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History of transformer
Why do we need transformers?
Working Principle
Basic Construction of Transformer.
Types of Transformer
Components of Transformer
Equivalent circuit transformers
 Losses in Transformers
Testing of transformers
Cooling of Transformer
Tap Changing in transformers
Properties of transformer oil
Insulating materials used in transformers
Power Transformer
Instruments Transformer
How does an Autotransformer work?
 protection of transformer
Parallel operation of transformer
Tertiary winding
Vector Group
Presentation outline
Neha Gethe 2

History of Transformer
 About
Transformer
 Timeline of
Transformer
Development
 The Problem of
Transmitting
Power
(transformer &
their role)
The original 1885 Stanley prototype transformer
at the Berkshire Museum.
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About Transformer
E.W. Rice Jr.(2nd president of General
Electric) powers a light bulb using William
Stanley's original 1885 transformer
What is a Transformer? A: A transformer is a
device that transfers electrical energy from one
circuit to another through inductively coupled
conductors—the transformer's coils.
"the heart of the alternating current system"
- William Stanley Jr.
How is it used? A: A transformer is used to
bring voltage up or down in an AC electrical
circuit. A transformer can be used to convert
AC power to DC power. There are
transformers all over every house, they are
inside the black plastic case which you plug
into the wall to recharge your cell phone or
other devices. These types are often called
"wall warts". They can be very large, as in
national utility systems, or it can be very small
embedded inside electronics. It is an
essential part of all electronics today.
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Who invented the transformer? Ottó Bláthy, Miksa Déri, Károly Zipernowsky of the
Austro-Hungarian Empire First designed and used the transformer in both
experimental, and commercial systems. Later on Lucien Gaulard, Sebstian Ferranti,
andWilliam Stanley perfected the design. See the next question for more details.
When was the transformer invented? A: The property of induction was discovered
in the 1830's but it wasn't until 1886 thatWilliam Stanley, working
for Westinghouse built the first reliable commercial transformer. His work was built
upon some rudimentary designs by the Ganz Company in Hungary (ZBD
Transformer 1878), and Lucien Gaulard and John Dixon Gibbs in England. Nikola
Tesla did not invent the transformer as some dubious sources have claimed. The
Europeans mentioned above did the first work in the field. George Westinghouse,
Albert Schmid, Oliver Shallenberger and Stanley made the transformer cheap to
produce, and easy to adjust for final use.
Where were the first transformers used? The first AC power system that used
the modern transformer was in Great Barrington, Massachusetts in 1886. Earlier
forms of the transformer were used in Austro-Hungary 1878-1880s and 1882 onward
in England. Lucien Gaulard (Frenchman) used his AC system for the
revolutionary Lanzo to Turin electrical exposition in 1884 (Northern Italy). In 1891
mastermind Mikhail Dobrovsky designed and demonstrated his 3 phase
transformers in theElectro-Technical Exposition at Frankfurt, Germany.
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William Stanley's First Transformer built
in 1885. Single phase AC power.
Stanley's first transformer which was
used in the electrification of Great
Barrington, Massachusetts in 1886.
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Transformer development timeline
1830s - Joseph Henry and Michael Faraday work with electromagnets and discover the property of
induction independently on separate continents.
1836 - Rev. Nicholas Callan of Maynooth College, Ireland invents the induction coil
1876 - Pavel Yablochkov uses induction coils in his lighting system
1878 -1883 - The Ganz Company (Budapest, Hungary) uses induction coils in their lighting systems with AC
incandescent systems. This is the first appearance and use of the toroidal shaped transformer.
1881 - Charles F. Brush of the Brush Electric Company in Cleveland, Ohio develops his own design of
transformer (source: Brush Transformers Inc.)
1880-1882 - Sebastian Ziani de Ferranti (English born with an Italian parent) designs one of the earliest AC
power systems with William Thomson (Lord Kelvin). He creates an early transformer. Gaulard and Gibbs
later design a similar transformer and loose the patent suit in English court to Ferranti.
1882 - Lucien Gaulard and John Dixon Gibbs first built a "secondary generator" or in today's terminology a
step downtransformer which they designed with open iron core, the invention was not very efficient to
produce. It had a linear shape which did not work efficiently. It was first used in a public exhibition in Italy
in 1884 where the transformer brought down high voltage for use to light incandescent and arc lights. Later
they designed a step up transformer. Gaulard (French) was the engineer and Gibbs (English) was the
businessman behind the initiative. They sold the patents to Westinghouse. Later they lost rights to the
patent when Ferranti (also from England) took them to court
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1884 - Use of Lucien Gaulard's transformer system (a series system) in the first large exposition of
AC power in Turin, Italy. This event caught the eye of William Stanley, working for Westinghouse.
Westinghouse bought rights to the Gaulard and Gibbs Transformer design. The 25 mile long
transmission line illuminated arc lights, incandescent lights, and powered a railway. Gaulard won an
award from the Italian government of 10,000 francs.
1885 - George Westinghouse orders a Siemens alternator (AC generator) and a Gaulard and Gibbs
transformer. Stanley begin experimenting with this system.
1885 - William Stanley makes the transformer more practical due to some design changes:
"Stanley's first patented design was for induction coils with single cores of soft iron and
adjustable gaps to regulate the EMF present in the secondary winding. This design was first used
commercially in the USA in 1886". William Stanley explains to Franklin L. Pope (advisor to
Westinghouse and patent lawyer) that is design was salable and a great improvement. Pope
disagrees but Westinghouse decides to trust Stanley anyway.
1886 - William Stanley uses his transformers in the electrification of downtown Great Barrington,
MA.This was the first demonstration of a full AC power distribution system using step and step
down transformers.
Later 1880s - Later on Albert Schmid improved Stanley's design, extending the E shaped plates to
meet a central projection.
1889 - Russian-born engineer Mikhail Dolivo-Dobrovolsky developed the first three-
phase transformer in Germany at AEG. He had developed the first three phase generator one year
before. Dobrovolsky used his transformer in the first powerful complete AC system (Alternator +
Transformer + Transmission + Transformer + Electric Motors and Lamps) in 1891.Neha Gethe 8

