ABSTRACT: transmission and distribution system. The major fault occur

ABSTRACT: Power quality is one of the major area of concern in recent
40 years both in transmission and distribution system. The major fault occur in
the system are voltage sag, voltage swell, harmonics, voltage fluctuation etc.
There are various custom power devices are used to compensate such problems. In
this paper we have discuss on the Multilevel inverter based DVR which is used
to protect the sensitive equipment such as computer, printer from line fault
such as voltage sag. The implementation of the DVR  in the distribution system is based on the
multilevel inverter topology against voltage sag by using microcontroller.


KEYWORDS: Multilevel inverter, Dynamic voltage restorer,
voltage sag

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The common characteristics in
most of the electronics apparatus is they are very sensitive to the voltage
variation. Now a days the use of power electronics devices, computer, printer
etc. can create power quality issue. Among those power quality issue the most
of them is related to the voltage such as voltage sag, voltage swell etc. As
there are many custom power devices has been used to compensate such problems.
Dynamic voltage restorer is one of the device that can be capable of generating
and absorbing active and reactive power in distribution system against the
voltage sag. In this paper the multilevel inverter based DVR is used. The
multilevel inverter provide much attention in recent year because they can
operate with PWM techniques also with amplitude modulation. It can also provide
the improve output voltage waveform. Multilevel inverter have number of
topologies such as 1. Cascade multilevel inverter 2. Diode clamped 3. Flying Capacitor

From using Multilevel inverter,
the output voltage waveform obtained is in staircase waveform but at the same
time increased in voltage levels can create circuit complexity. Multilevel
inverter based DVR provide the compensation of the voltage sag with less
voltage stress.



sag/swell is one of the challenging power quality problem in utility industry
can be compensated by injecting power in distribution system.DVR is the series
compensation device which inject the voltage in distribution system via
insertion transformer. A typical DVR system is show in fig1.The purpose of
improvement of the voltage quality depend on the magnitude, phase shift, wave
shape. The system can be designed for based on the dc-dc converter where dc
link voltage adjusted according to voltage sag. It offers variable dc voltage as
output that given to the dc-dc converter as an input voltage. H bridge topology
has used so that dv/dt stress are to be reduce 1.The implementation of the
single phase DVR is presented 2.Analysis and implementation of the Five level
cascaded H Bridge inverter is presented, the configuration is useful in the
constant frequency applications and reactive power compensation 3.There are
different topologies are used in the case of multilevel inverter that have been
discussed 4.The MATLAB modal have been invented for the system. The simulink
is developed for the POD control technique of 5 level diode clamp MLI
presented5.Interline dynamic voltage restorer can be used to mitigate voltage
sag, its performance is based on the load power factor, simulation modal
presented in PSCAD6.Simulation modal has been done for the closed loop and
open loop system, where the PQ theory has been used to to generate reference
voltage7. Z source converter has been implemented for the various power
conversions for that impedance circuit presented8. The simulink is done to
show its effectiveness.



1.  Typical DVR system




Multi level inverter
basically used of number of electronics switches such as MOSFET, GTO etc.
Sometimes capacitor, flying clamp diode can be used to obtain higher number of
stepped waveform. By introducing the concept of Multilevel inverter the output
voltage stress become reduce also harmonic profile improved.

Following are the
types of multilevel inverter:

Cascade multilevel inverter

Multilevel inverter using flying

Diode clamp multilevel inverter.


multilevel inverter

multilevel topology often used. It is always have separate dc source so it is
efficient because it does not require any capacitor or diode clamp. In this
method the soft switching could be used to avoid bulky and losses due to
capacitor diode switching snubber circuit.


Fig 2. Cascade multilevel inverter



2)       Multilevel inverter using flying

building block of capacitor clamp circuit is known as the flying capacitor
multilevel inverter. By applying the suitable combination of capacitor the
balancing of capacitor is possible. For this method the number of bulky
capacitor clamp are required.



Fig 3. Multilevel inverter using flying capacitor


3)     Diode
clamp multilevel inverter

    This type of inverter
uses the diode to produce the different voltage levels. It is having disadvantage
of the real power flow control is difficult to do also for higher number of
voltage levels diode clamp required in
large number. 













Fig 1.Block diagram of MLI based DVR system.


In this project we are
designing multi stage dynamic voltage restorer system (DVR) to protect
sensitive devices such as computer etc. from line faults such as voltage sag,
voltage swell etc. here we constantly monitor the input line voltage using
voltage reading circuit. This input is given to microcontroller unit. The ADC
of microcontroller reads real time voltage. If the voltage is less than 10% of
nominal voltage (Voltage Sag) then accordingly microcontroller triggers the
MOSFET driver and hence the MOSFET Bridge to produce constant voltage of 230V
to protect the load device from voltage sag condition. When voltage is normal
then microcontroller turns off triggering to the bridge. Step up transformers
are connected in series with the line source and load.

In this we are designing
the voltage reading circuit by using capacitor, diode and resistor.PIC
microcontroller is capable of sensing the line voltage directly. The operating
frequency of PIC is very high (20 MHz), so any small change in line voltage is
sensed quickly and action is taken in fraction of second. Microcontroller unit
i.e. PIC16F877A is used which is having operating voltage of 3.3 to 5 volt and
also it has 33input output pin. In MOSFET bridge circuit, All
four MOSFETs are triggered in Chris-Cross way i.e. ‘S1’ & ‘S3’ are
triggered ON simultaneously keeping ‘S2’ & ‘S4’ OFF while ‘S2’ & ‘S4’
are triggered ON simultaneously keeping ‘S1′ &’S3’ OFF. The
MOSFET IRF540 is used as switch for triggering purpose, it has fast switching
& it has simple drive requirement.










The multilevel inverter emerged
as one of the effective techniques due to its advantages it can be used for
many applications. The implementation of MLI based DVR system using
microcontroller has been presented. DVR is an effective custom power device for
voltage sag mitigation. The impact of voltage sag/swell on sensitive equipment
is severe. Therefore, DVR is considered to be an efficient solution due to its
small size and fast response.          



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