Dynamic Voltage Regulator

DEPARTMENTOF ELECTRICAL ENGINEERING Page 1
ABSTRACT
ANALYSIS, MODELING AND SIMULATION OF DYNAMIC
VOLTAGE RESTORER (DVR) FOR COMPENSATION OF VOLTAGE
QUALITY DISTURBANCES
Quality of the output power delivered from the utilities has become a major concern of the
modern industries for the last decade. These power quality associated problems are voltage
sag, surge, flicker, voltage imbalance, interruptions and harmonics problems. The use of
sensitive electronic equipment has increased now a day which has lead to power quality
problems. These power quality issues may cause problems to the industries ranging from
malfunctioning of equipments to complete plant shutdowns. To overcome the problems
caused by customer side abnormalities one such reliable power device used to address the
voltage sag, swell problem is the Dynamic Voltage Restorer (DVR). It is a series connected
custom power device, which is considered to be a cost effective alternative when compared
with other commercially available voltage sag compensation devices. The main function of
the DVR is to monitor the load voltage waveform constantly and if any sag or surge occurs,
the balance (or excess) voltage is injected to (or absorbed from) the load voltage. To achieve
the above functionality a reference voltage waveform has to be created which is similar in
magnitude and phase angle to that of the supply voltage.
Keywords: Power quality, Dynamic voltage restorer, Compensating strategies, Control
methods, Voltage source converter, Simulation.

CHAPTER 1
INTRODUCTION
Power quality is a very important issue due to its impact on electricity suppliers, equipment
manufactures and customers. “Power quality is described as the variation of voltage, current
and frequency in a power system. It refers to a wide variety of electromagnetic phenomena
that characterize the voltage and current at a given time and at a given location in the power
system”.
An important percentage of all power quality problems are of the voltage-quality type.
Voltage Sag (dip) is a momentary decrease in the root mean square (RMS) voltage magnitude
in the range of 0.1 to 0.9 per unit, with a duration ranging from half cycle up to 1 min. It is
considered as the most serious problem of power quality. It is often caused by balanced or
unbalanced faults in the distribution system or by the starting of large induction motors .
Though there are many different ways to mitigate voltage sags in power systems. Among
these, the distribution static compensator and the DVR are the most effective devices; both of
them based on the voltage source converter (VSC) principle.
A DVR is a series-connected solid-state device that injects voltage into the system in order to
regulate the load side voltage. It is normally installed in a distribution system between the
supply and a critical load feeder at the so called Point of Common Coupling (PCC) which is
defined as the point of the network changes. Its primary function is to rapidly boost up the
load-side voltage in the event of voltage sag in order to avoid any power disruption to that
load.
Although, the inverter used in the a DVR can have many different topologies, this paper uses
a traditional 2-level, 3- phase pulse width modulation (PWM) inverter since this topology is
still the most popular one.
The various power quality problems are due to the increasing use of non linear and power
electronic loads. Harmonics and voltage distortion occur due to these loads. The power
quality problems can cause malfunctioning of sensitive equipments, protection and relay

system. Distribution system is mainly affected by voltage sag and swell power quality issue.
Short circuits, lightning strokes, faults and inrush currents are the causes of voltage sags.
Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers,
single line to ground fault on the system lead to voltage swells. Voltage sag is a decrease of
the normal voltage level between 10 and 90% of the nominal rms voltage at the power
frequency, for durations of 0.5 cycle to 1 minute. Voltage swells are momentary increase of
the voltage, at the power frequency, outside the normal tolerances, with duration of more than
one cycle and typically less than a few seconds. The use of custom power devices is one of
the most efficient method to mitigate voltage sag and swells.
There are many custom power devices. Each of which has its own benefits and limitations.
Custom power device (CPD) is a powerful tool based on semiconductor switches concept to
protect sensitive loads if there is a disturbance from power line.
Power quality in the distribution system can be improved by using a custom power device
DVR for voltage disturbances such as voltage sags, swells, harmonics, unbalanced voltage
and etc. The Dynamic Voltage Restorer (DVR) is a device that detects the sag or swell and
connects a voltage source in series with the supply voltage in such a way that the load voltage
is kept inside the established tolerance limits. It is normally installed in a distribution system
between the supply and the critical load feeder at the point of common coupling (PCC). Other
than voltage sags and swells compensation, DVR also has added other features like: line
voltage harmonics compensation, reduction of transients in voltage and fault current
limitations.
Following shows some abnormal electrical conditions caused both in the utility end and the
customer end that can disrupt a process
1. Voltage sags
2. Phase outages
3. Voltage interruptions
4. Transients due to Lighting loads, capacitor switching, non linear loads etc..
5. Harmonics