1891
Early three phase transformer
(circular core type)
Siemens and Halske company
5.7 kVA 1000/100 V
This transformer was created at the
beginning of the modern electrical
grid, the same year as the Frankfurt
Electrical Exhibition which
demonstrated long distance
transmission of power.
William Stanley once wrote: " I have a very personal
affection for a transformer." "It is such a complete and
simple solution for a difficult problem. It so puts to
shame all mechanical attempts at regulation. It
handles with such ease, certainty, and economy vast
loads of energy that are instantly given to or taken
from it. It is so reliable, strong, and certain."
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The Problem of Transmitting Power
DC power was mainly in used in the 1880's and it was hard to transmit over distance
because:
-To transmit over long distance you need high voltage on a skinny wire or low
voltage on a wide wire. High voltage on DC is very dangerous, and with low voltage the
wire would have to be so thick that it would not be practical. Also with high voltage you
couldn't not step down the voltage so it could be used with home light bulbs.
Using the water analogy: imagine that a small wire with high voltage is like a garden
hose with high pressured water moving fast inside. Imagine that this hose fills 2 gallon
jugs of water in one minute. Now think of a 6" wide drain pipe filled with water. You can
deliver the same amount of water to the destination in the same time period without
needing so much pressure.
With AC power you also use high voltage to move the electricity down a long wire. AC
becomes more practical because once you send the power to the destination, you can
use a transformer to change the voltage down to a manageable level. The power is
stepped down several times by the time it reaches you home. The power line coming into
your home is at 240 volts, from your breaker box it is split into lines of 120 volts for most
of your home sockets and 240 for appliance sockets. In Europe and other regions 220 V
is the standard home socket.
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Transformer - a device that transfers electrical energy from one circuit to another
circuit using inductively coupled conductors. In other words by putting two coils of
wire close together while not touching, the magnetic field from the first coil called
the primary winding effects the other coil (called the secondary coil). This effect is
called "inductance". .
Now if you would like to change the voltage on a powerline, you could do this by
changing current going into the primary coil (voltage stays high). The current level
affects the induced voltage on the secondary coil. A changing magnetic field
induces a changing electromagnetic force (EMF) or "voltage". To put it simply:
by changing the current you can obtain the desired voltage on the other side.
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Why do we need transformers?
In most cases, machines and appliances using electricity are manufactured to operate using a specific
voltage and frequency. Sounds simple, right? Well, let me throw you a curve - voltage and frequency
vary from place to place. Not all countries - and sometimes not all regions within the same country -
generate the same voltage and frequency in their electricity.
That's where transformers come in. Transformers adjust the voltage coming into the appliance to the
proper level, and pump the electricity through the appliance to keep it operating properly.
The most common, and preferred, class of transformer is the autotransformer - particularly those with a
single tapped winding (as opposed to isolation transformers with 2 separate windings).
Autotransformers are smaller, lighter, and provide greater voltage stability and overload tolerance.
While transformers do adjust voltage, they do not - and cannot - change frequency. In most cases,
frequency is irrelevant to the proper operation of the appliance. Where frequency is an issue - such as
with clocks, stereo components, and timers - the appliance must have both a transformer to adjust
voltage, and physical adjustment of gears, pulleys, etc. to correct the speed of operation.
Frequency can, however, be an issue in cases of appliances whose motors run continuously, or
continuously stop and start, such as refrigerators and air conditioners. In these cases, it is a good idea to
adjust your voltage up or down according to the frequency. For example, 60 Hz motors should be run
at 10% less voltage when operated on 50 Hz, while 50Hz motors should be run at 10% more voltage
when operated on 60Hz.
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Working Principle
The basic principle behind working of a transformer is the phenomenon of mutual
induction between two windings linked by common magnetic flux.
 The figure at right shows the simplest form of a transformer. Basically
a transformer consists of two inductive coils; primary winding and
secondary winding.
 The coils are electrically separated but magnetically linked to each
other. When, primary winding is connected to a source of alternating
voltage, alternatingmagnetic flux is produced around the winding.
 The core provides magnetic path for the flux, to get linked with the
secondary winding. Most of the flux gets linked with the secondary
winding which is called as 'useful flux' or main 'flux', and the flux
which does not get linked with secondary winding is called as 'leakage
flux'.
 As the flux produced is alternating (the direction of it is continuously
changing), EMF gets induced in the secondary winding according
to Faraday's law of electromagnetic induction.
 This emf is called 'mutually induced emf', and the frequency of
mutually induced emf is same as that of supplied emf. If the secondary
winding is closed circuit, then mutually induced current flows through
it, and hence the electrical energy is transferred from one circuit
(primary) to another circuit (secondary).
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Basic Construction Of Transformer
 Basically a transformer consists of two inductive windings and
a laminated steel core. The coils are insulated from each other
as well as from the steel core.
 A transformer may also consist of a container for winding and
core assembly (called as tank), suitable bushings to take our the
terminals, oil conservator to provide oil in the transformer tank
for cooling purposes etc.
 The figure at left illustrates the basic construction of a
transformer.
In all types of transformers, core is constructed by assembling
(stacking) laminated sheets of steel, with minimum air-gap
between them (to achieve continuous magnetic path).
 The steel used is having high silicon content and sometimes
heat treated, to provide high permeability and low hysteresis
loss. Laminated sheets of steel are used to reduce eddy current
loss.
 The sheets are cut in the shape as E,I and L. To avoid high
reluctance at joints, laminations are stacked by alternating the
sides of joint.
 That is, if joints of first sheet assembly are at front face, the
joints of following assemble are kept at back face.Neha Gethe 14

Neha Gethe
15
aircell
Silica
gel
Iron core
& wdg
Core &
wdg
radiator
buchholz
Terminal
connector
bushing
Oil tank
pump
Cooler
fan
valve

