Transcript Power-Electronic Systems for the Grid Integration of

Power-Electronic Systems for
the Grid Integration
of Renewable Energy Sources
Based on: J.M. Carrasco, J.T Bialasiewicz, et al:Power-Electronic
Systems for the Grid Integration of Renewable Energy Sources: A
Survey, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,
VOL. 53, NO. 4, AUGUST 2006.
Zbigniew Leonowicz, PhD
Outline
• 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
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
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 semiconductor
switches
b. introduction of real-time controllers
Outline (detailed)
1. Current technology and future trends in
variable-speed wind turbines
2. Power-conditioning systems used in gridconnected photovoltaic (PV)
3. Research and development trends in
energy-storage systems
Wind turbine technology
• Wind-turbine market has been growing at
over 30% a year
• Important role in electricity generation
• Germany and Spain
New technologies - wind turbines
– Variable-speed technology – 5% increased
efficiency
– Easy control of active and reactive power
flows
– Rotor acts as a flywheel (storing energy)
– No flicker problems
– Higher cost (power electronics cost 7%)
DFIG
http://www.windsimulators.co.uk/images/DFIG.gif
Variable-speed turbine with DFIG
• Converter feeds the rotor winding
• Stator winding connected directly to the
grid
• Small
converter
• Low
price
Simplified semi-variable speed
turbine
• Rotor resistance of the squirrel cage
generator - varied instantly using fast
power electronics
Variable-Speed Concept Utilizing FullPower Converter
• Decoupled from grid
ENERCON
multipole
synchronous
generator
reduced
losses
lower
costs
increased
reliability
http://www.wwindea.org/technology/ch01/imgs/1_2_3_2_img1.jpg
Full converter
Energy storage
driver controlling the torque
generator, using a vector control
strategy
Energy Transfer
Control of the active
and reactive powers
total-harmonicdistortion control
Rectifier and chopper
step-up chopper is used to adapt the
rectifier voltage to the dc-link
voltage of the inverter.
Semiconductor-Device Technology
• Power semiconductor devices with better
electrical characteristics and lower prices
• Insulated Gate Bipolar Transistor (IGBT) is
main component for power electronics
Integrated gated control thyristor
(IGCT) - ABB
Comparison between IGCT and IGBT
• IGBTs have higher switching frequency
than IGCTs
• IGCTs are made like disk devices – high
electromagnetic emission, cooling
problems
• IGBTs are built like modular devices lifetime of the device 10 x IGCT
• IGCTs have a lower ON-state voltage droplosses 2x lower
Grid-Connection Standards for Wind
Farms
Voltage Fault Ride-Through Capability of
Wind Turbines
a. turbines should stay connected and
contribute 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.
Requirements
Power-Quality Requirements for GridConnected Wind Turbines
• - flicker + interharmonics
• Draft IEC-61400-21 standard for “powerquality requirements for Grid Connected
Wind Turbines”
IEC Standard IEC-61400-21
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
Other Standards
• High-frequency (HF)
according to the IEC
harmonics and
61000-4-7
interharmonics IEC
• switching frequency of
61000-4-7 and IEC
the inverter is not
61000-3-6
constant
• methods for summing • Can be not multiple of
harmonics and
50 Hz
interharmonics in the
IEC 61000-3-6
• To obtain a correct
magnitude of the
frequency components,
define window width,
Transmission Technology for the
Future
• Offshore installation.
HVAC
• Disadvantages:
• High distributed capacitance of cables
• Limited length
HVDC
More economic > 100 km and power 200-900 MW
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
disturbances
4) Power flow is fully defined and controllable.
5) Cable power losses are low.
6) Power-transmission capability per cable is higher.
HVDC LCC-based
• Line-commutated converters
• Many disadvantages
• Harmonics
HVDC VSC based
HVDC Light – HVDC Plus
Several advantages- flexible power control,
no reactive power compensation, …
High-Power Medium-Voltage
Converter Topologies
• Multilevel-converter
1) multilevel configurations with diode clamps
2) multilevel configurations with bidirectional
switch interconnection
3) multilevel configurations with flying
capacitors
4) multilevel configurations with multiple
three-phase inverters
5) multilevel configurations with cascaded
single-phase H-bridge inverters.
