplification) and the DC/AC inverter is dedicated to control the grid current by means of Pulse-Width Modulation (PWM), Space Vector Modulation (SVM) or bang-bang operation.
  • 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 DClink or zero voltage, to the grid plus grid inductor [42] .

B. Hydrogen

  • The purpose of this section is to analyze new trends in hydrogen storage systems for high quality back-up power.
  • The use of fuel cells in such applications is justified since they are a very important alternative power source due to their well-known specific 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.
  • An example of the hydrogen storage application to improve the grid power quality through smoothing large and quick fluctuations of wind energy is reported in [60] .
  • 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, which stores it in an air reservoir (i.e. an aquifer like 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.
  • 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.
  • Recent research is devoted to maximum efficiency point tracking control [64] or integrated technologies for power supplies applications [65] .

D. Supercapacitors

  • Supercapacitors, also known as ultracapacitors or electric double layer capacitors (EDLC), are built up with modules of single cells connected in series and packed with adjacent modules connected in parallel.
  • Single cells are available with capacitance values from 350F to 2700F and operate in the range of the 2V.
  • The module voltage is usually in the range from 200V to 400V.
  • They have a long life cycle and are suitable for short discharge applications and less than 100kW.
  • New trends focused on using ultracapacitors to cover temporary high peak power demands [66] , integration with other energy storage technologies and development of high−voltage applications.

E. Superconducting Magnetic Energy Storage (SMES)

  • 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.
  • Such devices require cryogenic refrigerators (to operate in liquid helium at -269°C) besides the solid-state power electronics.
  • When a load must be fed, the current is generated using the energy stored in the magnetic field.
  • Typical applications of SMES are corrections of voltage sags and dips at industrial facilities (1MW units) and stabilization of ring networks (2MW units).

F. Battery Storage

  • There are several types of batteries used in renewable energy systems: lead acid, lithium and nickel.
  • Batteries provide rapid response for either charge or discharge, although the discharge rate is limited by chemical reactions and type of battery.
  • New trends in the use of batteries for renewable energy systems focused on the integration with several energy sources (wind energy, photovoltaic systems, etc.) and also on the integration with other energy storage systems complementing them.
  • Also, there are attempts to optimize battery cells in order to reduce maintenance and to increment its lifetime [69] .

G. Pumped -Hydroelectric Storage (PHS)

  • 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 MW to 350MW, with efficiencies around 75%.
  • New trends in PHS are focused on the integration with variable speed drives (cycloconverters driven doubly-fed induction machine) [70] and the use of underground pumped hydroelectric storage (UPHS), where the lower reservoir is excavated from subterranean rock.
  • Such a system is more flexible, more efficient, but requires a higher capital cost.

V. CONCLUSION

  • The new power electronics technology plays a very important role in the integration of renewable energy sources into the grid.
  • It should be possible to develop the power electronics interface for the highest projected turbine rating; optimize energy conversion and transmission and control reactive power; minimize harmonic distortion; achieve at a low cost high efficiency over a wide power range; have a high reliability and tolerance to the failure of a subsystem component.
  • In addition, the applicable regulations favour the increasing number of wind farms due to the attractive economical reliability.
  • These systems are nowadays being studied and only research projects have been developed focused on the achievement of mature technologies.

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1
AbstractThe use of distributed energy resources (DER) is
increasingly being pursued as a supplement and an alternative to
large conventional central power stations. The specification of a
power electronics interface is subject to requirements related not
only to the renewable energy source itself but also to its effects on
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 power generators are presented. A
review of appropriate storage systems technology used for the
integration of intermittent renewable energy sources is also
introduced. Discussions about common and future trends in
renewable energy systems based on reliability and maturity of
each technology are presented.
Index Terms DFIG, multilevel converter topologies, direct
drives, flywheel, hidrogen, SMES, supercapacitors, wind diesel.
I. INTRODUCTION
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 electronics
technology 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 of applications become more integrated with grid-based
systems.
During the last few years, power electronics has been
undergoing a fast evolution, mainly due to two factors. The
first one is the development of fast semiconductor switches,
which 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 algorithms. These factors together have led to
the development of cost-effective and grid-friendly converters.
In this paper, new trends in power electronics technology
for the integration of renewable energy sources and energy
storage systems are presented. The paper is organized in the
following sections. In section II, we describe 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 electronics 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
generation plants. The continuously decreasing prices for 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
greater market penetration and result in primary energy and
emissions savings. In section IV, we present research and
development trends in energy storage systems used for the grid
integration of intermittent 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 1970’s to the wind turbine of the
2000’s using the latest in power electronics, aerodynamics and
mechanical 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
continuing growth of installed capacity. Wind power is quite
different from conventional electricity generation with
synchronous 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
Power Electronic Systems for the Grid Integration
of Renewable Energy Sources: a Survey
J. M. Carrasco, Member, IEEE, L. G. Franquelo, Fellow, IEEE, J.T.Bialasiewicz, Senior Member,
IEEE, E. Galvan, Member, IEEE, R. Portillo, M. M. Prats, Member, IEEE, J. I. León Student Member,
IEEE, and N. Moreno.
T
J. M. Carrasco, E. Galvan, R. Portillo, M. M. Prats, J. I. Leon, and L. G.
Franquelo are with the Department of Electronic Engineering, Seville
University, 41092-Seville, SPAIN, e-mail: carrasco@gte.esi.us.es.
N. Moreno is with Department of Electrical Engineering, Seville
University, 41011-Seville, SPAI, e-mail: narciso-ma@us.es.
J. T.Bialasiewicz is with the Department of Electrical Engineering,
University of Colorado at Denver and Health Sciences Center, Denver,
CO 80217, USA, e-mail: jan.bialasiewicz@cudenver.edu.