Types Of Transformers
Transformers can be classified on different basis, like types of construction, types of cooling etc.
(A) On the basis of construction, transformers can be classified into two types as;
(i) Core type transformer and
(ii) Shell type transformer
(I) Core Type Transformer
In core type transformer, windings are cylindrical former wound, mounted on the core limbs as
shown in the figure above. The cylindrical coils have different layers and each layer is insulated
from each other. Materials like paper, cloth or mica can be used for insulation. Low voltage
windings are placed nearer to the core, as they are easier to insulate.
(Ii) Shell Type Transformer
The coils are former wound and mounted in layers stacked with insulation between them. A
shell type transformer may have simple rectangular form (as shown in above fig), or it may have
a distributed form
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(B) On the basis of their purpose
I. Step up transformer: Voltage increases (with subsequent decrease in current) at secondary.
II. Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.
(C) On the basis of type of supply
I. Single phase transformer
II. Three phase transformer
(D) On the basis of their use
I. Power transformer: Used in transmission network, high rating
II. Distribution transformer: Used in distribution network, comparatively lower rating than that of
power transformers.
III. Instrument transformer: Used in relay and protection purpose in different instruments in industries
 Current transformer (CT)
 Potential transformer (PT)
(E) On the basis of cooling employed
I. Oil-filled self cooled type
II. Oil-filled water cooled type
III. Air blast type (air cooled)
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F. General Types of Transformers
I. Autotransformers
Autotransformers are different from traditional transformers because autotransformers share a
common winding. On each end of the transformer core is an end terminal for the winding, but there is
also a second winding that connects at a key intermediary point, forming a third terminal. The first and
second terminals conduct the primary voltage, while the third terminal works alongside either the first
or second terminal to provide a secondary form of voltage. The first and second terminals have many
matching turns in the winding. Voltage is the same for each turn in the first and second terminal.
II. Polyphase Transformers
This type of transformer is commonly associated with three phase electric power, which is a common
method of transmitting large amounts of high voltage power, such as the national power grid. In this
system, three separate wires carry alternating currents of the same frequency, but they reach their peak
at different times, thus resulting in a continuous power flow. Occasionally these “three-phase” systems
have a neutral wire, depending on the application.
II. Leakage Transformer
Leakage transformers have a loose binding between the primary and secondary winding, which leads
to a large increase in the amount of inductance leakage. All currents are kept low with leakage
transformers, which helps prevent overload. They are useful in applications such as arc welding and
certain high-voltage lamps, as well as in the extremely low-voltage applications found in some
children’s toys.
IV. Resonant Transformer
As a type of leakage transformer, resonant transformers depend on the loose pairing of the primary and
secondary winding, and on external capacitors to work in combination with the second winding. They
can effectively transmit high voltages, and are useful in recovering data from certain radio wave
frequency levels. Neha Gethe 21

V. Audio Transformer
Originally found in early telephone systems, audio transformers help isolate potential interference and send one signal
through multiple electrical circuits. Modern telephone systems still use audio transformers, but they are also found in
audio systems where they transmit analog signals between systems. Because these transformers can serve multiple
functions, such as preventing interference, splitting a signal, or combining signals, they are found in numerous
applications. Amplifiers, loudspeakers, and microphones all depend on audio transformers in order to properly perform.
VI .Custom Transformers
Custom transformers are usually required when a transformer needs to be able to perform special functions or requires
unusual features. If untraditional construction, rare material, or high voltage coils are involved, a custom transformer
may be the safest option to pursue.
There are several common problems and requisites that custom transformers can offer potential solution for, including:
 Unusual input or output voltage
 Line and load reactors are essential parts of regeneration and power factor control
 Protective enclosures are needed
 Ripple filters and armature chokes are required to reduce DC drive noise
 Unusual wiring
VII. Isolation Transformers
In an insulation transformer, the primary and secondary windings are isolated from one another by insulation. This
separation allows for an AC voltage to be transferred from one circuit to another while blocking DC signals and
interference associated with ground loops. Generally speaking, isolation transformers are effective at power transfer in
sensitive applications, such as computers and laboratory equipment. Hospital Grade isolation transformers can help
protect sensitive equipment in medical environments.
VIII. Zigzag Transformers
Zigzag transformers have a primary winding but lack a secondary winding. Because they fall under the category of
three-phase transformers, there are six coils on three separate cores. On each core, the first coil connects in a zigzag
pattern to the second coil on the next core. Then, the second coils are joined and connected to the primary coils,
comprising a neutral. Because the phases couple, the voltages cancel out, enabling the neutral pole to be secured to the
ground. They resemble a Y transformer, whose neutral point is grounded. If one phases fluxes, all other phases are
thrown out of balance; the zigzag formation, however, provides a path for earth faults (or zero sequence) to exit. Zigzag
transformers are often applied to ungrounded electrical systems to derive a reference point.Neha Gethe 22

