International Journal of Engineering Research & Technology (IJERT)

ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

Voltage Regulation by Solid State Tap Change

Mechanism for Distributing Transformer
Vigneshwaran. S Yuvaraja. T
Department Of Eee Department Of Eee
Nandha Engineering College Nandha Engineering College
Erode-52 Erode-52

Abstract—Distributed Generation has gained its B. Automatic Voltage Regulator

importance after considering the environmental hazards and A voltage regulator is designed to automatically
recent developments in the renewable energy sector. The maintain a constant voltage level. A voltage regulator may
penetration of Distributed Generation and varying load at the be a simple "feed-forward" design or may include negative
consumer side has its own demerits of maintaining the feedback control loops. It may use an
constant output voltage at the consumer side. Since voltage is electromechanical mechanism, or electronic components.
one of the most important parameter for the control of Depending on the design, it may be used to regulate one or
electric power system, hence it needs to be maintained atleast
near to its constant value. The method involve in maintaining more AC or DC voltages.
the output voltage within the acceptable limit at the consumer
side is by Tap Changing mechanism associated with the
C. Variable Shunt Reactor
distributing transformer. This project work deals with the Variable Shunt Reactors are used in high
replacement of conventional Electro-mechanical way of On- voltage energy transmission systems to stabilize the voltage
Load Tap changers with the Solid State Tap Changers for during load variations. A traditional shunt reactor has a
voltage regulation. With fully electronic control of tap fixed rating and is either connected to the power line all the
changing the problems associated with the mechanical On- time or switched in and out depending on the load. The
Load tap changing which includes excessive conduction losses, regulation speed is normally in the order seconds per step
slow operation and arcing in the diverter switch have been and around a minute from max to min rating. The VSR can
properly rectified. It also explains the sequential switching of continuously compensate reactive power as the load varies
the Thyristor to regulate the voltage, based on the voltage and thereby securing voltage stability.
feedback taken from the output side. Here Pulse Width
Modulation (PWM) technique is adopted for sequential D. Tap Changer
switching of the Thyristor, depending on the voltage feedback. A tap changer is a connection point selection
By adopting Solid State tap changers in the distribution
mechanism along a power transformer winding that allows
transformers fast and reliable operation is ensured and also
a variable number of turns to be selected in discrete steps.
encourages the proliferation of Distribution Generation.
A transformer with a variable turn‟s ratio is produced,
enabling stepped voltage regulation of the output. The tap
Keywords—On-Load Tap Changer, Pulse Width
selection may be made via an automatic or manual tap
Modulation, Silicon Controlled Rectifier (SCR), pulse
changer mechanism. Generally tap changers are classified
transformer.
into two types.
I.INTRODUCTION (i) Off-circuit or de-energized tap changing (DETC)
Voltage is one of the most important parameter for the is sometimes employed in high voltage transformer
control of power systemA distribution transformer is designs. Since the different tap points are at different
a transformer that provides the final voltage transformation voltages, the two connections cannot be made
in the electric power distribution system, stepping down the simultaneously, as this would short-circuit a number of
voltage used in the distribution lines to the level used by turns in the winding and produce excessive circulating
the customer. A voltage regulator is designed to current. Consequently, the power to the device must be
automatically maintain a constant voltage level. interrupted during the switchover event. It is only
applicable to installations in which the loss of supply
A. Distribution Transformer can be tolerated.
A distribution transformer is a transformer that provides
the final voltage transformation in the electric power (ii) On-Load Tap Changer (OLTC) is employed for
distribution system, stepping down the voltage used in the many power transformer applications, where supply
distribution lines to the level used by the customer. The interruption during a tap change is unacceptable. On-
invention of a practical efficient transformer made AC load tap changers may be generally classified as either
power distribution feasible. mechanical, or electronically assisted, or fully
electronic.