As a result of above abnormalities the industries may undergo burned-out motors, lost data
on volatile memories, erroneous motion of robotics, unnecessary downtime, increased
maintenance costs and burning core materials especially in plastic industries, paper mills &
semiconductor plants.
As the new technologies emerged, the manufacturing cost and the reliability of those solid
state devices are improved; hence the protection devices which incorporate such solid state
devices can be purchased at a reasonable price with better performance than the other
electrical or pneumatic devices available in the market [5]. Uninterruptible Power Supplies
(UPS), Dynamic Voltage Restorers (DVR) and Active commonly used custom power
devices.

CHAPTER: 2
POWER QUALITY PROBLELMS
2.1 Powerquality problems in distribution network
Maintaining the power quality is one of the major requirements, the electricity consumers are
demanding of. The reason is modern technology demands for an un-interrupted, high quality
electricity supply for the successful operation of voltage sensitive devices such as advanced
control, automation, precise manufacturing techniques. Power quality may be degraded due
to both the transmission and the distribution side abnormalities.
The abnormalities in the distribution system are load switching, motor starting, load
variations and non-linear loads. Whereas lightning and system faults can be regarded as
transmission abnormalities .To overcome the power quality related problems occurring in the
transmission system, FACTS (Flexible AC Transmission System) devices play a major role.
These are also referred to as Utility based solutions. One of the main advantages of the
FACTS devices is that they allow for increased controllability and optimum loading of the
lines without exceeding the thermal limits. Whereas Custom Power devices ensure a greater
reliability and a better quality of power flow to the load centers in the distribution system by
successfully compensating for voltage sags/dips, surges, harmonic distortions, interruptions
and flicker, which are the frequent problems associated with distribution lines.
However, failure of such custom power devices cause equipment failing, mal- operations,
tripping of protective relays and ultimately plant shut downs, which results huge financial
loss to the industry. Therefore proper design of control and selection of the custom power
device is very important.

2.1.1 Voltage sags and surges
The most frequent power quality associated problem in the distribution network is voltage
sags and surges .
Figure 1 Voltage Sags and surges
top left - Voltage sag occurs at the zero crossing point & without a phase shift
top right- Voltage surge occurs at zero crossing point & without a phase shift
bottom left- Voltage sag not at the zero crossing point & without a phase shift
bottom right -Voltage sag at zero crossing point with a phase shift
Voltage sag/surge can simply be defined as a sudden increase/decrease in the rms voltage
with duration of half a cycle to few cycles. In addition to the magnitude change of the supply
voltage, there can be a phase shift during the voltage sag / surge as shown in Figure 1. The
magnitude of the voltage sag will depend on the fault type and the location and also on the
fault impedance. The duration of the fault depends on the performance of the relevant

protective device. Further it has been found that the voltage sags with magnitude 70% of the
nominal value are more common than the complete outages.
Table 1 Definitions Of Voltage sag And swell
For a particular disturbance (voltage sag or swell), if the voltage and time duration it remains
is within the range, the custom power devices are the optimized solution to overcome the
problem and compensate for the abnormality during the time period.
2.1.2 Custom power devices
The most common custom power devices to compensate for the voltage sags and swells are
the Uninterruptible Power Supplies (UPS), Dynamic Voltage Restorers (DVR) and Active
Power Filters (APF) with voltage sag compensation facility. Among those the UPS is the well
known.
DVRs and APFs are less popular due to the fact that they are still in the developing stage,
even though they are highly efficient and cost effective than UPSs. DVR and APF are
normally used to eliminate two different types of abnormalities that affect the power quality.
They are discussed based on two different load situations namely linear loads and non-linear
loads.
The load is considered to be a linear when both the dependent variable and the independent
variable shows linear changes to each other. Resistor is the best example for a linear device.