IX. Pulse Transformer
When it comes to transmitting a pulse with a fast rise and fall time, pulse transformers are optimal
because they are specially designed for handling this type of electrical transfer. Because many different
kinds of applications exhibit this type of pulse pattern (also known as rectangular electrical pulses), pulse
transmitters can be made in a range of sizes to effectively handle a change in voltage magnitude. For
smaller applications, pulse transformers called signal types are preferred, and are commonly used in
applications such as telecommunications circuits. Medium-sized models are used for applications like
camera flash controllers, whereas larger models play an important role in the power distribution
industry.
X. Speaker Transformers
Speaker transformers are a type of electrical transformer that can power multiple loudspeakers with one
circuit, as long as the circuit is performing at higher than normal voltage. They are referred to as constant
volt or 70 volt speakers, despite the fact that the voltage is constantly changing. In terms of audio
engineering, audio transformers can act as amplifiers by helping increase low-voltage output to the
speaker circuit. At each speaker, an even smaller transformer alters the voltage and impedance, bringing
them back up to standard speaker levels. The volume of each speaker can often be readjusted as needed.
Using high-voltage and impedance speaker transformers minimizes power loss. All pulse transformers
have a cycle of less than one; this means that at the end of the cycle any energy left in the coil must be
disposed of before the pulse can run again.
XI. Rectifier transformers
They are combined with a diode or thyristor rectifier. Applications range from large aluminum
electrolysis to medium size operations. The transformers may have a built-in or separate voltage
regulation unit.Regulating and rectifier transformer combinations that are applied to primary aluminum
production (smelters) are commonly known as 'rectiformers'. A typical aluminum potline is built as a 60-
pulse system with five parallel 12-pulse rectiformers, each with different phase-shift windings; a 60-pulse
system can be achieved by the following phase shift angles: –12°, –6°, 0°, +6° and +12°.
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XII. Control Transformer
A control transformer is generally used in an electronic circuit that requires constant voltage or
constant current with a low power or volt-amp rating. Various filtering devices, such as capacitors,
are used to minimize the variations in the output. This results in a more constant voltage or current.
Designed for industrial applications where electromagnetic devices such as relays and solenoids are
used, the control transformer maximizes inrush capability and output voltage regulation when
electromagnetic devices are initially energized.Control transformers incorporate high-quality
insulating materials.
XIII. regulating transformer
Transformer having one or more windings excited from the system circuit or a separate source and on
e or more windings connected inseries with the system circuit for adjusting the voltage or the phase r
elation or both in steps, usually without interrupting the load.
G .There are many other transformer like
 Rectifier Transformers
 Regulating Transformers
 Ferroresonant Transformers
 Inverter Transformers
 Oil-Filled Transformers
 Encapsulated Transformers
 Toroidal Transformers
 Grounding transformer
 Welding transformer
 Isolation transformers
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COMPONENTS OF A TRANSFORMER
 Core
 Windings
 Transformer oil
 Tap changer
 Conservator
 Breather
 Cooling tubes
 Buchholz Relay
 Explosion vent
A -Core
Core is used to support the windings in the
transformer. It also provides a low reluctance path to
the flow of magnetic flux. It is made up of laminated
soft iron core in order to reduce eddy current loss and
Hysteresis loss. The composition of a transformer core
depends on such factors as voltage, current, and
frequency. Diameter of the transformer core is directly
proportional to copper loss and is inversely proportion
to the iron loss. If diameter of the core is decreased, the
weight of the steel in the core is reduced which leads to
less core loss of transformer and the copper loss
increase. The vice versa happen when the diameter is
increased. Neha Gethe 25

B-Windings
There are two windings wound over the transformer core which are insulated from each other. Windings
consists of several turns of copper coils bundled together an each bundles are connected in series to form a
winding.
Windings can be classified in two different ways.
Based on the supply the windings are classified into
a) Primary windings--It is the winding to which the input voltage is applied.
b) secondary windings.--It is the winding to which the output voltage is applied.
Based on the voltage the windings can be classified as follows
a) High voltage winding--High voltage windings are made up of copper coil. The number of turns in it is
the multiple of the number of turns in the low voltage windings. It has copper coils thinner than that of
the low voltage windings.
b) Low voltage windings--Low voltage winding has lesser number of turns than that of the high voltage
windings. It is made up of the thick copper conductors. This is because the current in the low voltage
windings is higher than that of high voltage windings.
Transformer can be supplied from either LV or HV windings based on the requirement.
Windings are made of copper due to the following reasons.
 High conductivity
1. minimizes amount of copper needed for winding (volume & weight of winding)
2. minimizes losses
 High ductility
1. Easy to bend conductors into tight winding around core thus minimizes amount of copper and volume
of winding
.
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c. Transformer oil
Transformer oil performs two important functions of insulation as well as cooling for the core and coil
assembly. Core and windings of the transformer must be completely immersed in the oil. Normally
hydrocarbon mineral oils are used as transformer oil. Oil contamination is a serious problem because it
robs its dielectric properties and renders it useless as an insulating medium.
d.Conservator
Conservator conserves the transformer oil. It is an airtight metallic cylindrical drum which is fitted above
the transformer. The conservator tank is vented to the atmosphere at the top and the normal oil level is
approximately in the middle of the conservator to allow expansion and contraction of oil during the
temperature variations. It is connected to the main tank inside the transformer which is completely filled
with transformer oil through a pipeline.
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e.Breather
The insulating oil of transformer is provided for cooling and
insulating purpose. Expansion and contraction of oil during the
temperature variations cause pressure change inside the
conservator. This change in pressure is balanced by the flow of
atmospheric air into and out of the conservator. Transformer
breather is a cylindrical container which is filled with silica gel.
Insulating oil reacts with moisture can affect the paper
insulation or may even lead to some internal faults. So it is
necessary that the air entering the tank is moisture free. It
consists of silica gel contained in a chamber. For this purpose
breather is used. When the atmospheric air passes through the
silica gel breather the moisture contents are absorbed by the
silica crystals. Silica gel breather is acts like an air filter for the
transformer and controls the moisture level inside a
transformer. It is connected to the end of breather pipe.

