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Power-Electronic Systems for
the Grid Integrationof Renewable Energy Sources
Zbigniew Leonowicz, PhD
BasedBased on: J.M.on: J.M. CarrascoCarrasco, J.T, J.T BialasiewiczBialasiewicz,, et aet al:l:Power Power--ElectronicElectronic
Systems for the Grid IntegrationSystems for the Grid Integration of Renewable Energy Sources: Aof Renewable Energy Sources: ASurveySurvey,, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,
VOL. 53, NO. 4, AUGUST 2006VOL. 53, NO. 4, AUGUST 2006..
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O
utline
New trends in power electronics for the
integration of wind and photovoltaic
Review of the appropriate storage-system
technology
Future trends in renewable energy systems
based on reliability and maturity
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Introduction
Increasing number of renewable energy
sources and distributed generators
New strategies for the operation and
management of the electricity grid
Improve the power-supply reliability and
quality Liberalization of the grids leads to new
management structures
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Power-electronics technology
Plays an important role in distributed
generation
Integration of renewable energy sources
into the electrical grid
Fast evolution, due to:
a. development of fast semiconductorswitches
b. introduction of real-time controllers
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O
utline (detailed)
1. Current technology and future trends in
variable-speed wind turbines
2. Power-conditioning systems used in grid-
connected photovoltaic (PV)
3. Research and development trends in
energy-storage systems
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W
ind turbine technology
Wind-turbine market has been growing at
over 30% a year
Important role in electricity generation
Germany and Spain
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² Variable-speed technology ² 5% increased
efficiency
² Easy control of active and reactive powerflows
² Rotor acts as a flywheel (storing energy)
² No flicker problems
² Higher cost (power electronics cost 7%)
N
ew technologies - wind turbines
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DFIG http://www.windsimulators.co.uk/images/DFIG.gif
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Variable-speed turbine with DFIG
Converter feeds the rotor winding
Stator winding connected directly to the
grid
Small
converter
Low
price
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Simplified semi-variable speed
turbine Rotor resistance of the squirrel cage
generator - varied instantly using fast
power electronics
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Variable-Speed Concept Utilizing Full-
Power Converter
Decoupled from grid
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ENERCON
multipolemultipole
synchronoussynchronous
generator generator
reduced
losses
lower costs
increased
reliabilityhttp://www.wwindea.org/technology/ch01/imgs/1_2_3_2_img1.jpg
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Full converter
driver controlling the torque
generator, using a vector controlstrategy
Energy Transfer
Control of the activeand reactive powers
total-harmonic-distortion control
Energy storage
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Rectifier and chopper
step-up chopper is used to adapt the
rectifier voltage to the dc-link
voltage of the inverter.
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Semiconductor-Device Technology
Power semiconductor devices with better
electrical characteristics and lower prices
Insulated Gate Bipolar Transistor (IGBT) ismain component for power electronics
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Integrated gated control thyristor
(IGCT) - ABB
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Comparison between IGCT and IGBT
IGBTs have higher switching frequency
than IGCTs
IGCTs are made like disk devices ² highelectromagnetic emission, cooling
problems
IGBTs are built like modular devices -lifetime of the device 10 x IGCT
IGCTs have a lower ONON--state voltage dropstate voltage drop-
losses 2x lower
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Grid-Connection Standards for Wind
FarmsVoltage Fault RideVoltage Fault Ride--Through Capability ofThrough Capability of
Wind TurbinesWind Turbines
a. turbines should stay connected andcontribute to the grid in case of a
disturbance such as a voltage dip.
b. Wind farms should generate like conventional
power plants, supplying active and reactive
powers for frequency and voltage recovery,
immediately after the fault occurred.
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Requirements
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Power-Quality Requirements for Grid-
Connected Wind Turbines - flicker + interharmonicsinterharmonics
Draft IEC-61400-21 standard for ´power-
quality requirements for Grid ConnectedWind Turbinesµ
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IEC Standard IECIEC--6140061400--2121
1. Flicker analysis
2. Switching operations. Voltage and current
transients
3. Harmonic analysis (FFT) - rectangular
windows of eight cycles of fundamental
frequency. THD up to 50th harmonic
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Other Standards
High-frequency (HF)harmonics andinterharmonics IEC
61000-4-7 and IEC61000-3-6
methods for summingharmonics andinterharmonics in the
IEC 61000-3-6 To obtain a correct
magnitude of thefrequency components,define window width,
according to the IEC61000-4-7
switchingswitching frequencyfrequency ofof
the inverter is notthe inverter is notconstantconstant
CanCan be notbe not multiplemultiple ofof50 Hz50 Hz
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Transmission Technology for the
Future Offshore installation.