The non-linear load on the other hand does not show a linear change. Capacitors and
inductors are examples for non-linear devices.
(A) when the supply voltage/current consists of abnormalities, while the load is linear
In this case the custom power device together with the defected supply should be capable of
supplying a defect free voltage/current to the load. To be precise the device should be able to
supply the missing voltage/current component of the source. A reliable device that can be
used for the above case (for voltage abnormalities) is the DVR.
(B) power supplied is in normal condition with a non linear load
When non-linear loads are connected to the system, the supply current also becomes non-
linear and this will cause harmonic problems in the supply waveform. In such situation to
make the supply current as sinusoidal, a shunt APF is connected. This APF injects/absorbs a
current to make the supply current sinusoidal. Hence the supply treats both the non-linear
load and the APF as a single load, which draws a fundamental sinusoidal current.

CHAPTER 3
DYNAMIC VOLTAGE RESTORER
3.1 Introduction
Among the power quality problems (sags, swells, harmonics…) voltage sags are the most
severe disturbances. In order to overcome these problems the concept of custom power
devices is introduced recently. One of those devices is the Dynamic Voltage Restorer (DVR),
which is the most efficient and effective modern custom power device used in power
distribution networks. DVR is a recently proposed series connected solid state device that
injects voltage into the system in order to regulate the load side voltage. It is normally
installed in a distribution system between the supply and the critical load feeder at the point
of common coupling (PCC). Other than voltage sags and swells compensation, DVR can also
added other features like: line voltage harmonics compensation, reduction of transients in
voltage and fault current limitations.
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
Figure 2 Location of DVR

3.2 Structure of the DVR
The DVR basically consists of a power circuit and a control circuit. Control Circuit is used to
derive the parameters (magnitude, frequency, phase shift, etc.) of the control signal that has to
be injected by the DVR. Based on the control signal, the injected voltage is generated by the
switches in the power circuit. Further power circuit describes the basic structure of the DVR
and is discussed in this section. Power circuit mainly comprising of five units.
Figure 3 DVR Power circuit
3.3 Basic Configurationof DVR
The general configuration of the DVR consists of:
i. An Injection/ Booster transformer
ii. A Harmonic filter
iii. Storage Devices
iv. A Voltage Source Converter (VSC)
v. DC charging circuit

vi. A Control and Protection system
3.3.1 Energy storage unit
Energy storage device is used to supply the real power requirement for the compensation
during voltage sag. Flywheels, Lead acid batteries, Superconducting magnetic energy storage
(SMES) and Super-Capacitors can be used as energy storage device. For DC drives such as
SMES, batteries and capacitors, ac to dc conversion devices (solid state inverters) are needed
to deliver power, whereas for others, ac to ac conversion is required.
3.3.2 Voltage source inverter
Generally Pulse-Width Modulated Voltage Source Inverter (PWMVSI) is used. The basic
function of the VSI is to convert the DC voltage supplied by the energy storage device into an
AC voltage. In the DVR power circuit step up voltage injection transformer is used.The
common inverter connection methods for three phase DVRs are 3 phase Graetz bridge
inverter, Neutral Point Clamp inverter and H Bridge inverter for single phase DVRs.
A) Three-phase graetz bridge
This is often called as two-level three-phase inverter. Each leg is switched according to the
PWM technique used. In the case of fundamental switching is used then the switches are on
for a period of 180˚ with a duty ratio of 50%. The inverter configuration, switching and
output waveforms for the fundamental switching are shown in Figure 2.4. This is referred to
as two-level since the phase output voltage waveform consists of two output levels; +Vd and
0 Volts.

