Inverters for Grid Connected Pv Systems

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 [email protected]

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”