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HVAC
Disadvantages:
High distributed capacitance of cables
Limited length
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HVDC
MoreMore economiceconomic > 100 km and> 100 km and powerpower 200200--900 MW900 MW
1) Sending and receiving end frequencies areindependent.
2) Transmission distance using dc is not affected bycable charging current.
3) Offshore installation is isolated from mainlanddisturbances
4) Power flow is fully defined and controllable.
5) Cable power losses are low.
6) Power-transmission capability per cable is higher.
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HVDC LCC-based
Line-commutated converters
Many disadvantages
Harmonics
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HVDC VSC based
HVDC Light ² HVDC Plus
Several advantages- flexible power control,
no reactive power compensation, «
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High-Power Medium-Voltage
Converter Topologies Multilevel-converter
1) multilevel configurations with diode clamps
2) multilevel configurations with bidirectionalswitch interconnection
3) multilevel configurations with flyingcapacitors
4) multilevel configurations with multiplethree-phase inverters
5) multilevel configurations with cascaded
single-phaseH
-bridge inverters.
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Comparison
0 0.005 0.01 0.015 0.02
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time /s
A m p l i t u d e
0 0.005 0.01 0.015 0.02
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time /s
A m p l i t u d e
http://hermes.eee.nott.ac.uk/teaching/h5cpe2/http://hermes.eee.nott.ac.uk/teaching/h5cpe2/
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Multilevel back-to-back converter for
direct connection to the grid
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Low-speed permanent-magnet
generators
power-electronic buildingblock (PEBB)
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Direct-Drive Technology for Wind
TurbinesReduced size
Lower installation and maintenance cost
Flexible control methodQuick response to wind fluctuations and load
variation
Axial flux machines
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Future Energy-Storage Technologies
in Wind FarmsZincZinc brominebromine batterybattery
High energy density relativeto lead-acid batteries
100% depth of dischargecapability
High cycle life of >2000cycles at No shelf life Scalable capacities from
10kWh to over 500kWhsystems The ability to store energyfrom any electricitygenerating source
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Hydrogen as a vehicle fuel
Electrical energy can be produced and
delivered to the grid from hydrogen by a
fuel cell or a hydrogen combustiongenerator.
The fuel cell produces power through a
chemical reaction and energy is releasedfrom the hydrogen when it reacts with the
oxygen in the air.
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Variable-speed wind turbine with
hydrogen storage system
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PV Photovoltaic Technology
PV systems as an alternative energy
resource
Complementary Energy-resource in hybridsystems
NecessaryNecessary::
high reliability
reasonable cost
user-friendly design
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PV-module connections
TThe standardshe standards
EN61000-3-2, IEEE1547,
U.S. National Electrical Code (NEC) 690 IEC61727
power quality, detection of islandingoperation, grounding
structure and the features of the presentand future PV modules.
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IEC 61000-3-2
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Islanding
PV Generator Converter AC-DC Local Loads Grid
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Market Considerations PV
Solar-electric-energy growth consistently
20%²25% per annum over the past 20 years
1) an increasing efficiency of solar cells1) an increasing efficiency of solar cells
2) manufacturing2) manufacturing--technology improvementstechnology improvements
3) economies3) economies ofof scalescale
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PV growth
2001, 350 MW of solar equipment was sold
2003, 574 MW of PV was installed.
In 2004 increased to 927 MW
Significant financial incentives in Japan,
Germany, Italy and France
triggered a huge growth in demand In 2008, Spain installed 45% of all
photovoltaics, 2500 MW in 2008 to an drop
to 375 MW in 2009
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Perspectives
World solar photovoltaic (PV) installations
were 2.826 gigawatts peak (GWp) in 2007,
and 5.95 gigawatts in 2008
The three leading countries (Germany,
Japan and the US) represent nearly 89% ofthe total worldwide PV installed capacity.
2012 are and 12.3GW- 18.8GW expected
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Efficiency
Market leader in solar panel efficiency(measured by energy conversion ratio) isSunPower, (San Jose USA) - 23.4%
market average of 12-18%. Efficiency of 42% achieved at the University
of Delaware in conjunction with DuPont(concentration) in 2007.
The highest efficiency achieved withoutconcentration is by Sharp Corporation at35.8% using a proprietary triple-junctionmanufacturing technology in 2009.