2
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 a
5% greater than fixed speed technology, and that the active
and reactive power 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 a
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 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 doubly fed
induction generator [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
frequency 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 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
power 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
design, the semi-variable 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 transient 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 it is 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.
c
b
a
c’
b’
a’
Gear
Box
Three Winding
Transformer
Grid
Fig. 1. Single doubly fed induction machine with two fully controlled AC-
DC power converters.
CONTROL
CIRCUIT
Stator
winding
Rotor
winding
FIRING
UNIT
OPTICAL
COUPLING
Variable
resistor
ROTOR
GB
Fig. 2. Single doubly fed induction machine controlled with slip power
dissipation in an internal resistor.
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 this converted to a suitable AC energy for the
grid. The majority of these wind turbines are equipped with a
multi-pole 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 mechanical components. Enercon supplies such

3
technology.
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 power delivered to the grid.
It also keeps the total harmonic distortion 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
dc
V
Fig. 3. Double three phase voltage source inverter.
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 low
cost as compared to an induction generator connected to a
voltage source inverter 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 bandwidth 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 Devices Technology: Improvements in
the performance and reliability of power electronics variable
frequency 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 IGBT is now the main component for power electronics
and also for wind turbine applications. They are now mature
technology turn-on components adapted to very high power
(6kV-1.2kA), and they are in competition with GTOs (Gate
turn-off thyristor) for high power applications [6].
dc
i
dc
C
dc
V
GB
dc
L
ac
L
Fig. 4. Step-up converter in the rectifier circuit and full power inverter
topology used in wind turbine applications.
Recently, the IGCT (Integrated Gated Control Thyristor)
has been developed as a mechanical integration of a GTO plus
a delicate hard drive circuit that transforms the GTO into a
modern high performance component with a large SOA (Safe
Operation Area), lower switching losses, and a short storage
time [7]. The comparison between IGCT and IGBT for
frequency converters, used especially in wind turbines is
explained below:
a) IGBTs have higher switching frequency than IGCTs, so
they introduce less distortion in the grid.
b) 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, which has
exactly the same thermal behaviour as silicon, so nearly
no thermal stress occurs. This increases the lifetime of the
device by 10-fold approximately.
c) The main advantage of IGCTs versus IGBTs is that they
have a lower on-state voltage drop, which is about 3.0V
for a 4500V device. In this case the power dissipation due
to a voltage drop for a 1500kW converter will be 2400W
per phase. On the other hand, in the case of IGBT the
voltage drop is higher than IGCTs. For a 1700V device
having a drop of 5V the power dissipation due to the
voltage drop for a 1500 kW condition will be 5kW 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 wind capacity increases, network operators have
to ensure that consumer power quality is not compromised. To
enable large-scale application of wind energy without
compromising power system stability, the turbines should stay

