Transcript
INVERTERS FOR THE GRID CONNECTED PV SYSTEMS
Presented by DEEPTI GUPTA
(POWER ELECTRONICS & DRIVES)
(11SETMT104001)
Schematic diagram of grid connected PV System
GRID CONNECTED PV SYSTEMS
Grid-Connected PV system consists of PV modules, grid-connected inverters, metering device and power distribution system
The solar energy is converted into DC current by PV modules and then feeding into the local power grid network by the grid-connected inverters which synchronizing the frequency, phase and pure sine waveform with the power network.
The synchronized power is partially used for local load demands and
partially is fed to the power company.
Why we use a Grid Connected PV System?
Grid connected means that our system is connected to the utility lines, or the "grid".
A grid connected PV system is designed to meet all, or a portion of our daily energy needs.
This connection enables us to obtain the balance of our electricity from our local utility.
It also allows us to send excess solar electricity back to our power company for later use.
Use of Inverters in Grid connected PV Systems
PV systems provide direct current (DC) voltage.
To feed to the grid, this DC voltage has to be inverted to the grid alternating current (AC) voltage by a grid-tied inverter, synchronizing automatically its AC output to the exact AC voltage and frequency of the grid.
continued…..
The second important job of the solar power inverter is to control the PV system to run near its Maximum Power Point (MPP),
the operating point where the combined values of the current and voltage of the solar modules result in a maximum power output.
This MPP fluctuates during operation in an interval depending on the radiation, the cell temperature and the cell type and has so to be tracked by the inverter controlling unit
GRID CONNECTED INVERTER
The generated power in PV cells can be used in a standalone
system or can be fed to the AC main grid.
In standalone systems, the output power of the PV system can also be stored in batteries.
However, the battery systems are expensive, bulky and require high maintenance.
Where utility power is also available, another solution is to feed the power into the grid, which requires a grid connected inverter (GCI).
With a GCI, excess power is bought and credited by the utility, and grid power is available at times when the local demand exceeds the PV system output.
CURRENT SOURCE INVERTER FOR GRID CONNECTED PV SYSTEMS
The proposed two-stage PV GCI topology is based on a current-source inverter and
illustrated in the figure
This circuit topology is an extension of a Switched-Mode rectifier (SMR) circuit that was originally proposed for automotive applications , where it acted as a DC-DC converter.
The circuit uses a DC link inductor (L) in series with the PV panel to produce a constant-current source (Figure)
A boost switch (will be named as a current wave shaper, WS) is used to produce a PWM output current that resembles a rectified sine wave that is in-phase with the grid.
The thyristor based H-bridge inverter in the circuit “unfolds” the output of the current wave-shaper to produce a sinusoidal AC output current.
An output LC filter (CF and LF) is used to remove the PWM switching components (Figure).
The H-bridge inverter (unfolding circuit) in the circuit is controlled by a microcontroller, which is also used to detect zero-crossings of the mains voltage and to control the duty-cycle of the WS switch.
CONVERETER SYSTEM CONFIGURATIONS
Line frequency transformer configuration
Transform less configurationwith DC/DC boosting and stabilizing converter
CENTRALIZED INVERTER
Application in three-phase PV systems
Power ranging from 10 kW
Connection of the modules to a DC bus
High power losses
Voltage high enough to avoid the use of transformers or boost converters
For high power applications (few MW) several inverters are shunted
STRING INVERTER
Small domestic applications
Power ranging from 0.5kW to 1 kW
Reduced version of centralized inverter with one string connected to an inverter, facilitating the MPPT
Very flexible configuration
Each group have different space orientation
Parallel work of several inverters boost the reliability of the system
MULTISTRING INVERTERS
Further development of the string inverter Each PV module is interfaced by its own dc/dc converter and then connected to an inverter
Further enlargements of the PV plant are easily done because of the dc/dc converters
Reduced power losses
Lack of redundancy and scalability for grid connection
AC MODULE CONCEPT
Complex topology
Small-scale residential applications
DC/DC converter modules
DC/AC inverter modules
DC bus works as a current sharing carried parallel and redundant operation of dc/ac inverter modules are realized without communication link between them
Each module unit has independent functions
High efficiency
Flexible design
POWER ELECTRONIC CONDITIONINGSYSTEMS
Two stage topologies for single module- ex. DC/ grid-connected voltage source PWM
inverter
FLY-BACK INVERTER TOPOLOGY
Composed of buck-boost converter, fly back converter, common transformer and cyclo-converter at the output
Low number of components
FLY-BACK CURRENT FEDINVERTER TOPOLOGY
Provide rectified sine-waveoutput current Keep the MPPT PV voltage
Grid inverter may beimplemented with thyristors
The current into fly-back isdiscontinuous
Buffer capacitor is either forlow and high frequencyripple
SERIES RESONANT DC/DC CONVERTERAND BRIDGE GRID-CONNECTED INVERTER
The inverter is modified by addingtwo diodes
Dc/dc converter has fixed voltagetransfer ratio
Switching losses reduced
Grid inverter- high and lowswitching frequencies
Left leg- controlled by hysteresisband controller, operates at 20-80kHz
Right led- controlled according thepolarity of the grid voltage, gridswitching frequency
SINGLE-STAGE TOPOLOGY FORMULTIPLE MODULES
Consists of standard voltagesource PWM inverter and LCLfilter
High efficiency -97% High power losses duringpartial shading- all modules areconnected to the same MPPTdevice
Power decoupling between PVand grid- large capacitor
HALF-BRIDGE DIODE CLAMPEDTHREE-LEVEL TOPOLOGY
The three- level inverter can be expanded into 5, 7 and more levels, by adding more modules and switches
Possible further reduction of the harmonic distortion
High number of required semiconductors
Imbalanced loading of the different strings
TWO STAGE TOPOLOGIES FORMULTIPLE MODULES
Can be realized by:
Series connected modules- similar to the two stage topology for single modules
Separate dc/dc converter for each module string and common dc/ac inverter
MULTISTRING TOPOLOGY
DC/DC boost converter for each one of the strings of PV
Common half-bridge inverter The circuit also can be realized with galvanic isolated push-pull or full bridge converter
CONCLUSION
Different topologies for different applications and referring to
different combinations of power electronic devices for optimal
required parameters
Different topologies with regards to different number of PV
modules connected
Different topologies to research the inverter operation under
certain conditions- input/output active/reactive power voltage etc.
REFERENCES
Current-Source Grid-Connected Converter Topology
for Photovoltaic Systems. Ertasgin, D.M. Whaley, N. Ertugrul and W.L. Soong School of Electrical and Electronic Engineering
The University of Adelaidegurhan@eleceng.adelaide.edu.au
Zacharias P. Use of Electronics- Based Power Conversion for Distributed and Renewable Energy Sources, ISET 2008
Wikipedia
http://en.wikipedia.org/wiki/Photovoltaic_system
DER lab Young researchers and PhD seminar
“Distributed generation and renewable energy sources”
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