f.Tap changer
The output voltage may vary according to the input voltage and the load. During loaded conditions the
voltage on the output terminal fall and during off load conditions the output voltage increases. In order
to balance the voltage variations tap changers are used. Tap changers can be either on load tap changer
or off load tap changer. In on load tap changers the tapping can be changed without isolating the
transformer from the supply and in off load tap changers it is done after disconnecting the transformer.
Automatic tap changers are also available
.
g.Cooling tubes
Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling
tubes. The circulation of the oil may either be natural or forced circulation. In natural circulation, when
the temperature of the oil raises the hot oil naturally moves to the top and the cold oil moves
downwards. Thus the oil keeps on circulating through the tubes. In forced circulation, an external pump
is used for circulating the oil.
h.Buchholz Relay
It is a protective device container housed over the connecting pipe from main tank to conservator tank.
It is used to sense the faults occurring inside the transformer. It is a simple relay which is operated by
the gases emitted due to the decomposition of transformer oil during internal faults. It helps in sensing
and protecting the transformer from internal faults.
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Equivalent circuit transformers
Ideal transformer
I. A transformer that posses the following properties are considered to be an ideal
transformer.
II. Primary and secondary resistances are assumed to be zero. Hence there is no power
loss and voltage drop in an ideal transformer.
III. Leakage flux is completely absent.
IV. The permeability of the core is infinite and so the magnetizing current is zero.
V. The core losses are considered to be zero.
The figure shows the diagrammatic representation of an ideal transformer. It is represented in such a way
that all the above conditions are satisfied.
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Real Transformer
Transformer windings are made mainly of copper. Even though copper is a good conductor, it posses
a finite resistance. Both the primary and the secondary have finite resistances R1 and R2.
These resistances are uniformly spread through out the windings. These resistances give rise to the
copper losses (I2R). Consider that Φl1 be the leakage flux caused by the MMF I1N1 in the primary
windings and Φl2 be the leakage flux caused by the MMF I2N2 in the secondary windings.
Both the resistance and the leakage reactance of the transformer windings are series effects and at
operating frequencies, which is very low (50Hz / 60Hz) these can be regarded as the lumped
parameters. Hence the transformer consists of lumped resistances R1 and R2 and reactance X l1 and
X l2 in series with corresponding windings. Because of the presence of these lumped quantities the
induced emf s E1 and E2 may vary from the secondary voltages V1 and V2because of the small voltage
drops in the winding resistances and leakage reactance.
The transformer ratio can be given by:
a= (N1/N2) = (E1/ E2) ≈( V1 / V2)
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The exciting current I0‾ can be resolved into two components, whose magnetizing
component Im‾ creates mutual flux Φ‾ and whose core loss component Ii‾ provides the
loss associated with alternation of flux.
I0‾ = Im‾ + Ii‾
Note: vector form is indicated by the symbol ‾.
Equivalent circuit
Hence the equivalent circuit can be represented
as shown in the figure.
Here,
Gi = conductance
Bi = Susceptance
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Losses in Transformers
Transformer is the most efficient electrical machine. Since the transformer has no moving parts, its
efficiency is much higher than that of rotating machines. The various losses in a transformer are
enumerated as follows:
1. Core loss or iron loss or constant loss
2. Copper loss or variable loss
3. Load (stray) loss
4. Dielectric loss
Lets see about each one of then in detail.
1. Core loss
When the core of the transformer undergoes cyclic magnetization power losses occur in it. There losses
are together called as core loss. There are two kinds of core losses namely hysteresis loss and eddy
current loss. Core loss is important in determining heating, temperature rise, rating and efficiency of
transformers. The core losses comprises of two components:
 Hysteresis loss
 Eddy current loss
Hysteresis loss
This phenomenon of lagging of magnetic induction behind the magnetising field is called hysteresis.
In the process of magnetisation of a ferromagnetic substance through a cycle, there is expenditure of
energy. The energy spent in magnetising a specimen is not recoverable and there occurs a loss of
energy in the form of heat. This is so because, during a cycle of magnetisation, the molecular magnets
in the specimen are oriented and reoriented a number of times. This molecular motion results in the
production of heat. It has been found that loss of heat energy per unit volume of the specimen in each
cycle of magnetisation is equal to the area of the hysteresis loop.
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The shape and size of the hysteresis loop is
characteristic of each material because of the
differences in their retentivity, coercivity, permeability,
susceptibility and energy losses etc.
The net unrecoverable energy lost in the process is area of abco
which is lost irretrievably in the form of heat is called the
hysteresis loss. the total hysteresis loss in one cycle is easily
seen to be the area of one complete loop abcdefa.
If wh indicates the hysteresis loss/ unit volume, then hysteresis
loss in volume V of material when operated at f Hz is given by
the following equation.
Ph=whVf W
Steinmetz gave an emprical formula to simplify the
computation of the hysteresis loss based on his experimental
studies. The formula given by him is as follows:
Ph=khfBn
m W
where kh is a characteristic constant of the core material, Bm is
the maximum flux density and n is caller steinmetz constant
Neha Gethe 34

Eddy current Loss
When the magentic core flux varies in a magnetic core with respect to
time, voltage is induced in all possible paths enclosing the flux. This
will result in the production of circulating currents in the transformer
core. These currents are known as eddy currents.
These eddy currents leads to power loss called Eddy current loss.
This loss depends upon two major factors. The factors affecting the
eddy currents are:Resistivity of the core andLength of the path of the
circulating currents for a given cross section.
The eddy currents can be expressed as
Pe = kef2B2 W/m3
ke = ke'd2/p
Where,
d is the thickness of the lamination
p is the resistivity of material of the core
Pe = ke'd2f2B2/p W/m3
Hence from the above equations it is evident that Eddy current loss is
directly proportional to the square of the thickness of the lamination
and that of the frequency of supply voltage.
Total core loss = Hysteresis loss + Eddy current loss.
Permissible copper losses at 75
degree Centigrade
kVA Copper losses (W)
16 500
25 700
40 975
50 1180
63 1400
75 1600
88 1650
100 2000
125 2350
160 2840
200 3400
250 4000
315 4770
400 5700
500 6920
860 8260
1000 11880
All the above losses are
All the above losses are subjected to
positive or negative variation of 10%
Neha Gethe 35

2.Copper Loss
It is a well known fact that whenever there is a resistance to the flow of
current in a conductor, power loss occurs in the conductor due to its
resistance. Copper loss occurs in the winding of the transformer due to
the resistance of the coil. When the winding carries current, power loss
occurs due to its internal resistance. This loss is known as copper loss.
The copper loss can be expressed as below
Pcu = I2R
Where I is the current through the winding and R is the resistance of the
winding.
Copper loss is proportional to the square of current flowing through the
winding.
3.Stray Loss.
Stray loss results from leakage fields including Eddy currents in the
tank wall and conductors. The winding of the transformers should be
designed such that the stray loss is small. This can be achieved by the
spliting of conductors in to small strips to reduce Eddy currents in the
conductors. The radial width of the strips should be small and they
should be transposed.
Permissible copper losses at
75 degree Centigrade
kVA Copper losses (W)
16 500
25 700
40 975
50 1180
63 1400
75 1600
88 1650
100 2000
125 2350
160 2840
200 3400
250 4000
315 4770
400 5700
500 6920
860 8260
1000 11880
All the above losses are subjected
to positive or negative variation
of 10%
4.Dielectric Loss
This loss occurs in the transformer oil and other solid insulating materials in the transformer.
The major losses occurring in the transformer are Core loss and copper loss. Rest of the losses are very
small compare to these two. All the losses occurring in transformer are dissipated in the form of heat in
the winding, core, insulating oil and walls of the transformer. Efficiency of the transformer increases with
decrease in the losses.
Neha Gethe 36