4
connected and contribute to the grid in case of a disturbance
such as a voltage dip. Wind farms should generate like
conventional power plants, supplying active and reactive
power for frequency and voltage recovery, immediately after
the fault occured
Thus, several utilities have introduced special grid
connection codes for wind farm developers, covering reactive
power control, frequency response and fault ride-through,
especially in places where wind turbines provide for a
significant part of the total power. Examples are Spain,
Denmark and part of Northern Germany.
Fig. 5. E.On Netz requirements for wind farm behavior during faults
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
requirements, fault ride-through is regarded as the main
challenge to the wind turbine manufacturers. Though the
definition of fault ride through varies, the E.ON (German
Transmission and Distribution Utility) regulation is likely to
set the standard [8]. This stipulates that a wind turbine should
remain stable and connected 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
reactive power to the grid in order to support grid voltage
restoration.
2) Power Quality Requirements for Grid-Connected Wind
Turbines: The grid interaction and grid impact of wind
turbines has been focussed on during the past few years. The
reason behind this interest is that wind turbines are among
utilities considered to be potential sources of bad power
quality. Measurements 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 turbines produce not only
harmonics but also inter-harmonics.
IEC initiated the standardization on power quality for wind
turbines in 1995 as a 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. Voltage and current transients are measured during
the switching operations of the wind turbine (start-up at cut
wind speed and start-up at rated wind speed). The last one is
the harmonic analysis, which is carried out by the FFT
algorithm. Rectangular windows of 8 cycles of fundamental
frequency width, with no gap and no overlapping between
successive windows are applied. Furthermore, the current total
harmonic distortion (THD) is calculated up to 50th harmonic
order.
Recently, high frequency harmonics and inter-harmonics are
treated in the IEC 61000-4-7 and IEC 61000-3-6 [10][11]. The
methods for summing harmonics and inter-harmonics 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 it has
been reported in [12]. Wind turbines not only produce
harmonics, they also produce inter-harmonics, i.e. harmonics,
which 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 on 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 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, around 25 to 30
years. The reason is that the low turbulence at sea gives lower
fatigue loads on the wind turbine.
Conventional HVAC transmission systems are a simple and
cost-efficient solution for the grid connection of wind farms.

5
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 capacity 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 transmission distances over
100 km or power levels of between approximately 200 and
900MW. The HVDC transmission offers many advantages
over HVAC [13]:
a) Sending and receiving end frequencies are independent
b) Transmission distance using DC is not affected by cable
charging current.
c) Offshore installation is isolated from mainland
disturbances, and vice versa
d) Power flow is fully defined and controllable.
e) Cable power losses are low.
f) Power transmission capability per cable is higher.
Classical HVDC transmission systems (as shown in Fig. 6a)
are based on current source converters with naturally
commutated thyristors, so called line-commutated converters
(LCC). 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 connection with
wind farms as the offshore AC grid needs to be powered up
prior to a possible start-up. Further disadvantages of LCC
based HVDC transmission systems is the lack of the possibility
to provide 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 Inverter (VSC) based HVDC transmission
systems are gaining more and more attention, not only for the
grid connection of large offshore wind farms. Today, VSC
based solutions are marketed by ABB under the name “HVDC
Light” [14] and by Siemens under the name “HVDC Plus".
Fig. 6b shows the schematic of a VSC based HVDC
transmission 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 power can be controlled
independently, which may reduce the need for reactive power
compensation and can contribute to stabilization of the AC
network at their connection points [15]
.
2) High Power Medium-Voltage Converter Topologies: In
order to decrease the cost per MW and to increase the
efficiency of wind energy conversion, nominal power of wind
turbines has been continuously growing in last years [16].
Fig. 6. Two HVDC transmission solutions: a) Classical LCC based system
with STATCOM, b) VSC based system.
The different proposed multilevel converter topologies can
be classified in the following five categories [17]:
a) Multilevel configurations with diode clamps.
b) Multilevel configurations with bi-directional switch
interconnection.
c) Multilevel configurations with flying capacitors.
d) Multilevel configurations with multiple three-phase
inverters.
e) Multilevel configurations with cascaded single phase H-
Bridge inverters
A common feature of the five different topologies of
multilevel 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 increases 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 voltage is highlighted, together with the
reduced EMI [18]. Moreover, the multilevel converters have
the lowest demands for the input filters or alternatively
reduced number of switchings [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
software 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 to
solve the balancing problem directly modifying the reference
voltage with relatively low computational burden [21][22].

Citations
More filters
Journal ArticleDOI
TL;DR: This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters.
Abstract: This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.

2,254 citations


Cites background from "Power-Electronic Systems for the Gr..."

  • ...industry [8], grid integration of renewable-energy sources [9]– [11], reactive-power compensation [12]–[14], and other applications [15], [16]....

    [...]

Journal ArticleDOI
TL;DR: The idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed, and the inverters that are operated in this way are called synchronverters.
Abstract: In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.

2,115 citations


Cites background from "Power-Electronic Systems for the Gr..."