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Design of PV-Converters
IGBT technology
Inverters mustmust be able to detect an
islanding situation and take appropriatemeasures in order to protect persons and
equipment
PV cells - connected to the grid
PV cells - isolated power supplies
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Converter topologies
Central inverters
Module-oriented or module-integrated
inverters
StringString invertersinverters
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Multistring converter
Integration of PV strings of different
technologies and orientations
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Review of PV Converters
S. B. Kjaer, J. K. Pedersen, F.Blaabjerg ÅA Review of Single-Phase Grid-
Connected Inverters for Photovoltaic Modulesµ, IEEE TRANSACTIONS ON
INDUSTRY APPLICATIONS, VOL. 41, NO. 5, SEPTEMBER/OCTOBER 2005
Demands Defined by the Grid
- standards (slide 37) EN standard (applied
in Europe) allows higher current harmonics the corresponding IEEE and IEC standards.
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Islanding
Islanding is the continued operation of the
inverter when the grid has been removed
on purpose, by accident, or by damage Detection schemes - active and passive.
1. The passive methods -monitor grid
parameters.2. The active schemes introduce a
disturbance into the grid and monitor the
effect.
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Grounding & ground faults
The NEC 690 standard - system grounded
and monitored for ground faults
Other Electricity Boards only demandequipment ground of the PV modules in
the case of absent galvanic isolation
Equipment ground is the case when framesand other metallic parts are connected to
ground.
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Power injected into grid
Decoupling is necessary
p ²instantaneous
P - average
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Demands Defined by the Photovoltaic
Module
Voltage in the range from 23 to 38 V at a power generation of approximate 160 W, and their open-circuit
voltage is below 45 V.
New technolgies - voltage range around 0.5 -1.0 V at
several hundred amperes per square meter cell
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Maximum Power Point Tracker
EX.: ripple voltage should be below
8.5% of the MPP voltage in order to
reach a utilization ratio of 98%
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Cost
Cost effectiveness
using similar circuits as in single-phasepower-factor-correction (PFC) circuits
variable-speed drives (VSDs)
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High efficiency
wide range of input voltage and input
power
very wide ranges as functionsof solar irradiation and ambient
temperature.
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Meteorological data
.
(a) Irradiation distribution
for a reference year.(b) Solar energy distribution
for a reference year.
Total time of
irradiation equals 4686 h
per year.Total potential energy is
equal to 1150 kWh=(m2
year) 130 W/m2
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Reliability
long operational lifetime
most PV module manufacturer offer a
warranty of 25 years on 80% of initialefficiency
The main limiting components inside the
inverters are the electrolytic capacitorsused for power decoupling between the PV
module and the single-phase grid
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Topologies of PV inverters
Centralized Inverters
String Inverters
Multi-string Inverters
AC modules & AC cell technology
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Centralized Inverters
PV modules as series
connections (a string)
series connections thenconnected in parallel, through
string diodes
Disadvantages !
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String Inverters
Reduced version of the
centralized inverter
single string of PV modules isconnected to the inverter
no losses on string diodes separate MPPTs
increases the overall efficiency
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AC module
inverter and PV module
as one electrical device
No mismatch losses
between PV modules
Optimal adjustment ofMPPT
high voltage-
amplification necessary
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Future topologies
Multi-String Inverters
AC Modules
AC Cells
«
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Multi-string Inverters
Flexible
Every string can be controlled
individually.
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AC cell
One large PV cell connected to a dc²ac
inverter
Very low voltage New converter
concepts
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Classification of Inverter Topologies
Single-stage inverter
Dual stage inverter
Multi-string inverter
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Power Decoupling
Capacitors
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Transformers and Types of
Interconnections Component to avoid (line transformers=
high size, weight, price)
High-frequency transformers Grounding,
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Types of Grid Interfaces
Inverters operating in current-source mode
Line-commutated CSI switching at twice the
line frequency
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Voltage-Source Inverters
standard full-bridge three-level VSI
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VSI
Half-bridge diode-clamped three-level VSI
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AC Modules
1. 100-W single-transistor flyback-type HF-
link inverter
100 W, out 230 V, in 48 V, 96%, pf=0,955
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AC modules
2. 105-W combined flyback and buck²boost
inverter
105 W, out 85V, in 35V, THD <5%
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AC modules
3. Modified Shimizu Inverter (160W, 230,
28V, 87%)
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AC modules
4. 160-W buck²boost inverter
in 100V out 160V
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AC modules
5. 150-W flyback dc²dc converter with a
line-frequency dc²ac unfolding inverter
in 44V, out 120V
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AC modules
6. 100-W flyback dc²dc converter with a
PWM dc²ac inverter
30V ² 210 V
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AC modules
110-W series-resonant dc²dc converter
with an HF inverter toward the grid
30-230V , 87%
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AC modules
dual-stage topology Mastervolt Soladin 120
in 24-40V, out 230V, 91%, pf=0,99
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String Inverters
Single-stage
Dual-stage
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String Inverter
a transformerless half-bridge diode-
clamped three-level inverter
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String Inverter
two-level VSI, interfacing two PV strings
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SMA Sunny Boy 5000TL
three PV strings, each of 2200 W at 125-
750 V, with own MPPT
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PowerLynx Powerlink PV 4.5 kW
three PV strings, each 200-500 V, 1500 W
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Evaluation and Discussion
component ratings
relative cost
lifetime
efficiency
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Results
Dual-stage CSI = large electrolytic
decoupling capacitor
VSI = small decoupling electrolytic
capacitor.