NO LOAD LOSSES CANBE CONTROLLED BY:
 Grade of CRGO material
 Thickness of sheet
 Flux density
 Method of core claming
 Handling of CRGO laminations
 Type of core whether interleaved or mitred
burr levels produced during cutting of sheets
 Thickness of steel varies from 0.23 mm to 0.35 mm
 Different grade of steel: M4,M5,M6,MOH,M2H,M3H,ZH100,ZDKH,Hi - B etc
 M4, Hi-B grade steel CRGO(3% silicon)
 Hysteresis loss  intesity of B of the core material
 Eddy current loss  thickness of the laminations
Insulation coating is as CARLITE.
 The iron loss are minimized by using steel of high silicon content for the core and by using thin
laminations
Neha Gethe 37

Why Transformer Rating In kVA, Not in KW?
Copper losses ( I²R)depends on Current which passing through transformer
winding while Iron Losses or Core Losses or Insulation Losses depends on
Voltage.
So the Cu Losses depend on the rating current of the load so the load type
will determine the powerfactor P.F ,Thats why the rating of Transformer in
kVA,Not in kW.
Designer doesn’t know the actual consumer power factor while
manufacturing transformers and generators i.e. the P.F (Power factor) of
Transformer and Generator/Alternator depends on the nature of connected
load such as resistive load, capacitive load, and inductive load as Motors, etc.
But Motor has fixed Power factor, i.e. motor has defined power factor and the
rating has been mentioned in KW on Motor nameplate data table. That’s why
we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.
Neha Gethe 38

For confirming the specifications and performances of an electrical power transformer it has to go
through numbers of testing procedures. Some tests are done at manufacturer premises before delivering
the transformer.
Mainly two types of transformer testing are done at manufacturer premises-
 type test of transformer
 routine test of transformer.
In addition to that some transformer tests are also carried out at the consumer site before
commissioning and also periodically in regular & emergency basis through out its service life.
Testing of TRANSFORMER
1. Type of Transformer Testing
Tests done at factory
 Type tests
 Routine tests
 Special tests
Tests done at site
 Pre-commissioning tests
 Periodic/condition monitoring tests
 Emergency tests
Type Test of Transformer
To prove that the transformer meets customer’s specifications and design expectations, the transformer
has to go through different testing procedures in manufacturer premises. Some transformer tests are
carried out for confirming the basic design expectation of that transformer. These tests are done mainly
in a prototype unit not in all manufactured units in a lot. Type test of transformer confirms main and
basic design criteria of a production lot. Neha Gethe 39

2.Routine Tests of Transformer
Routine tests of transformer is mainly for confirming operational performance of individual unit in a
production lot. Routine tests are carried out on every unit manufactured.
3.Special Tests of Transformer
Special tests of transformer is done as per customer requirement to obtain information useful to the
user during operation or maintenance of the transformer.
4.Pre Commissioning Test of Transformer
In addition to these, the transformer also goes through some other tests, performed on it, before actual
commissioning of the transformer at site. The transformer testingperformed before commissioning the
transformer at site is called pre-commissioning test of transformer. These tests are done to assess the
condition of transformer after installation and compare the test results of all the low voltage tests with
the factory test reports.
 Type tests of transformer includes
 Transformer winding resistance measurement
 Transformer ratio test
 Transformer vector group test
 Measurement of impedance voltage/short circuit impedance and load loss (Short circuit test).
 Measurement of no load loss and current (Open circuit test).
 Measurement of insulation resistance.
 Dielectric tests of transformer
 Temperature rise test of transformer.
 Tests on on-load tap-changer.
 Vacuum tests on tank and radiators.
Neha Gethe 40

 Routine tests of transformer include
 Transformer winding resistance measurement
 Transformer ratio test
 Transformer vector group test
 Measurement of impedance voltage/short circuit impedance and load loss (Short circuit test).
 Measurement of no load loss and current (Open circuit test)
 Measurement of insulation resistance.
 Dielectric tests of transformer
 Tests on on-load tap-changer.
 Oil pressure test on transformer to check against leakages past joints and gaskets.
That means Routine tests of transformer include all the type tests except temperature rise and vacuum
tests. The oil pressure test on transformer to check against leakages past joints and gaskets is included.
 Special Tests of transformer include
 Dielectric tests.
 Measurement of zero-sequence impedance of three-phase transformers
 Short-circuit test.
 Measurement of acoustic noise level.
 Measurement of the harmonics of the no-load current.
 Measurement of the power taken by the fans and oil pumps.
 Tests on bought out components / accessories such as buchhloz relay, temperature indicators,
pressure relief devices, oil preservation system etc.
Neha Gethe 41

1. Open circuit (OCC) test.
Open circuit test is conducted on the transformer for the following purposes
I. To determine the shunt parameters in the equivalent circuit
II. To determine core loss
III. To determine the magnetizing current (Im)
During this test, the rated voltage is supplied to one of the winding while the other winding is kept open.
Normally LV side is provided with the rated voltage and the LV side is kept open. If the transformer is
used at voltages other than the rated voltage, then the test should be conducted at that voltage. The
meters are connected to the transformer as shown in the circuit diagram. After applying the voltage the
meter readings are noted. The ammeter reading corresponds to the no load current Io and the watt meter
reading corresponds to the core loss or iron loss Pi.
Pi = Po (Iron loss)
Shunt parameters in the equivalent circuit can be calculated from the following formula.
Yo = Gi - jBm
Yo = Io / Vi
Vi
2 Gi = Po Hence, The conductance Gi = Po / Vi
2
The susceptance Bm = √ (Yo
2 - Gi
2)
Neha Gethe 42

2. Short circuit (SC) test
Short circuit test is conducted to determine the following
I. The full load cu- loss (Copper loss).
II. Leakage reactance and equivalent resistance.
In short circuit test supply arrangements are made at the HV side and the LV side is short circuited. The
voltage needed for the short circuit test is 5 - 8% of rated voltage of the transformer.
Since the test on the HV side requires less current than that on the LV side supply is provided on the HV
side. The supply voltage is gradually raised from zero till the transformer draws its full load current.
Voltage = Vs; current = Isc; Power input = Psc
The iron loss during the short circuit test is negligible due to very low excitation voltage. Therefore power
drawn will be sufficient to satisfy the copper loss.
Hence the watt meter reading corresponds to the full toad copper loss.
Psc = Pcu (Copper loss)
Z = V sc / I sc= √ (R2 + X2),Equivalent resistance R = Pcu/ I sc
2
Equivalent reactance X = √ (Z2 - R2)
Since the iron loss is neglected the shunt branch in the equivalent circuit can also be neglected.
Neha Gethe 43