Comparison
http://hermes.eee.nott.ac.uk/teaching/h5cpe2/
1
0.8
0.6
0.2
1
0
0.8
-0.2
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0.6
-0.6
0.4
-0.8
0.2
-1
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0.005
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Time /s
0.015
0.02
Amplitude
Amplitude
0.4
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-0.2
-0.4
-0.6
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-1
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Time /s
0.015
0.02
Multilevel back-to-back converter for
direct connection to the grid
Low-speed permanent-magnet
generators
power-electronic building
block (PEBB)
Direct-Drive Technology for Wind
Turbines
•Reduced size
•Lower installation and maintenance cost
•Flexible control method
•Quick response to wind fluctuations and load
variation
•Axial flux machines
Future Energy-Storage Technologies
in Wind Farms
Zinc bromine battery
• High energy density relative
to lead-acid batteries
• 100% depth of discharge
capability
• High cycle life of >2000
cycles at
• No shelf life
• Scalable capacities from
10kWh to over 500kWh
systems
• The ability to store energy
from any electricity
generating source
Hydrogen as a vehicle fuel
• Electrical energy can be produced and
delivered to the grid from hydrogen by a
fuel cell or a hydrogen combustion
generator.
• 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.
Variable-speed wind turbine with
hydrogen storage system
PV Photovoltaic Technology
• PV systems as an alternative energy
resource
• Complementary Energy-resource in hybrid
systems
Necessary:
• high reliability
• reasonable cost
• user-friendly design
PV-module connections
The standards
• EN61000-3-2, IEEE1547,
• U.S. National Electrical Code (NEC) 690
• IEC61727
• power quality, detection of islanding
operation, grounding
• structure and the features of the present
and future PV modules.
IEC 61000-3-2
Islanding
PV Generator
Converter AC-DC
Local Loads
Grid
Market Considerations PV
• Solar-electric-energy growth consistently
20%–25% per annum over the past 20 years
1) an increasing efficiency of solar cells
2) manufacturing-technology improvements
3) economies of scale
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
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% of
the total worldwide PV installed capacity.
• 2012 are and 12.3GW- 18.8GW expected
Efficiency
• Market leader in solar panel efficiency
(measured by energy conversion ratio) is
SunPower, (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 without
concentration is by Sharp Corporation at
35.8% using a proprietary triple-junction
manufacturing technology in 2009.
Design of PV-Converters
• IGBT technology
• Inverters must be able to detect an
islanding situation and take appropriate
measures in order to protect persons and
equipment
• PV cells - connected to the grid
• PV cells - isolated power supplies
Converter topologies
• Central inverters
• Module-oriented or module-integrated
inverters
• String inverters
Multistring converter
• Integration of PV strings of different
technologies and orientations
Review of PV Converters
• S. B. Kjaer, J. K. Pedersen, F.Blaabjerg „A Review of Single-Phase GridConnected Inverters for Photovoltaic Modules”, 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.
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.
Grounding & ground faults
• The NEC 690 standard - system grounded
and monitored for ground faults
• Other Electricity Boards only demand
equipment ground of the PV modules in
the case of absent galvanic isolation
• Equipment ground is the case when frames
and other metallic parts are connected to
ground.
Power injected into grid
• Decoupling is necessary
• p –instantaneous
• P - average
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
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%
Cost
• Cost effectiveness
• using similar circuits as in single-phase
power-factor-correction (PFC) circuits
• variable-speed drives (VSDs)
High efficiency
• wide range of input voltage and input
power
• very wide ranges as functions
of solar irradiation and ambient
temperature.
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
Reliability
• long operational lifetime
• most PV module manufacturer offer a
warranty of 25 years on 80% of initial
efficiency
• The main limiting components inside the
inverters are the electrolytic capacitors
used for power decoupling between the PV
module and the single-phase grid
Topologies of PV inverters
•
•
•
•
Centralized Inverters
String Inverters
Multi-string Inverters
AC modules & AC cell technology
Centralized Inverters
• PV modules as series
connections (a string)
• series connections then
connected in parallel, through
string diodes
• Disadvantages !
String Inverters
• Reduced version of the
centralized inverter
• single string of PV modules is
connected to the inverter
• no losses on string diodes
• separate MPPTs
• increases the overall efficiency
AC module
• inverter and PV module
as one electrical device
• No mismatch losses
between PV modules
• Optimal adjustment of
MPPT
• high voltageamplification necessary
Future topologies
•
•
•
•
Multi-String Inverters
AC Modules
AC Cells
…
Multi-string Inverters
• Flexible
• Every string can be controlled
individually.
