IRJET- Mollification Parameter Control by Dynamic Voltage Restorer (DVR)

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1190
Mollification Parameter Control By Dynamic Voltage Restorer (DVR)
Sunny Dhoke1, Roshan Mohabe2, Dhiraj Satkar3, Nagmanaj Pathan4
1,2,3 Student, Dept. of Electrical Engineering, W.C.E.M. Nagpur, Maharashtra, India
4Professor, Dept. of Electrical Engineering, W.C.E.M. Nagpur, Maharashtra, India
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Abstract -For electrical engineering, the noteworthy topic
is power quality. Power quality problem is an occurrence
demonstrated as a nonstandard current, voltage orfrequency.
Electrical failures and service disruption canbefoundinutility
distribution networks, detracting commercial operation and
sensitive industrial loads which causes heavy financial losses.
Voltage sag is one of the major problems that will be dealt
here.
Dynamic Voltage Restorer is a series connected device based
on power electronics which can quickly mollify the voltagesag
in the system and restore the load voltage to the pre-fault
value.
Mollification of the power quality problem is made possibleby
the increasing development in power electronics. This work
focuses on the major problem of voltage sag. Voltage sag
problems are mollified by devices such as UPFC, tap changing
transformer, STATCOM and DVR. Between these devices
dynamic voltage restorer provides the most economical and
commercial answer to mollify voltage sag by injecting power
as well as voltage into the system. This thesis gives an
introduction to significant power quality problems for DVR
and power electronics controllers for voltagesag mollification.
Then operation hardware and elements in DVR is explained.
This thesis gives the idea of using of the error signal to control
the triggering of the switches of an inverter using Sinusoidal
Pulse Width Modulation (SPWM) technique.
Key Words: Static Series Compensator (SSC), Dynamic
Voltage Restorer (DVR), Mollification, Peripheral
Interface Controller (PIC).
1. INTRODUCTION
The typical power quality disturbances are voltage sags,
voltage swells, intermission, phase shifts, harmonics and
transients. Among the interference, voltagesagisconsidered
the most severe since the sensitive loads are very
impressionable to temporary changes in the voltage.
“Authenticity” is a key word for utilities and their
customers in general, and it is very important to companies
operating in a highly competitive business environment, it
affectsprofitability and also is a driving force intheindustry.
A very high level of authenticity has been reached by the
electrical transmission and distribution systems, there
cannot be any disturbances that can be avoided. Problems
related with operation of electrical and electronic devices
can be caused by any disturbancesto voltage.Thereisaneed
of constant frequency, constant sine wave shape and
symmetrical voltage with a constant peak to peak value to
continue the production.
The wide area solution is required to alleviate voltage
sags and improve power quality. One of thenewestapproach
is using a DVR . The basic operational principle is injecting
the missing voltage in series to the bus and detecting the
voltage sag by zero crossing detector. DVR hasbecomeacost
effective solution for the protection of non-linear loadsfrom
voltage sags. The DVR is efficient, quick and flexible solution
to voltage sag problems. DVR consists of capacitor bank
storage unit, PWM inverter, and filter and booster
transformer.
It is well known that power quality is facing various
difficultiessuch asvoltage sags/swells, flicker,surge,voltage
imbalance, harmonic distortion and interruptions, fretting
fatigue. Voltage sags/swells can occur more regularly than
other power quality problems, also these sags/swellsarethe
most serious power quality disturbances in the power
distribution system. Dynamic Voltage Restorer (DVR) is one
of the most important custom power devicesthat have been
created to improve the performance of power quality. The
DVR balances the load voltage at a nominal magnitude and
phase by settling the voltage sag/swell. These systems can
compensate voltage sags by increasing the appropriate
voltages in cascade with the supply voltage, and therefore
prevent loss of power.
2. METHODOLOGY
Fig -1: Block diagram of the role of dynamic voltage
restorer in improving power quality.