  • ...The current paradigm in the control of wind- or solar-power generators is to extract the maximum power from the power source and inject them all into the power grid (see, for example, [1]–[3])....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications is presented, where the MIC topologies are classified into three different arrangements based on the dc link configurations.
Abstract: The annual world photovoltaic (PV) cell/module production is growing at almost an exponential rate and has reached 1727 MW in 2005. Building integrated PV (BIPV) projects are emerging as the strongest part of the PV market and grid interactive inverters are a key component in determining the total system cost. Module integrated converter (MIC) technology has become a global trend in grid interactive PV applications and may assist in driving down the balance of system costs to secure an improved total system cost. This paper concentrates on the topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications. The MIC topologies are classified into three different arrangements based on the dc link configurations. A systematic discussion is also provided at the end of the paper that focuses on the major advantages and disadvantages of each MIC arrangement. These are considered in detail and will provide a useful framework and point of reference for the next generation MIC designs and applications.

1,158 citations


Cites background from "Power-Electronic Systems for the Gr..."

  • ...Among these, the MIC system offers “plug and play” concept and greatly optimizes the energy yield [4], [12]....

    [...]

  • ...2) The dual grounding becomes a difficult issue in the transformerless inverters [4]....

    [...]

  • ...However, compared with large converters, MICs have smaller power ratings and tend to have lower efficiencies [4], [31]....

    [...]

  • ...Electronic power inverter is one of the enabling technologies required for utilizing PV energy and its cost is becoming more visible in the total price of the PV system [3], [4]....

    [...]

  • ...However, a low-power line-frequency transformer is bulky and may not be very efficient [4], [51]....

    [...]

Journal ArticleDOI
TL;DR: This paper presents a review of ESSs for transport and grid applications, covering several aspects as the storage technology, the main applications, and the power converters used to operate some of the energy storage technologies.
Abstract: Energy storage systems (ESSs) are enabling technologies for well-established and new applications such as power peak shaving, electric vehicles, integration of renewable energies, etc. This paper presents a review of ESSs for transport and grid applications, covering several aspects as the storage technology, the main applications, and the power converters used to operate some of the energy storage technologies. Special attention is given to the different applications, providing a deep description of the system and addressing the most suitable storage technology. The main objective of this paper is to introduce the subject and to give an updated reference to nonspecialist, academic, and engineers in the field of power electronics.

1,115 citations


Cites background from "Power-Electronic Systems for the Gr..."

  • ...introduces some new issues on the operation of the power system, such as potential unbalancing between generation and demand [98]....

    [...]

Journal ArticleDOI
TL;DR: In this article, the authors provide a detailed analysis of real life application and performance of different energy storage technologies, and highlight some of the challenges hindering the commercial deployment of energy storage technology.

1,106 citations

References
More filters
Journal ArticleDOI
TL;DR: The most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-Clamped (flying capacitor), and cascaded multicell with separate DC sources are presented and the circuit topology options are presented.
Abstract: Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

6,472 citations


"Power-Electronic Systems for the Gr..." refers background in this paper

  • ...Sometimes they are a multiple of 50 Hz, and sometimes they are not....

    [...]

Book
31 Jul 1997
TL;DR: Converters in Equilibrium, Steady-State Equivalent Circuit Modeling, Losses, and Efficiency, and Power and Harmonics in Nonsinusoidal Systems.
Abstract: Preface. 1. Introduction. I: Converters in Equilibrium. 2. Principles of Steady State Converter Analysis. 3. Steady-State Equivalent Circuit Modeling, Losses, and Efficiency. 4. Switch Realization. 5. The Discontinuous Conduction Mode. 6. Converter Circuits. II: Converter Dynamics and Control. 7. AC Equivalent Circuit Modeling. 8. Converter Transfer Functions. 9. Controller Design. 10. Input Filter Design. 11. AC and DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode. 12. Current Programmed Control. III: Magnetics. 13. Basic Magnetics Theory. 14. Inductor Design. 15. Transformer Design. IV: Modern Rectifiers and Power System Harmonics. 16. Power and Harmonics in Nonsinusoidal Systems. 17. Line-Commutated Rectifiers. 18. Pulse-Width Modulated Rectifiers. V: Resonant Converters. 19. Resonant Conversion. 20. Soft Switching. Appendices: A. RMS Values of Commonly-Observed Converter Waveforms. B. Simulation of Converters. C. Middlebrook's Extra Element Theorem. D. Magnetics Design Tables. Index.

