Power-Electronic Systems for the Grid
Integration of Renewable Energy Sources
M A ISLAM
EEE, IIUC
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 energystorage 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 totalharmonic-distortion
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)
harmonics and
interharmonics IEC 61000- •
4-7 and IEC 61000-3-6
• methods for summing
harmonics and
•
interharmonics in the IEC
61000-3-6
• To obtain a correct
magnitude of the
frequency components,
define window width,
according to the IEC
61000-4-7
switching frequency of
the inverter is not
constant
Can be not multiple of 50
Hz
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 threephase inverters
5) multilevel configurations with cascaded singlephase H-bridge inverters.
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 Grid-Connected
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 powerfactor-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 voltage-amplification
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 HF-link
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 linefrequency 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 diode-clamped
three-level inverter
String Inverter
• two-level VSI, interfacing two PV strings
SMA Sunny Boy 5000TL
• three PV strings, each of 2200 W at 125-750 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 low- and
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