2.1.1 Basic Configuration:
Some of the basic elements of a DVR are as follows:
 Converter
 L&C filter
 Booster transformer DC-link and paper capacitor
storage unit
 DC-link and energy storage
 By-pass equipment
 Disconnecting equipment

The converter is most likely a Voltage Source Converter
(VSC), which sinusoidal Pulse Width modulates(SPWM) the
DC from the DC-link/storage to AC-voltagesinjectedintothe
system. A VSC is a power electronic system,whichconsistsof
capacitor storage and switching devices, which can
generated a sinusoidal voltage at required constant
frequency, magnitude, and phase angle. In the DVR
application, the VSC is used to transitory replace the supply
voltage or to generate the part of the supply voltage whichis
missing. There are four main types of switching devices:
Light Activated Silicon Controlled Rectifier (LA-SCR), Gate
Turn-Off thyristor (GTO), Power Metal OxideSemiconductor
Field Effect Transistors (P-MOSFET), Integrated Gate
Commutated Thyristor (IGCT) and Insulated Gate Bipolar
Transistors (IGBT). Each type of power electronicdevicehas
its own advantages and limitation. The IGCT is a newly
compact device which is having authenticity and enhanced
performance that allows VSC to build with very large power
ratings. Because of the highly practiced converter design
with IGCTs, the DVR can balance dips which are above the
capability of the past DVRs using conventional devices. The
purpose of such devices is to supply the necessary energy to
the VSC using a dc link for the generation of injected
voltages.
2.1.2 L&C Filter:
To reduce the switching harmonics generated by the
SPWM VSC L&C filters are used.
2.1.3 Booster Transformer:
In most DVR applications the DVR is equipped with
injection transformers to certify galvanic isolation and
protection equipment and for simplification oftheconverter
topology. The Booster transformer is a specially designed
transformer that tries to limit the coupling of transient
energy and noise from the primary side to the secondary
side.
The key tasks are:
1) It connects the DVR to the distribution network using
the HV-windings. The voltage source converter to the
incoming supply voltage generates injecting series
compensating voltage.
2) Also, the Booster transformer can be used for the
purpose of isolating the load from the system.
2.1.4 DC-Link and Energy Storage:
The VSC uses a DC-link voltage to synthesize an AC
voltage into the grid. During a majorityofvoltageimmersion,
restoration of supply voltage is necessary for active power
injection. The dc charging circuit has two important tasks.
1) The first task of the DC-link and storage is to charge the
energy source after a sag compensation incident.
2) The second task is to maintain dc link voltage at the
apparent dc link voltage.
2.1.5 By-Pass Equipment:
During faults, overload and service a circumvent pathfor
the load current has to be secured. When the sag on line is
detected, DC storage unit is fed to PWM inverter.
2.1.6 DVR topology with no energy storage:
DVR topologies used with no energy storage on dc link,
Part of the supply voltage remains present during the sag
and this residual supply can be used to provide thecapacitor
boost energy required to maintain full load power at rated
voltage. A passive shunt converter is used because only
unidirectional power flow is required necessary and it is
cheap solution for voltage sag. Two main topologies can be
used, which are required to be categorized hereaccordingto
the location of shunt converter with series compensation.
2.1.7 Dynamic Voltage Restorer:
In year of 1994, L. Gyugyi proposed a device and a
method for dynamic voltage restoration of utility as per
required in distribution network. This method uses active
power in order to inject the faulted supply voltages and is
locally known as the Dynamic Voltage Restorer. In this
paper, a DVR design is essentially contains a voltage source
inverter (VSI), an series injection transformer connected
between the AC voltage supply and the non -linear load,aDC
energy storage capacitor, and a control system as shown in
Figure 2.