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 International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

A mechanical tap changer physically makes the new

connection before releasing the old using multiple tap
selector switches, but avoids creating high circulating
currents by using a diverter switch to temporarily place
large diverter impedance in series with the short-circuited
turns. This technique overcomes the problems with open or
short circuit taps. In a typical diverter switch powerful
springs are tensioned by allow power motor, and then
rapidly released to affect the tap changing operation. To
reduce arcing at the contacts, the tap changer operates in a
chamber filled with insulating transformer oil, or inside an
SF6vessel.
Thyristor-assisted tap changers use thyristors to take the
on-load current while the main contacts change over from Fig. 1. Traditional electric power (left) and electric power system with
distributed generations (right).
one tap to the previous. This prevents arcing on the main
contacts and can lead to a longer service life between
Voltage is one of the most important parameters for the
maintenance activities.
control of electric power systems. Hence the voltage
Solid State Tap Changers are typically employed only stability cannot be compromised. An easily implemented
on smaller power transformers. Solid state tap changers and cost-efficient method for grid integration of DG and
which uses thyristors both to switch the load current and to regulating the voltage at consumer side is by improving the
pass the load current in the steady state. transformer control concept of MV/LV-transformer, which
is typically an On-Load Tap Changing transformer control.
E. Distributed Generation The On-Load Tap Changer (OLTC) transformers are used
The development of Distributed Generation in India has between these multiple voltage levels to regulate and
been quite impressive. Over the last few years, a number of maintain the voltage which is supplied to consumers within
influences have been combined to lead to the increased statutory limits. The OLTC voltage regulation is naturally
interest in the use of small-scale generation, connected to operated by changing the number of turns in one winding
local distribution systems, which is commonly called of the transformer to physically alter the ratios of the
„Distributed Generation‟ (DG).The DG on the networks transformers.
will make a significant reduction of the total consumption
of fossil fuelled electricity, hence allowing a substantial Voltage regulation at the sub-transmission and
minimisation of Carbon-di-oxide emission. distribution levels strongly relies on the use of transformer
tap changers, implemented now-a-days by means of
F. Overview of penetration of distributed generation sophisticated electromechanical mechanisms. The
Control of Distributed Generation (DG) systems in possibility of replacing such slow and prone-to-wear
power distribution systems is very important task that must switches by electronic devices has been the subject of much
be considered carefully. The presence of local generation in interest in the last few years.
a distribution system will affect the distribution system. G. Existing model
Distribution networks have not been designed to cope with
power injections from DG, therefore the proliferation of The conventional OLTC transformer adopts the
DG on the electric networks results in a number of adverse mechanical on-load tap-changer. The major parts of the
effects. Typically, one the most severe situation is that mechanical tap changer are on-off selector or tap-changer,
voltage magnitude at the proximity of DG exceeds the diverter switch, transition resistance. A mechanical tap
statutory limits during maximum power output from DG changer physically makes the new connection before
and minimum power demand from the network. In such releasing the old using multiple tap selector switches, but
case the DG will alter the power flow in the distribution avoids creating high circulating currents by using a diverter
system, and the distribution system can no longer be switch to temporarily place large diverter impedance in
considered as a system with unidirectional power flow. series with the short-circuited turns. In a typical diverter
Here the network experiences the largest reverse power switch powerful springs are tensioned by a low power
flow and large voltage change; hence the network has motor (motor drive unit (MDU)), and then rapidly released
become active distribution network. An active distribution to effect the tap changing operation. To reduce arcing at the
network is defined as distribution network with system in contacts, the tap changer operates in a chamber filled with
place to control a combination of distributed energy insulating transformer oil, or inside an SF6vessel.To
resource comprising of both generator and storage which prevent contamination of the tank oil and facilitate
affects the network safety and stability. maintenance operations, the diverter switch usually
operates in a separate compartment from the main
transformer tank, and often the tap selector switches will be
located in the compartment as well. All of the winding taps
will then be routed into the tap changer compartment
through a terminal array.