B) Neutral Point Clamped Inverter:
This Neutral Point Clamped (NPC) inverter can be used for higher voltage levels than the
Volts. The inverter configuration and the ingle phase output waveforms
are shown in Figure 5.
C) H bridge inverter
In the H bridge inverter, four switches are used. When it used for multilevel arrangement
specially for high voltage application, it is commonly called as chain circuits.For fundamental
switching each switch is on for a duty cycle of 50% and shown in Figure 2.6.
Figure 6: H-bridge inverter configuration and its switching arrangement

3.3.3 Passive filters
Low pass passive filters are used to convert the PWM inverted pulse waveform into a
sinusoidal waveform. This is achieved by removing the unnecessary higher order harmonic
components generated from the DC to AC conversion in the VSI, which will distort the
compensated output voltage.
When the filters are in the inverter side higher order harmonics are prevented from passing
through the voltage transformer. And it will reduce the stress on the injection transformer.
But there can be a phase shift and voltage drop in the inverted output. This can be reduced by
placing the filter in the load side. But in this case since the higher order harmonic currents do
penetrate to the secondary side of the transformer, a higher rating of the transformer is
necessary.
Figure 7: Different Filter Placement
3.3.4 By-pass switch
Since the DVR is a series connected device, any fault current that occurs due to a fault in the
downstream will flow through the inverter circuit. The power electronic components in the
inverter circuit are normally rated to the load current as they are expensive to be overrated.

Therefore to protect the inverter from high currents, a by-pass switch (crowbar circuit) is
incorporated to by-pass the inverter circuit.
3.4.5 Voltage injection transformers
The high voltage side of the injection transformer is connected in series to the distribution
line, while the low voltage side is connected to the DVR power circuit. For a three-phase
DVR, three single-phase or three-phase voltage injection transformers can be connected to
the distribution line, and for single phase DVR one single-phase transformer is connected.
The basic function of the injection transformer is to increase the voltage supplied by the
filtered VSI output to the desired level while isolating the DVR circuit from the distribution
network. The transformer winding ratio is pre-determined according to the voltage required
in the secondary side of the transformer.
A higher transformer winding ratio will increase the primary side current, which will
adversely affect the performance of the power electronic devices connected in the VSI. The
rating of the injection transformer is an important factor when deciding the DVR
performance, since it limits the maximum compensation ability of the DVR. The winding
configuration of the injection transformer mainly depends on the upstream distribution
transformer.
If the distribution transformer is connected in Δ-Y with the grounded neutral, during an
unbalance fault or an earth fault in the high voltage side, there will not be any zero sequence

currents flow in to the secondary. Thus the DVR needs to compensate only the positive and
negative sequence components. As such, an injection transformer which allows only positive
and negative sequence components is adequate.

CHAPTER 4
DVR OPERATING STATES
4.1 During a voltage sag/swellonthe line
The DVR injects the difference between the pre-sag and the sag voltage, by supplying the
real power requirement from the energy storage device together with the reactive power. The
maximum injection capability of the DVR is limited by the ratings of the DC energy storage
and the voltage injection transformer ratio.
4.2 During the normal operation
Since the network is working under normal condition the DVR is not injecting any voltages
to the system. In that case, if the energy storage device is fully charged then the DVR
operates in the standby mode or otherwise it operates in the self- charging mode. The energy
storage device can be charged either from the power supply itself or from a different source.
4.3 During a short circuit or fault in the downstreamof the distribution line
In this particular case as mentioned in section 2.2.4 the by-pass switch is activated to provide
an alternate path for the fault currents. Hence the inverter is protected from the flow of high
fault current through it, which can damage the sensitive power electronic components.
4.3.1 Protection mode:
If the over current on the load side exceeds a permissible limit due to short circuit on the load
or large inrush current, the DVR will be isolated from the systems by using the bypass
switches (S2 and S3 will open) and supplying another path for current (S1 will be closed).