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Results - Efficiency
Low efficiency=87%
C=68 QF 160V
High efficiency=93%
C=2,2 mF 45V
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Discussion - String Inverters
The dual-grounded multilevel invertersp.82 ² good solution but quite largecapacitors 2x640QF 810V -> half-periodloading
bipolar PWM switching toward the gridp.83 & 84 (no grounding possible, large
ground currents) ² 2x1200 QF 375 V current-fed fullbridge dc²dc converters
with embedded HF transformers, for eachPV string ² p.85 ² 3x 310 QF 400V
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Resume ² PV Inverters
Large centralized single-stage inverters should beavoided
Preferable location for the capacitor is in the dc
link where the voltage is high and a largefluctuation can be allowed without compromisingthe utilization factor
HFTs should be applied for voltage amplification inthe AC module and AC cell concepts
Line-frequency CSI are suitable for low power,e.g., for ac module applications.
High-frequency VSI is also suitable for both low-and high-power systems, like the ac module, thestring, and the multistring inverters
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Converter topologies (general)
PV inverters with dc/dc converter (with or
without isolation)
PV inverters without dc/dc converter (withor without isolation)
Isolation is acquired using a transformerthat can be placed on either the grid or
low frequency (LF) side or on the HF side
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HF dc/dc converter
full-bridge
single-inductor push²pull
double-inductor push²pull
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Another classification
number of cascade power processing
stages
-single-stage -- dual-stage
-----multi-stage
There is no any standard PV inverter topology
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Future
very efficient PV cells
roofing PV systems
PV modules in high building structures
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Future trends
PV systems without transformers -
minimize the cost of the total system
cost reduction per inverter watt -make PV-generated power more attractive
AC modules implement MPPT for PV
modules improving the total systemefficiency
Å plug and play systemsµ
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Research
MPPT control
THD improvements
reduction of current or voltage ripple
standards are becoming more and morestrict
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STORAGE
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Energy Storage Systems
Improvement of Q uality
Support the Grid during Interruption
Flywheels ² spinning mass energy
(commercial application with active
filters)
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Flywheel-energy-storage
low-speed flywheels (< 6000 r/min) with
steel rotors and conventional bearings
modern high-speed flywheel systems (to60 000 r/min) advanced composite wheels
ultralow friction bearing assemblies, such
as magnetic bearings
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Applications of flywheels
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Research
Experimental alternatives for wind farms
=flywheel connected to the dc link
Control strategy = regulate the dc voltageagainst the input power surges/sags or
sudden changes in the load demand
Similar approach applied to PV systems, wave
energy
D-static synchronous compensator (STATCOM)
Frequency control using distributed flywheels
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Hydrogen-storage systems
Storable
transportable,
highly versatile efficient
clean energy carrier
fuel cells to produce electricity
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Hydrogen technology
Storage
² compressed or liquefied gas
² by using metal hydrides or carbon nanotubes Technologies
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Compressed-Air Energy Storage -CAES
Energy storage in compressed air
Gas turbines
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Supercapacitors
350 to 2700 F at of 2 V.
modules 200 -to 400 V
long life cycle suitable for short discharge applications
<100 kW.
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Superconducting Magnetic Energy
Storage (SMES) energy in a magnetic field without
resistive losses
ability to release large quantities of powerduring a fraction of a cycle
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Battery Storage
Several types of batteries
Discharge rate limited by chemistry
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Pumped-Hydroelectric Storage (PHS)
variable-speed drives
30 - 350 MW, efficiencies around 75%.
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Conclusions
power-electronic technology plays a very
important role in the integration of
renewable energy sources optimize the energy conversion and
transmission
control reactive power minimize harmonic distortion
to achieve at a low cost a high efficiency
over a wide power range
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Conclusions
Achieve a high reliability
tolerance to the failure of a subsystemcomponent.
common and future trends for renewableenergy systems have been described.
Wind energy is the most advanced technology
Regulations favor the increasing number of
wind farms. The trend of the PV energy leads to consider
that it will be an interesting alternative inth f t