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 International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

H. Problems in existing model III. VOLTAGE CONTROL IN ELECTRIC

 The mechanical regulating tap-changer of the DISTRIBUTION NETWORK
traditional OLTC transformer produces the electric arc The voltage variation ∆V across the line can be
in the tap changing process, and the tap-changer's approximated and represented by the following equation:
movement speed is slow, the regulating response time 𝑃𝑅+𝑄𝑋
∆V = (1)
is long. 𝑉
 Mechanical drive components, brushes and contractors
require regular maintenance and/or replacement. Where ∆V indicates voltage variation, P and Q represent
active and reactive power output of DG, X and R are
 Frequent overloads can damage brushes. reactance and resistance of the line connecting to DG, V is
 Speed of voltage correction correct may not be fast nominal voltage at the terminal of DG. An on-load tap
enough for electronic loads changer (OLTC) transformer, a local load, a reactive power
 It has the high failure rate, maintains difficultly and is compensator, an automatic voltage controllers (AVCs), a
unable to accurately control regulating time. line drop compensator (LDC) and a energy storage device
So this kind of mechanical on-load tap changer has not are also connected on the network.
been able to completely satisfy the security of the modern
electrical network and the request of economical movement
I. Proposed model
Like the conventional electro-mechanical OLTC, the
solid state tap changer also needs a transformer with
several tapings. The solid state tap changer is connected in
series to an MV or LV feeder. In addition to a common
MV/LV transformer the solid state tap changer is an
alternative to an conventional OLTC in the MV/LV
transforming station. The solid state tap changer operates as Fig. 2. Simple radial feeders with connected DG
follows:
Generally, compared with transmission line, the X/R
In order to increase the voltage during a voltage drop on
ratio is relatively low in a distribution network. According
the output side, the equipment extracts voltage feedback via
to equation, any significant amount of power injected by
the Transformer. Through controlling the thyristor using
DG will result in voltage rise/drop on the distribution
Pulse Width Modulation (PWM) technique, the solid state
network, especially in a weak distribution feeder with high
tap changer regulates the voltage by changing the tap on
impedance. The voltage variation would also depend on
the primary side of the transformer with the aim of
several factors including DG size and location, and method
obtaining a constant operating voltage on the output side
of voltage regulation.
according to the reference value. If there is a voltage
increase in the output side, the solid state tap changer A. Role of Oltc
operates correspondingly. The solid state tap changer has The most common voltage control technique on the
several advantages compared to the conventional OLTC. distribution network is to use OLTCs which maintain a
The solid state tap changer regulates the voltage within stable secondary voltage by selecting the appropriate tap
milliseconds, while the conventional OLTC needs at least position. It is an effective way to control the voltage by
several seconds. Furthermore, the solid state tap changer is shifting phase angle and adjusting voltage magnitude. It is
normally used with a timing relay to reduce the number of usually in conjunction with AVC relay and LDC. The AVC
tap settings. The solid state tap changer regulates the relay continuously monitors the output voltage from the
voltage continuously, while the voltage is set in stages by transformer; a tap change command will be initiated when
the conventional OLTC. Furthermore, the solid state tap the voltage is above the pre-set limits. The LDC is used to
changer is able to regulate each single phase. compensate additional voltage drop on the line between the
II. VOLTAGE STABILITY transformer and load location, particular, in the far end of
the feeder.
Voltage stability refers to the ability of a power system
to maintain steady voltages at all buses in the system after  In order to cope with dramatic changes on network
being subjected to a disturbance from a given initial management system, intelligent distributed controllers
operating condition. It depends on the ability to will be widespread on the network to minimise.
maintain/restore equilibrium between load demand and  The voltage impacts. The control structure will move
load supply from the power system. Instability that may from simple control strategy to two hierarchical level
result occurs in the form of a progressive fall or rise of operations. The fundamental level is local level and the
voltages of some buses. A possible outcome of voltage second level is coordinated level. The local voltage
instability is loss of load in an area, or tripping of control aims to maintain voltage at DG units in a fast
transmission lines and other elements by their protections control response. The coordinated level considers a
leading to cascading outages that in turn may lead to loss of system wide perspective for voltage control of a
synchronism of some generators. distribution network.