Figure 9 Protection mode
4.3.2 Standby Mode: (VDVR= 0)
In the standby mode the booster transformer’s low voltage winding is shorted through the
converter. No switching of semiconductors occurs in this mode of operation and the full load
current will pass through the primary.
Figure 10 Standby mode

4.4 Equations relatedto DVR
Figure 11 Equivalent ckt diagram of DVR
The system impedance Zth depends on the fault level of the load bus. When the system
voltage (Vth) drops, the DVR injects a series voltage VDVR through the injection
transformer so that the desired load voltage magnitude VL can be maintained. The series
injected voltage of the DVR can be written as

CHAPTER 5
DVR COMPENSATION TECHNIQUES
The compensation control technique of the DVR is the mechanism used to track the supply
voltage and synchronized that with the pre-sag supply voltage during a voltage sag/swell in
the upstream of distribution line. Generally voltage sags are associated with a phase angle
jump in addition to the magnitude change.
Basically the type of load connected influences the compensation strategy. For example, for a
linear load, only magnitude compensation is required as linear loads are not sensitive to phase
angle changes. Compensation is achieved via real power and reactive power injection.
Depending on the level of compensation required by the load, three types of compensation
methods are defined and discussed below namely pre-sag compensation, in-phase
compensation and energy optimization technique.
When the system is in normal condition, the supply voltage (Vs) is identified as pre-sag
voltage and denoted by Vpre-sag. In such situation since the DVR is not injecting any voltage
to the system, load voltage (Vload) and the supply voltage will be the same.
During voltage sag, the magnitude and the phase angle of the supply voltage can be changed
and it is denoted by Vsag. The DVR is in operative in this case and the voltage injected will
12

be VDVR. If the voltage sag is fully compensated by the DVR, the load voltage during the
voltage sag will be Vpre-sag.
5.1 Pre-sag compensation
This compensation strategy is recommended for the non-linear loads (e.g. thyristor controlled
drives) which needs both the voltage magnitude as well as the phase angle to be
compensated. In this technique the DVR supplies the difference between the pre-sag and the
sag voltage, thus restore the voltage magnitude and the phase angle to that of the pre-sag
value. The pre-sag method tracks the supply voltage continuously and if it detects any
disturbances in supply voltage it will inject the difference voltage between the sag or voltage
at PCC and pre-fault condition, so that the load voltage can be restored back to the pre-fault
condition. Compensation of voltage sags in the both phase angle and amplitude sensitive
loads would be achieved by pre-sag compensation method. In this method the injected active
power cannot be controlled and it is determined by external conditions such as the type of
faults and load conditions
VDVR = Vprefault - Vsag
13

5.2 In-phase compensation
The DVR compensates only for the voltage magnitude in this particular compensation
method, i.e. the compensated voltage is in-phase with the sagged voltage and only
compensating for the voltage magnitude. Therefore this technique minimizes the voltage
injected by the DVR. Hence it is recommended for the linear loads, which need not to be
compensated for the phase. This is the most straight forward method. In this method the
injected voltage is in phase
with the supply side voltage irrespective of the load current and pre-fault voltage. The phase
angles of the pre-sag and load voltage are different but the most important criteria for power
quality that is the constant magnitude of load voltage are satisfied.
It should be noted that the techniques mentioned need both the real and reactive power1 for
the compensation, and the DVR is supported by an energy storage device.
|VL|=|Vprefault|
One of the advantages of this method is that the amplitude of DVR injection voltage is
minimum for a certain voltage sag in comparison with other strategies. Practical application
of this method is in non-sensitive loads to phase angle jump.
14

5.3 Energy optimization technique
In this particular control technique the use of real power is minimized (or made equal to zero)
by injecting the required voltage by the DVR at a 90° phase angle to the load current. Figure
2.11 depicts the energy optimization technique. However in this technique the injected
voltage will become higher than that of the in-phase compensation technique. Hence this
technique needs a higher rated transformer and an inverter, compared with the earlier cases.
It is even possible to combine different compensation techniques described earlier, to achieve
better efficiency and ease of controllability. One such technique is combining both the pre-
sag and in-phase compensation method. In the combined technique the system initially
restores the load voltage to the same phase and magnitude of the nominal pre-sag voltage
(pre-sag compensation) and then gradually changes the injected voltage towards the sag
voltage phasor.
15