Fig -2: Basic DVR topology
The main function of the DVR is the protection of non-
linear loads from voltage sags/swells expected from the
network. The DVR is connected in series between thesource
voltage or grid of distribution and no-linear loads through
series injection transformer. There are one of several types
of energy storage that has been used in the DVR such as
battery, paper capacitance and superconducting coil. These
types of energy storages are very important in order to

supply real and imaginary power to the DVR. The controller
is more important part of the DVR for switching purposes.
The switching converter is responsible to conversion
process from DC to AC and to make sure that’s only the swell
or sag voltage is injected to the series injection transformer.
The three-phase transformersconnection used in the three-
phase DVR can be constructed either in delta/open or
star/open connection. In case of asymmetrical fault on high
voltage side, the zero sequence current flowing almost zero,
if the distribution transformer connection in Ä-Y with the
grounded neutral. In such a way of connection, the DVR is
only used for the allegation of the positive and negative
sequence.
3. COMPENSATION METHODS OF DVR
The type of the compensation technique mainly depends
upon the specified factors such as DVR power ratings,
various conditions of load, voltage sag type & swell. Some
loads are non-linear towards phase angle l jump and some
are sensitive towards change in magnitude and others are
tolerant to these. Therefore, the control strategies mainly
depend upon the type of load characteristics; therearethree
different methods of DVR seriesvoltage injection which are:
(a) Pre-sag compensation
(b) In-phase compensation
(c)Voltage tolerance method with minimumenergy
injection
3.1.1 Pre-Sag/Dip Compensation Method:
The pre-sag method detects the supply voltage
continuously and if it detects any disturbances in supply
voltage by micro-controller it will inject the difference
voltage between the sag and pre-fault condition, so that the
load voltage can be restored by back to the pre-fault
condition. Compensation of voltage sagsin both phase angle
and amplitude. Non-linear loads would be achieved by pre-
sag compensation method. In this technique the series
injected real power cannot be controlled and it is calculated
by external conditions such as the type of faults and load
conditions. The voltage of DVR is given below:
VDVR = Vpre fault – Vsag
Fig -3: Phasor diagram of pre-sag/dip method
3.1.2 In-Phase Compensation Method:
In this method the injected voltage is in phase with the
supply side voltage regardless of the load current and pre-
fault voltage. The phase angles of the pre-sag and load
voltage are not similar but the vital criteria forpowerquality
that is the constant magnitude of load voltage are fulfilled.
The load voltage is given below:
|VL| = |Vpre-fault|
One of the advantagesof this technique is that the amplitude
of DVR injection voltage is minimum for some voltage sag in
comparison with different strategies. Practicalapplicationof
this technique is in non-sensitive loads to phase angle jump.
Fig -4: Phasor diagram of in-phase method
3.1.3 Voltage Tolerance Method with Minimum
Energy Injection:
A small jump in phase angle and a small drop in voltage
can be tolerated by the load itself. If the phase angle
variations between 5% -10% of nominal state and voltage
magnitude lies between 90%-110% of nominal state that
will not disturb the operational featuresof loads.Bothphase
and magnitude are the control parameter for this method
and can be achieved by small energy injection. In this
technique, the magnitude and phase angle of corrected load
voltage inside the area of load voltage tolerancearechanged.
The phase angle jump and voltage drop on load can be
accepted by load itself. The delicacy of loads to voltage
magnitude and phase angle jump is different.
4. PROPOSED CONTROL TECHNIQUE
The control system of a DVR plays a vital role, with the
operational requirements of fast response when voltage
sag/swells are occur. When voltage sag or swells are
detected, the DVR must react as fast as possible and should
inject AC voltage to the grid. It can be implemented by using
a Space Vector PWM control technique which isbasedonthe
voltage reference and instantaneous values of load voltage
and supply. There are many basic rules of a controller in a
DVR that is the detection of the voltage sag or swell
occurrencesin the system; generation of thetriggerpulsesof

PWM inverter, calculation of the compensating voltage and
stop triggering pulses when the occurrence has passed. The
dq method gives the information of the phase shift (q) and
depth (d) of voltage sag with start and end time. The load
voltage is converted to VD, VQ and V0 based on park
transformation according equations (1), (2) & (3).