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 International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

B. Coordinated voltage and Reactive power control

The voltage and reactive power control in distribution
systems has for many years been based on local operation
of the OLTC and shunt capacitors. However in line with
the mode mization of electricity distribution, coordinated
voltage and reactive power control with short term
operation planning has been adopted in to distribution
systems. The OLTC is controlled by considering the
dispatch schedule of all capacitors in order to reduce the
number of OLTC operations.
Fig. 4. Firing Angle Control of Voltage
IV. VOLTAGE CONTROL BY STATIC OLTC
In this method of control, output voltage can be
A. Basic control mechanism represented as a function of α (firing angle), as given below
Vrms is f (α)

𝛼 𝜋
1 2
Vlt sin 𝜔𝑡 𝑑 𝜔𝑡 + (Vup sin(𝜔𝑡))2 𝑑(𝜔𝑡)
𝜋
0 𝛼

(2)
Fig. 3. Voltage Control representation
α - firing angle, Vlt - Peak Value of Lower taps voltage
The control scheme of a conventional OLTC Vup - Peak Value of Upper taps voltage
transformer is briefly explained in this chapter. Automatic
OLTC controls within ±18% change of nominal voltage. Vrms – Resultant rms value
The upper most tap represents -18% changes to nominal Using above stated equation by varying the firing angle,
voltage, while lower most tap represents +18% changes. voltage control in between taps can be obtained.
The Vmin refers to -18% and Vmax to +18%. OLTC
transformer reduces the error in voltage till secondary Condition: 1
voltage is within the dead band operating tap positions. Tap
changing results step change in voltage. Vref> Present selected Tap Voltage
The voltage in between two steps i.e., between two taps
can be obtained through static/semiconductor tap changing
systems with sequence control with voltage error less than
±0.1%.
The voltage at load end / secondary side of automatic
OLTC transformer is measured and compared with the pre-
set value. If the difference is within the dead band, no
operation takes place and if the difference lies outside the
dead band an appropriate lower or raise correction will start Fig. 5. Firing control between upper tap voltage & selected tap voltage
after a pre-determined delay. This process will be repeated
until the secondary voltage is within the inner dead band. In this condition firing control is being performed
The main purpose of time delay is to prevent unnecessary between present selected tap and lower voltage tap, to
tap operations due to temporary voltage fluctuations. Tap reduce the voltage further to reach pre-set voltage.
changing is done by switching Thyristor in respective taps. Condition: 2
In addition to the above automatic tap control; a sequence
control is introduced between taps for obtaining fine Vref< Present selected Tap Voltage
secondary voltage.
B. Sequence voltage control
An attempt is made to adjust output voltage by
controlling Thyristors in between taps. The control of
Thyristors is achieved through firing angle control. A
typical output waveform using sequence modulation
method.

Fig. 6. Firing control between lower tap voltage & selected tap voltage

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ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

In this condition firing control is being performed A. Pulse transformer

between present selected tap and lower voltage tap, to Pulse transformer designers usually seek to minimize
reduce the voltage further to reach pre-set voltage. voltage droop, rise time, and pulse distortion. Droop is the
V.BLOCK DIAGRAM DESCRIPTION decline of the output pulse voltage over the duration of one
pulse. It is caused by the magnetizing current increasing
The proposed model consists of many blocks. Each and during the time duration of the pulse. It is used to avoid
every block has its own features and applications core saturation and therefore needs to understand the
1. Full Wave Rectifier voltage-time constant. Pulse Transformers are small in size
and provide all the desirable qualities expected from the
2. LM324 (Operational Amplifier) transformers designed to handle square pulses. The
Magnetic flux in a typical AC transformer core alternates
3. Flip flop
between positive and negative values. But the Magnetic
4. Timer flux in a pulse transformer does not. Pulse transformer
operates in a unipolar mode.
5. Counter
B. Voltage regulator
6. Decoder
Any Electrical or Electronic device that maintains the
7. Pulse Transformer voltage of a power source within the acceptable limits. The
8. SCR Controlled Regulator voltage regulator is needed to keep the voltages within the
prescribed range that can be tolerated by the electrical
9. Voltage Regulator equipment using that voltage. Voltage Regulators used in
10. SCR control board Electronic equipment in which excessive variations in
voltage would be determined.
Each of the blocks is individually and elaborately
mentioned in this chapter. The proposed model consists of
varying load but the output voltage is constant.