5.4 In-phase advanced compensationmethod
In this method the real power spent by the DVR is decreased by minimizing the power angle
between the sag voltage and load current. In case of pre-sag and in-phase compensation
method the active power is injected into the system during disturbances. The active power
supply is limited stored energy in the DC links and this part is one of the most expensive
parts of DVR. The minimization of injected energy is achieved by making the active power
component zero by having the injection voltage phasor perpendicular to the load current
phasor. In this method the values of load current and voltage are fixed in the system so we
can change only the phase of the sag voltage. IPAC method uses only reactive power and
unfortunately, not al1 the sags can be mitigated without real power, as a consequence, this
method is only suitable for a limited range of sags.
5.5 Voltage tolerance method with minimum energy injection
A small drop in voltage and small jump in phase angle can be tolerated by the load itself. If
the voltage magnitude lies between 90%-110% of nominal voltage and 5%-10% of nominal
16

state that will not disturb the operation characteristics of loads. Both magnitude and phase are
the control parameter for this method which can be achieved by small energy injection.
Figure 17 Voltage tolerance method with minimum energy injection
5.6 Control techniques used in commerciallyavailable DVRs
Most of the commercially available DVRs use either the in-phase compensation technique or
energy optimization technique, owing to minimal requirement of real power injection: hence
it reduces the capacity of the energy storage needed. Control technique describes the method
used to quantify the DVR control voltage injected during the compensation. In simple terms
it basically detects the occurrence of voltage sag.
Irrespective of the compensation techniques used, there should be a scheme to track the phase
angle and the magnitude of the supply voltage during normal operation (more specifically
positive sequence component of the supply voltage) and to detect the occurrence of voltage
sag. In other words there should be a voltage sag detection technique.
5.7 Voltage sag detectiontechniques
(i) Fourier transform
(ii) Phase Locked Loop (PLL)

(iii) Vector control (Software Phase Locked Loop –SPLL)
(iv) Peak value detection
(v) Applying the wavelet transform to each phase
Out of the techniques mentioned above only the Fourier transform, Vector control and
wavelet transform methods provide both the voltage magnitude and phase shift information.
PLL method can provide only the phase shift information while peak value detection
technique enables to get the magnitude change (voltage sag) information. Hence it is possible
to combine one or more techniques mentioned above to obtain accurate voltage sag
compensation.

CHAPTER 6
REALIZATION OF COMPENSATION TECHNIQUE
6.1 Discrete PWM-BasedControlScheme
In order to mitigate the simulated voltage sags in the test system of each compensation
technique, also to compensate voltage sags in practical application, a discrete PWM-based
control scheme is implemented, with reference to DVR.
The aim of the control scheme is to maintain a constant voltage magnitude at the sensitive
load point, under the system disturbance. The control system only measures the rms voltage
at load point, for example, no reactive power measurement is required.
The DVR controller scheme implemented in MATLAB/SIMULINK. The DVR control
system exerts a voltage angle control as follows: an error signal is obtained by comparing the
reference voltage with the rms voltage measured at the load point. The PI controller processes
the error signal and generates the required angle δ to drive the error to zero, for example; the
load rms voltage is brought back to the reference voltage.
It should be noted that, an assumption of balanced network and operating conditions are
made. The modulating angle δ or delta is applied to the PWM generators in phase A, whereas
the angles for phase B and C are shifted by 240° or -120° and 120° respectively.
VA = Sin (ωt +δ)
VB=Sin (ωt+δ-2π/3)
VC = Sin (ωt +δ+2π/3)
6.2 Testsystem for DVR
Single line diagram of the test system for DVR is composed by a 13 kV, 50 Hz generation
system, feeding two transmission lines through a 3- winding transformer connected in Y/S/S,
13/115/115 kV. Such transmission lines feed two distribution networks through two

transformers connected in S/Y, 115/11 kV. To verify the working of DVR for voltage
compensation a fault is applied at point X at resistance 0.66 U for time duration of 200 ms.
The DVR is simulated to be in operation only for the duration of the fault.
Figure 18 Single line diagram of test system