4.1.1 Components and their rating:
Sr. No. Component Rating
1 PIC Microcontroller 32-Bit
2 Series Inverter 1kV
3 Thyristor 0.5kV
4 Capacitor Bank 300kVAR
5 Potential Transformer 230/5V
6 Zero Crossing Detector 5V
7 Opto-coupler 12V
8 Regulator IC 5V
9 Line Frequency 50Hz
10 Filter Inductance 7mH
11 Filter Capacitance 10μF
4.1.2 Design Criteria and Rated Power
Calculations:
(1) Design criteria:
The design of the DVR is affected by the sensitive
load, the supply characteristicsand by the expected
voltage-dip characteristics. When designing for a
DVR for certain application, the following items
should be considered:
Maximum load power and power factor:
The load size strongly affects the current rating of
the voltage-source converter and the injection
transformer as well as the need of energy storage
needed.
Maximum depth and duration of voltage dips to
be corrected:
These characteristics together with the load size,
dictates the necessary storage capacity of the
energy storage device. The maximum duration and
penetration of voltage dips to be corrected is
determined by the statistics of the voltage dips at
the DVR location and by the acceptable number of
equipment trips.
Maximum allowed voltage drop of the DVR
during the standby mode:
This effect is only for controlling mode during
normal operation and indirectly the reaction speed
at the beginning of a voltage dip.
Parameters of the step-down transformers:
Coupling of the step-down transformer (CIA or YIY,
etc.) at input and output sides of the DVR.
Harmonic requirements of the load and of the
system:
These affect the harmonic filtering needed for the
DVR and also influence the choice of charging
method for the capacitors. At the first instance
when designing a DVR, some assumption could be
made to simplify the analysis, such as:
 Ideal switches
 DC-side capacitors are large enough to maintain a
ripple free DC bus voltage, even for unbalanced
input voltage.
 Series transformer and output filter components
are ideal.
In order to design a DVR, the concept of “boost
rating” is introduced to define themaximumvoltage
that the DVR is capable to inject into the power line
with respect to the nominal distribution system
voltage.
(2) Rated power calculations:
The DVR function, in case of voltage collapse, is for
exchange real power between the power system
and the energy storage device. The real power
injected by the DVR is an important feature to
precede its design process. To calculate the active
and the reactive power, a factor is defined to
indicate the reduction of the positive sequence
voltage with respect to the nominal voltage of the
load. For a certain supply voltage U, and required
load voltage UL, the DVR injected voltage is written
as:
Uc = UL –Us = (1-MF) UL
The range of the modulus of MF is defined by the
maximum variation of Us, for which the DVR is
designed. So, in normal operation MF should be
unity and Uc is zero.
MF = Us/ UL

Considering the fact that the DVR current shouldbe
designed to be the same as the rated load current,
the apparent power required by the DVR is then
calculated in terms of the apparent load power, SL
and MF by the following formula.
Sc = SL (1- MF)
Consequently, the active and reactive powers are
calculated by separating Sc into its real and
imaginary parts as
Where cos (ΦS) is the source power factor and
cos(ΦL) is the load power factor.
5. HARDWARE DESIGN
This is the hardware implementation of Dynamic Voltage
Restorer (DVR) designed for mollification of voltage sags.