3.Sumpner’s test or back to back test
This test is conducted to determine the steady state temperature rise in if the transformer is full loaded
continuously. This test is called Sumpner's after its inventers William Sumpner. In Short circuit and
open circuit tests the power loss is due to either copper loss or core loss but never both. Sumpners test
provides a way to find the steady rise in temperature of a fully loaded transformer without conducting
an actual loading test. In this test two identical transformers required to determine the steady state
temperature rise. But in case of very large sizes two identical transformers may not be available as these
are custom built.
Neha Gethe 44

Cooling of Transformers
The Transformer is a device used to convert the energy at one voltage level to the energy at another
voltage level. During this conversion process, losses occur in the windings and the core of the
transformer. These losses appear as heat. The transformer’s output power is less than its input power.
The difference is the amount of power converted into heat by core loss and winding losses. The losses
and the heat dissipation increases with increase in the capacity of the transformer.
The temperature rise of a transformer can be estimated by the following formula:
ΔT = (PΣ/AT)0.833
Where:
ΔT = temperature rise in °C
PΣ = total transformer losses (power lost and dissipated as heat) in mW;
AT = surface area of transformer in cm2.
Cooling of transformers
Cooling of transformer is the process of dissipation of heat developed in the transformer to the
surroundings. The losses occurring in the transformer are converted into heat which increases the
temperature of the windings and the core. In order to dissipate the heat generated cooling should be
done.
How to cool the transformer?
There are two ways of cooling the transformer:
First, the coolant circulating inside the transformer transfers the heat from the windings and the core
entirely to the tank walls and then it is dissipated to the surrounding medium
Second, along with the first technique the heat can also be transferred by coolants inside the
transformer.
The choice of method used depends upon the size, type of applications and the working conditions
Neha Gethe 45

.Coolants
The coolants used in the transformer are air and oil. In dry type transformer air coolant is used and in oil
immersed one, oil is user. In the first said, the heat generated is conducted across the core and windings
and is dissipated from the outer surface of the core and windings to the surrounding air. In the next,
heat is transferred to the oil surrounding the core and windings and it is conducted to the walls of the
transformer tank. Finally the heat is transferred to the surround air by radiation and convection.
Methods of cooling of transformer
Based on the coolant used the cooling methods can be classified into:
 Air cooling
 Oil and Air cooling
 Oil and Water cooling
1. Air cooling (Dry type transformers)
 Air Natural(AN)
 Air Blast (AB)
2. Oil cooling (Oil immersed transformers)
 Oil Natural Air Natural (ONAN)
 Oil Natural Air Forced (ONAF)
 Oil Forced Air Natural (OFAN)
 Oil Forced Air Forced (OFAF)
3. Oil and Water cooling (For capacity more than 30MVA)
 Oil Natural Water Forced (ONWF)
 Oil Forced Water Forced (OFWF)
Neha Gethe 46

1. Air cooling (Dry type transformers)
In this method, the heat generated is conducted across the core and windings and is dissipated from the
outer surface of the core and windings to the surrounding air.
Air Natural(AN)
This method uses the ambient air as the cooling medium.
The natural circulation of the air is used for dissipation of
heat generated by natural convection. The core and the
windings are protected from mechanical damage by
providing a metal enclosure. This method is suitable for
transformers of rating up to 1.5MVA. This method is
adopted in the places where fire is a great hazard.
Air Blast (AB)
In this method, the transformer is cooled by circulating
continuous blast of cool air through the core and the
windings. For this external fans are used. The air
supply must be filtered to prevent accumulation of
dust particles in the ventilating ducts.
Neha Gethe 47

2. Oil cooling (Oil immersed transformers)
In this method, heat is transferred to the oil surrounding the core and windings and it is conducted to
the walls of the transformer tank. Finally the heat is transferred to the surround air by radiation and
convection.
 Oil coolant has two distinct advantages over the air coolants.
 It provides better conduction than the air
 High coefficient of conduction which results in the natural circulation of the oil.
Oil Natural Air natural (ONAN)
The transformer is immersed in oil and the heat generated in the cores and
the windings is passed on to oil by conduction. Oil in contact with the
surface of windings and core gets heated up and moves towards the top
and is replaced by the cool oil from the bottom. The heated oil transfers its
heat to the transformer tank through convection and which in turn transfers
the heat to the surrounding air by convection and radiation.
This method can be used for the transformers having the ratings up to
30MVA.
Oil Forced Air Natural (OFAN)
In this method, copper cooling coils are mounted above the transformer core. The copper coils will be
fully immersed in the oil. Along with the oil natural cooling the heat from the core passes to the
copper coils and the circulating water inside the copper coil takes away the heat. The disadvantage in
this method is that since water enters inside the transformer any kind of leakage will contaminate the
transformer oil.
Neha Gethe 48

Oil Natural Air Forced (ONAF)
In this method, the heated oil transfers its heat to the transformer
tank. The tank is made hollow and air is blown to cool the
transformer. This increases the cooling of transformer tank to five
to six time its natural means. Normally this method is adopted by
externally connecting elliptical tubes or radiator separated from
the transformer tank and cooling it by air blast produced by fans.
These fans are provided with automatic switching. When the
temperature goes beyond the predetermined value the fans will be
automatically switched on.
Oil Forced Air Forced (OFAF)
In this method the oil is cooled in the cooling plant using air blast
produced by the fans. These fans need not be used all the time.
During low loads fans are turned off. Hence the system will be similar
to that of Oil Natural Air natural (ONAN). At higher loads the pumps
and fans are switched on and the system changes to Oil Forced Air
Forced (OFAF). Automated switching methods are used for this
conversion such that as soon as the temperature reaches a certain
level the fans are automatically switched on by the sensing elements.
This method increases the system efficiency. This is a flexible method
of cooling in which up to 50% of rating ONAN can be used and OFAF
can be used for higher loads. This method is used in transformers
having ratings above 30MVA.
Neha Gethe 49