CHAPTER 7
FURTHER DEVELOPMENTS AND LIMITATIONS
6.1 Further developments
It is clear that there will be an injected voltage present even when the sag is not presented. In
the above simulations, this injected voltage waveform with a maximum peak value of 17V. It
was neglected assuming it’s a small value compared with the load voltage and the load
voltage was within the acceptable limits. As a future work and a further development an
additional control can be added to neutralize the injected voltage component during the
normal operation, by generating a similar sinusoidal waveform with a phase shift of 180˚,
which is basically the drop across the DVR internal impedance.
In the above work, due to the time limitation hardware implementation was not carried out.
The control circuit can be implemented using electronic components and power electronic
switches can be used to generate DVR injected voltages. Then the simulation results can be
compared with that of the hardware and the effectiveness of the simulated model can be
ensured.
The above simulated work was done without giving much attention to the cost factor of the
components (such as PWM components, injection transformer) involved. By selecting the
ratings of the components with worst case analysis, the cost and the performance are
optimized, better results could be obtained.
6.2 Limitations
It has been identified that the simulation results heavily dependent on the time step
considered in the simulation software. By reducing the time step beyond 1μs (for a run time
of 1.5s) the oscillatory and the stepped nature of the output waveform can be minimized. Due
to the limitations in the PSCAD simulation software and also the limitations in the processing
speed of the computer the time step could not be reduced as desired.

CHAPTER 8
CONCLUSION
Voltage sags and surges are a common problem faced by the electricity consumers. As many
industries have already making their product from the row materials, solution to this
electricity problem has been identified as the potential issue to reduce their production cost.
When considering the scenario in Sri Lanka it has been identified that single phase voltage
sags and surges are the most common than the three phase voltage abnormalities. The
commonest solution for the above problem is moving into a full UPS system, which is a
costly alternative.
In the above, voltage sag compensation using Dynamic Voltage Restorer was considered.
Even though three phase DVR system and its control techniques are popular among the
researchers, very less consideration was given to single phase DVRs and its control
techniques. This describes a voltage sag compensation technique for a single phase DVR.
The control technique was designed by combining both the in-phase and pre-sag
compensation techniques to minimize the requirement of real power and voltage ratings of
the DVR when the voltage sag prevails for a longer period of time .It uses a closed loop
control system to detect the phase angle and magnitude errors between the voltages during
and before the sag.
The system was simulated for several cases. To cover all possible voltage sags, the sags were
created with and without phase angle shift, and it was initiated at different point of the supply
voltage waveform. Finally the supply voltage with harmonic content also checked in the
simulation. In all results, the developed control technique with the proposed single phase
DVR circuit has shown a very good level of voltage compensation.

CHAPTER 9
REFERENCES
[1] http://en.wikipedia.org/wiki/Phase-locked_loop “Wikipedia the free encyclopaedia”,
accessed on January, 2007
[2] http://www.du.edu/~etuttle/electron/elect12.htm, “The phase locked loop”, accessed on
January, 2007
[3] http://perso.orange.fr/polyvalens/clemens/clemens.html, accessed on January, 2007
[4] http://www.w3.org “PI Controller” , accessed on January, 2007
[5] C. S. Chang, Zhemin Yu, “Distributed Mitigation of Voltage Sag by Optimal Placement
of Series Compensation Devices Based on Stochastic Assessment”, IEEE transactions on
Power Systems, Vol.19, No.2, May 2004, pg.788-795.
[6] Peradeniya University Annual Research Sessions (PURSE – 2006) held at University of
Peradeniya, Sri Lanka on November 30th 2006.
Topic of the Technical paper and presentation: Compensation techniques of the
Dynamic Voltage Restorer for single phase voltage sags.
[7] Second International Conference on Information and Automation 2006 (ICIA 2006)
held at Galadari Hotel, Colombo, Sri Lanka on 14-17th December 2006.
Topic of the Technical paper and presentation: Automated control technique for a
single phase Dynamic Voltage Restorer

CHAPTER 10
A REVIEW OF DIFFERENT TECHNIQUES
[1] The content of this file is taken by the Control OF A DYNAMIC VOLTAGE
RESTORER TO COMPENSATE SINGLE PHASE VOLTAGE SAGS written by the
M.V.KASUNI PERERA Master of Science Thesis Stockholm, Sweden 2007.
[2] Some of the tables¸ figures & contents are taken by the same.
[3] Some of the titles & sub titles are also taken by the help of same.

Dynamic Voltage Regulator

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