Fig. 8. Pin diagram of 7912 and 7812

Fig. 7. Block diagram of the proposed system

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Vol. 4 Issue 02, February-2015

Fig. 9. Circuit Diagram of the Proposed Model.

In this project we are using 7812 as a positive regulator. The modulated pulse signal is given to the pulse
The input may be 16V or even 20V, output will be 12V. transformer to switch the thyristor to increase/decrease the
Here the negative side is taken as common line, common to tap based upon the voltage feedback to regulate the voltage
both input and output. 7912 is a negative regulator. You on the consumer side.
can derive -12 from -16 to -20V. The positive of the input
is common to both input and output. VII.CIRCUIT OPERATION
The implementation of the hardware falls into two
VI. CIRCUIT DESCRIPTION categories, it either increase the tap position or decrease the
The hardware of the proposed model consists of three tap position depending upon the under load or overload
taps on the winding of the transformer to regulate the condition. This can be briefly explained by the following
voltage at the consumer side. The circuit diagram of the two cases.
proposed model is briefly explained.
Case 1: Consider a case if the loads connected to the
The power source is connected to the cathode K1 and supply are suddenly switched off, the voltage of the lines
K2 and anode G1 and G2 of the thyristor switch. Thyristor feeding the loads will be increased; hence the voltage
gets triggered and the voltage is fed to the load. The feedback gets increased. The feedback voltage is stepped
voltage feedback is taken from the lines feeding the load. down by 230/12 V transformer and fed to full wave
Then voltage feedback is connected to the 230/12 V step rectifier. This causes the output from full wave rectifier to
down transformer followed by rectifier circuit with filter. rise. This voltage is compared with the reference DC
Hence the output from the full wave rectifier will be voltage by using a comparator LM324. The comparator
smooth DC voltage. This DC voltage is compared with the produces negative error at the output of the comparator.
reference voltage with the help of LM324 comparator. The This error is taken by the upper negative reference
error voltage from the comparator circuit is used to comparator to increase the gain of the error signal. This
modulate the counter values. Depending upon counter error signal forward biases the diode and fed to reset
value the pulse width is modulated, where the pulses are terminal of the flip flop. Because of the reset terminal gets
generated with the help of IC555 and the modulated pulse triggered, output of the flip flop will be LOW. Hence the
width output is taken from the JAM lines of the counter. counter counts down. Clock pulses generated from the
The output from the counter is binary; hence it is IC555 is given to the counter to count the number of clock
connected to the decoder which makes only one pin to pulses. The output of the counter will be binary output. It is
HIGH at any time. So the corresponding AND gate then given to the input to the decoder. Only one pin of the
connected to the output pin of the counter will be triggered. decoder is enabled as HIGH. This high pulse waveform is

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Vol. 4 Issue 02, February-2015