6,136 citations

01 Jan 1980
TL;DR: In this article, a neutral-point-clamped PWM inverter composed of main switching devices which operate as switches for PWM and auxiliary switching devices to clamp the output terminal potential to the neutral point potential has been developed.
Abstract: A new neutral-point-clamped pulsewidth modulation (PWM) inverter composed of main switching devices which operate as switches for PWM and auxiliary switching devices to clamp the output terminal potential to the neutral point potential has been developed. This inverter output contains less harmonic content as compared with that of a conventional type. Two inverters are compared analytically and experimentally. In addition, a new PWM technique suitable for an ac drive system is applied to this inverter. The neutral-point-clamped PWM inverter adopting the new PWM technique shows an excellent drive system efficiency, including motor efficiency, and is appropriate for a wide-range variable-speed drive system.

4,432 citations

Journal ArticleDOI
TL;DR: The neutral-point-clamped PWM inverter adopting the new PWM technique shows an excellent drive system efficiency, including motor efficiency, and is appropriate for a wide-range variable-speed drive system.
Abstract: A new neutral-point-clamped pulsewidth modulation (PWM) inverter composed of main switching devices which operate as switches for PWM and auxiliary switching devices to clamp the output terminal potential to the neutral point potential has been developed. This inverter output contains less harmonic content as compared with that of a conventional type. Two inverters are compared analytically and experimentally. In addition, a new PWM technique suitable for an ac drive system is applied to this inverter. The neutral-point-clamped PWM inverter adopting the new PWM technique shows an excellent drive system efficiency, including motor efficiency, and is appropriate for a wide-range variable-speed drive system.

4,328 citations


"Power-Electronic Systems for the Gr..." refers methods in this paper

  • ...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]....

    [...]

Journal ArticleDOI
TL;DR: In this article, the authors focus on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid and categorize the inverters into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the kind of grid-connected power stage.
Abstract: This review focuses on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid. The inverters are categorized into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single-phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the type of grid-connected power stage. Various inverter topologies are presented, compared, and evaluated against demands, lifetime, component ratings, and cost. Finally, some of the topologies are pointed out as the best candidates for either single PV module or multiple PV module applications.

3,530 citations


Additional excerpts

  • ...[39], [40]....

    [...]

Frequently Asked Questions (19)
Q1. What is the purpose of the storage system in a wind farm?

The storage system in a wind farm will be used for bulk power storage from wind during time-averaged 15-minute periods of high availability and to absorb or to inject energy over shorter time periods in order to contribute to the grid frequency stabilization. 

In this paper, new trends in power electronics for the integration of wind and photovoltaic power generators are presented. A review of appropriate storage systems technology used for the integration of intermittent renewable energy sources is also introduced. 

Finally, for the energy storage systems ( flywheels, hydrogen, compressed air, supercapacitors, superconducting magnetic and pumped-hydroelectric ) the future presents several fronts and actually they are in the same development level. 

The current control scheme is employed more frequently, because 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 particular, multilevel cascade converters seem to be a good solution to increase the voltage in the converter in order to eliminate the high frequency transformer. 

The advantages of variable speed turbines are that their annual energy capture is about a 5% greater than fixed speed technology, and that the active and reactive power generated can be easily controlled. 

Due to improvement of roofing PV systems, residential neighbourhoods are becoming a target of solar panels and some current projects involve installation and set-up of PV modules in high building structures [45]. 

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 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. 

The most commonly reported disadvantage of the multilevel converters with split DC-link is the voltage unbalance between the capacitors that integrate it. 

As compared to a conventional gearbox-coupled wind turbine generator, a direct drive generator has reduced overall size, lower installation and maintenance cost, a flexible control method and quick response to wind fluctuations and load variation. 

For approximately 20 years it has been a primary technology 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. 

As the ratings of the components increases and the switching and conducting properties improve, the advantages of applying multilevel converters become more and more evident. 

The continuous reduction of the cost per kW of PEBBs, is making the multilevel cascaded topologies to be the most commonly used by the industrial solutions. 

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 mechanical components. 

The hydrogen-fuel economy has been rapidly increasing in industrial application due to the advantages of the hydrogen of being storable, transportable, highly versatile, efficient and clean energy carrier to supplement or replace many of the current fuel options. 

New trends in the use of batteries for renewable energy systems focused on the integration with several energy sources (wind energy, photovoltaic systems, etc.) and also on the integration with other energy storage systems complementing them. 

Because these electrolysers require a constant minimum load, wind turbines must be integrated with grid or energy systems to provide power in the absence of wind [28]. 

As disadvantages, variable speed wind turbines need a power converter that increases the component count and make the control more complex. 

Trending Questions (1)
What are the system requirements for power bi?

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.