AC cell
• One large PV cell connected to a dc–ac
inverter
• Very low voltage
• New converter
concepts
Classification of Inverter Topologies
• Single-stage inverter
• Dual stage inverter
• Multi-string inverter
Power Decoupling
• Capacitors
Transformers and Types of
Interconnections
• Component to avoid (line transformers=
high size, weight, price)
• High-frequency transformers
• Grounding,
•
Types of Grid Interfaces
• Inverters operating in current-source mode
Line-commutated CSI switching at twice the
line frequency
Voltage-Source Inverters
• standard full-bridge three-level VSI
VSI
• Half-bridge diode-clamped three-level VSI
AC Modules
1. 100-W single-transistor flyback-type HFlink inverter
• 100 W, out 230 V, in 48 V, 96%, pf=0,955
AC modules
2. 105-W combined flyback and buck–boost
inverter
• 105 W, out 85V, in 35V, THD <5%
AC modules
3. Modified Shimizu Inverter (160W, 230,
28V, 87%)
AC modules
4. 160-W buck–boost inverter
• in 100V out 160V
AC modules
5. 150-W flyback dc–dc converter with a
line-frequency dc–ac unfolding inverter
• in 44V, out 120V
AC modules
6. 100-W flyback dc–dc converter with a
PWM dc–ac inverter
• 30V – 210 V
AC modules
• 110-W series-resonant dc–dc converter
with an HF inverter toward the grid
• 30-230V , 87%
AC modules
• dual-stage topology Mastervolt Soladin 120
• in 24-40V, out 230V, 91%, pf=0,99
String Inverters
• Single-stage
• Dual-stage
String Inverter
• a transformerless half-bridge diodeclamped three-level inverter
String Inverter
• two-level VSI, interfacing two PV strings
SMA Sunny Boy 5000TL
• three PV strings, each of 2200 W at 125750 V, with own MPPT
PowerLynx Powerlink PV 4.5 kW
• three PV strings, each 200-500 V, 1500 W
Evaluation and Discussion
•
•
•
•
component ratings
relative cost
lifetime
efficiency
Results
• Dual-stage CSI = large electrolytic
decoupling capacitor
• VSI = small decoupling electrolytic
capacitor.
Results - Efficiency
• Low efficiency=87%
• C=68 mF 160V
• High efficiency=93%
• C=2,2 mF 45V
Discussion - String Inverters
• The dual-grounded multilevel inverters
p.82 – good solution but quite large
capacitors 2x640mF 810V -> half-period
loading
• bipolar PWM switching toward the grid
p.83 & 84 (no grounding possible, large
ground currents) – 2x1200 mF 375 V
• current-fed fullbridge dc–dc converters
with embedded HF transformers, for each
PV string – p.85 – 3x 310 mF 400V
Resume – PV Inverters
• Large centralized single-stage inverters should be
avoided
• Preferable location for the capacitor is in the dc
link where the voltage is high and a large
fluctuation can be allowed without compromising
the utilization factor
• HFTs should be applied for voltage amplification in
the 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 lowand high-power systems, like the ac module, the
string, and the multistring inverters
Converter topologies (general)
• PV inverters with dc/dc converter (with or
without isolation)
• PV inverters without dc/dc converter (with
or without isolation)
• Isolation is acquired using a transformer
that can be placed on either the grid or
low frequency (LF) side or on the HF side
HF dc/dc converter
• full-bridge
• single-inductor push–pull
• double-inductor push–pull
Another classification
• number of cascade power processing
stages
• -single-stage
• -- dual-stage
• -----multi-stage
• There is no any standard PV inverter
topology
Future
• very efficient PV cells
• roofing PV systems
• PV modules in high building structures
Future trends
• PV systems without transformers minimize the cost of the total system
• cost reduction per inverter watt -make PVgenerated power more attractive
• AC modules implement MPPT for PV
modules improving the total system
efficiency
• „ plug and play systems”
Research
• MPPT control
• THD improvements
• reduction of current or voltage ripple
• standards are becoming more and more
strict
STORAGE
Energy Storage Systems
• Improvement of Quality
• Support the Grid during Interruption
• Flywheels – spinning mass energy
• (commercial application with active
filters)
Flywheel-energy-storage
• low-speed flywheels (< 6000 r/min) with
steel rotors and conventional bearings
• modern high-speed flywheel systems (to
60 000 r/min) advanced composite wheels
ultralow friction bearing assemblies, such
as magnetic bearings
Applications of flywheels
Research
• Experimental alternatives for wind farms
=flywheel connected to the dc link
• Control strategy = regulate the dc voltage
against 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
Hydrogen-storage systems
•
•
•
•
•
Storable
transportable,
highly versatile
efficient
clean energy carrier
• fuel cells to produce electricity
Hydrogen technology
• Storage
– compressed or liquefied gas
– by using metal hydrides or carbon nanotubes
• Technologies
Compressed-Air Energy Storage -CAES
• Energy storage in compressed air
• Gas turbines
Supercapacitors
•
•
•
•
350 to 2700 F at of 2 V.
modules 200 -to 400 V
long life cycle
suitable for short discharge applications
<100 kW.
Superconducting Magnetic Energy
Storage (SMES)
• energy in a magnetic field without
resistive losses
• ability to release large quantities of power
during a fraction of a cycle
Battery Storage
• Several types of batteries
• Discharge rate limited by chemistry
Pumped-Hydroelectric Storage (PHS)
• variable-speed drives
• 30 - 350 MW, efficiencies around 75%.
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
Conclusions
• Achieve a high reliability
• tolerance to the failure of a subsystem
component.
• common and future trends for renewable
energy 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 in
the near future