The arrangement is of 10 sub-circuits as shown in the LA-
SCR’s, DVR coupling network, Opto coupler circuit, Isolated
power supply circuit, Lamp load, Line Impedance, PIC
microcontroller circuit,1:1 isolated transformer, and the 3
transformers are:
1) For DVR DC Supply,
2) For trigger circuit,
3) For control circuit
In the below diagram, the 230V, 50 HZ supply is isolated by
means of a 1:1 (230/230), 150VA isolationtransformer.This
isolated voltage is further steady state as the line voltage,
which supplies power to the load, through simulated line
impedance. The simulated line impedance is simply a wire
wound resistor. To have maximum effect of voltage drop
acrossthe line impedance, the resistance valueconsideredis
of very high value, 120 Ohms. After this simulated line
impedance, a switch is connected, through which the load
can be connected or disconnected. In the recurring path, the
DVR output is connected through the output winding of the
coupling transformer developed.ThisoutputfromtheDVRis
connected in series with the load, and mitigates the line
impedance effect. The DVR circuit has to check the load
voltage and need to be connected to the micro controller
circuit and correct the voltage if there is any excess or
absence when compared to a Reference voltage, by injecting
appropriate voltage into the circuit by means of thecoupling
network.
The authors can acknowledge any person/authorities in
this section. This is not mandatory.
Fig -5: Hardware demonstration of Dynamic Voltage
Restorer (DVR).
6. DYNAMIC VOLTAGE RESTORER
The functioning and designing of different blocksofdynamic
voltage restorer is discussed herewith.
a) Single phase voltage source bridge inverter:
The voltage source inverter used in the DVR circuit makes
the induction of required voltage with required phase
possible. This inverter uses dc capacitors as the supply and
can switch at a high frequency to generate asignalwhichwill
mitigate the voltage sags and swells across the load.
b) Micro controller circuit:
The microcontroller circuit is the heart of the system, and is
responsible for generating the reference voltage waveform
from the voltage waveform that is sampled from the sensing
network, which it has obtained from the reference voltage
sensing circuit. This reference voltage waveform is
generated keeping the zero crossing as the reference to
maintain the phase relationship of the load and correcting
voltage. As the voltage induced through the coupling
transformer, generated by the bridge inverter, the resulted
voltage across the load will be a pure sinusoid of required
voltage.
c) Opto coupler and driver circuits:
Opto couplers are having the capability of transferring an
electrical signal between two circuits while electrically
isolating the circuitsfrom each other. They generallyconsist
of an infrared LED, light emitting section at the input and a
silicon photo detector at the output. The input for opto-
couplers can be either AC or DC, which can drive the LED.
d) Coupling network:
The voltage waveform for mitigating the voltage variations
in the load circuit is achieved with the voltage source
inverter, coupling transformer and an interfacing filter. The
coupling transformer needs to transfer energy from the

voltage source inverter to the load and at thesametimeneed
to provide low impedance on the load side, so that the
transformer winding itself doesn‟t provide a voltage drop
across the load.
e) Inverter driving circuitry:
In order to trigger the LA-SCR, it is required to apply a +12V
pulse to make it turn ‘ON’ and -12V pulse to turn it ‘OFF’ to
the gate with respect to its emitter. Thus, in order to provide
triggering pulses to each and every MOSFET, it is requiredto
have four isolated power supplies of ± 12V which can be
used to apply trigger pulses to the respective MOSFETs.
This inverter provides the required outputs, which can be
used to drive the opto-coupler secondary side, which is
meant for driving the LA-SCR gate.
f) Isolated power supplies for filter elements:
Three different power supplies are required, to provide
power to various blocks of the over-all DVR circuit.
7. CONCLUSION
In this paper, hardware model of DVR is setup.Theproposed
method is used for identification of the voltage sag and is
capable of mollification of the sag by keeping and
maintaining the magnitude of load voltage at the voltage
which is desired and THD within limits. The proposed
method is very easy to understand and is reliable, has been
used only for one switching per phase. Hence the system is
easy, simple, but requires energy storage device as
compared to commonly usedDVRorSTATCOM.Theworking
performance of the proposed device is verifiedbytheoretical
results and is found to be satisfactory. This is the best
control technique for non-linear loadswhichcan’twithstand
for phase angle jumps is pre-sag compensation. For
minimum voltage injection, in-phase injectioncompensation
is the best. For minimum energy insertion by the DVR,phase
advance compensation is best but requires more voltage
injection.
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