connected to the one of the input of AND gate and the VIII. SIMULATION RESULTS
other input is connected from the timer IC555. So for each
clock cycle at some period of time, the two inputs will be
same. At that time the pulse wave form is generated at the
output of the AND gate and is given to the pulse
transformer. Pulse transformer generates pulses to trigger
the gate of the thyristor and then the tap gets decreased to
maintain the constant output voltage.
Case 2: Consider a case if the loads connected to the
supply are suddenly increased, the voltage of the lines
feeding the loads will be decreased; hence the voltage
feedback gets decreased. The feedback voltage is stepped
down by 230/12 V transformer and fed to full wave
rectifier. This causes the output from full wave rectifier to
lower. This voltage is compared with the reference DC
voltage by using a comparator LM324. The comparator Fig. 10. Simulation Model Using MATLAB.
produces positive error at the output of the comparator.
This error is taken by the lower positive reference
comparator to increase the gain of the error signal. This
error signal forward biases the diode and fed to set terminal
of the flip flop. Because of the set terminal gets triggered,
output of the flip flop will be HIGH. Hence the counter
counts up. Clock pulses generated from the IC555 is given
to the counter to count the number of clock pulses. The
output of the counter will be binary output. It is then given
to the input to the decoder. Only one pin of the decoder is
enabled as HIGH. This high pulse waveform is connected
to the one of the input of AND gate and the other input is
connected from the timer IC555. So for each clock cycle at
some period of time, the two inputs will be same. At that
time the pulse wave form is generated at the output of the
AND gate and is given to the pulse transformer. Pulse
transformer generates pulses to trigger the gate of the Fig. 11. Simulation Of Output Voltage.
thyristor and then the tap gets increased to maintain the
constant output voltage.
A. Scr Controlled Regulator
A Thyristor (silicon controlled rectifier or SCR) is a
little like a transistor. It consists of four layers of silicon in
a p−n−p−n structure. Its circuit symbol shows that it is
basically a diode, but with an additional terminal, called the
GATE. The purpose of the gate is to enable the device to
be switched from a non-conducting (forward blocking)
mode into a low resistance, forward conducting state. Thus
a small current applied to the gate is able to switch a much
larger current (at a much higher voltage) applied between
anode and cathode. Once the thyristor is conducting
however, the gate current may be removed and the device
will remain in a conducting state. Fig. 12. Complete System Output (Output Side To Maintain the Constant
Output Voltage)
To turn the thyristor off, the current flowing between
anode and cathode must be reduced below a certain critical IX. CONCLUSION
"holding current" value, (near to zero); alternatively the The way distributed networks are operated now-a-days
anode and cathode may be reverse biased. The thyristor is is properly conditioned by the need to keep the voltage
normally made to conduct by applying a gating pulse, magnitude within the acceptable limits especially at the LV
while the main anode and cathode terminals are forward levels on penetration of the Distributed Generation. This
biased. When the device is reverse biased the gating pulse requirement is achieved by Mechanical tap changers.
has no effect. Power Electronic semiconductor switches which are
becoming cheaper and more reliable, hence offer better
opportunities to provide added control flexibility. In this
project solid state tap changer employing thyristor switch
control with 3 tap positions is proposed to regulate the
voltage at the consumer side depending on the voltage

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 International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 02, February-2015

feedback taken from the load side. Pulse Width Modulation

(PWM) technique is adopted for sequential switching of the
thyristor.
X. FUTURE ENHANCEMENT
The magnitude of output voltage which needs to be
maintained constant cannot be altered in the proposed
model. But it is possible by changing the reference point.
Changing the reference point in the proposed model is not
possible because of the technical difficulties. A fuzzy
algorithm can be adopted to provide an adaptive reference
of the OLTC controller which can mitigate the effect of the
high penetration of the DG units. The motivations behind
using the fuzzy logic are:
 It can map nonlinear relations behind its inputs
and output.
 It can provide a smooth transition, which lead to a
more relaxed tap operation.
 It requires less remote data measurements
compared to the centralized approaches.

REFERENCES
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2) Cigre, Working Group B4.35, “Thyristor controlled voltage
regulators”, February 2004.
3) R. Shuttleworth, A.J. Power, X. Tian, H. Jiang, and B. A. T. Al
Zahawi, “A novel thyristor-assisted tap changer scheme,” in CIRED
97, IEE Conference Publication No. 438, pp.1.28.1-1.28.5, June
1997.
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Power electronic technology. Power Electric,24(4): 43-45,June
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