Oil and Water cooling
In this method along with oil cooling, water is circulated through copper tubes which enhance the cooling
of transformer. This method is normally adopted in transformers with capacities in the order of several
MVA.
Oil Forced Water Forced (OFWF)
In this method, copper cooling coils are mounted above the
transformer core. The copper coils will be fully immersed in the
oil. Along with the oil natural cooling the heat from the core
passes to the copper coils and the circulating water inside the
copper coil takes away the heat. The disadvantage in this method
is that since water enters inside the transformer any kind of
leakage will contaminate the transformer oil. Since heat passes
three times as rapidly from copper cooling tube to water as from
oil to copper tubes, the tubes are provided with fans to increase
the conduction of heat from oil to tubes. The water inlet and outlet
pipes are lagged in order to prevent the moisture in the ambient
air fro condensing on the pipes and getting into the oil.
Oil Forced Water Forced (OFWF)
In this method hot oil is passed though a water heat exchanger. The pressure of the oil is kept higher
than that of the water therefore there will be leakage from oil to the water alone and the vise versa is
avoided. This method of cooling is employed in the cooling of transformers with very larger capacity in
the order of hundreds of MVA. This method is suitable for banks of transformers. Maximum of three
transformers can be connected in a single pump circuit. Advantages of this method over ONWF are
that the transformer size is smaller and the water does not enter into the transformer. This method is
widely used for the transformers designed for hydro electric plants.
Neha Gethe 50

Tap changing in Transformers
It is a normal fact that increase in load lead to decrease in the supply voltage. Hence the voltage
supplied by the transformer to the load must be maintained within the prescribed limits. This can be
done by changing the transformer turns ratio.
The taps are leads or connections provided at various points on the winding. The turns ratio differ from
one tap to another and hence different voltages can be obtained at each tap.
Need for system voltage control
1. Adjusting the terminal voltage of consumer within the prescribed limits
2. Adjustment of voltage based on change in load.
3. In order to control the real and reactive power.
4. For varying the secondary voltage based on the requirement.
Types of taps
Taps may be principal, positive or negative. Principal tap is one at which rated secondary voltage can
be obtained for the rated primary voltage. As the name states positive and negative taps are those at
which secondary voltage is more or less than the principle tap.
Taps are provided at the HV windings of the transformer because of the following reasons.
1. The number of turns in the High voltage winging is large and hence a fine voltage variation can be
obtained.
2. The current on the low voltage winding of large transformers are high. Therefore interruption of
high currents is a difficult task.
3. LV winding is placed nearer to the core and HV winding is placed outside. Therefore providing taps
on the HV winding is comparatively easier than that of the LV winding.
Neha Gethe 51

Location of Taps
The taps can be provided at the phase ends, at the neutral point, or in the middle of the winding. The
number of bushing insulators can be reduced by providing taps at the phase ends. When the taps are
provided at the neutral point the insulation between various parts will be reduced. This arrangement is
economical particularly important for the large transformer.
Tap changing methods
Tap changing causes change in leakage reactance, core loss, copper loss and perhaps some problems in the
parallel operation of dissimilar transformer. There are two methods of tap changing.
1. Off load tap changing
2. On load tap changing
The winding is tapped at various points. Since the taps are provided at
various points in the winding single tap must be connected at a time
otherwise it will lead to short circuit. Hence the selector switch is operated
after disconnecting the load.
To prevent unauthorized operation of an off load tap changer, mechanical
lock is provided. To prevent inadvertent operation, electromechanical
latching devices are provided to operate the circuit breakers and de-
energize the transformer as soon as the tap changer handle is moved.
Off load (No load or off circuit) tap changing--As the name indicates, in this method tap changing is
done after disconnecting the load from the transformer.
Off load tap changing is normally provided in low power, low voltage transformers. It is the cheapest
method of tap changing. The tap changing is done manually though hand wheel provided in the cover. In
some transformers arrangements to change the taps by simply operating the mechanical switches are also
provided.
Neha Gethe 52

On load tap changing
On load tap changers are used to change the turns ratio without disconnecting the load
from it. Tap changing can be done even when the transformer is delivering load. On load
tap changers considerable increases the efficiency of the system. Nowadays almost all the
large power transformers are provided with on load tap changers.
The reason for providing On load tap changer in power transformers are
1. During the operation of on load tap changers the main circuit remains unaffected.
2. Dangerous sparking is prevented.
Procedure
Consider a high speed resistor type on load tap changers provided at neutral end of each
phase as shown. The load is now supplied from the tap 1.
The selector switches 1 and 2 are in contact with the taps 1 and 2. Now to switch over to the
tap 2, the selector switch follows the following steps:
1. Contacts a and b are closed. The load current flows from tap 1 through contact b.
2. The external mechanism moves the diverter switch S3 from b, now load is supplied from
contact a through resistor R1.
3. When diverter switch moves further it closes the contact d and both R1 and R2 are
connected across taps 1 and 2 and the load current flows through these resistances to its mid
point.
4. When S3 moves further to the left, contact a is opened and the load current flows from tap
2 through resistor R2 and d.
Neha Gethe 53

5. Finally the contact reaches the contact c and
resistor R2 is short circuited. The load current
flows from tap 2 through contact c.
Now to change the tap from 2 to 3, the selector
switch S1 is first moved to tap 3 and the above
steps are reverse.
In order to limit the power loss it is necessary
that the transformers are kept in the circuit for
as minimum time as possible.
More compact tap changers with high reliability
and performance are being made by employing
vacuum switches in the diverter switch.
Neha Gethe 54

What is a Buchholz relay ? How does it work?
What is a Buchholz relay?
Buchholz relay is a type of oil and gas actuated protection
relay universally used on all oil immersed transformers
having rating more than 500 kVA. Buchholz relay is not
provided in relays having rating below 500 kVA from the
point of view of economic considerations.
Why Buchholz relay is used in transformers?
Buchholz relay is used for the protection of transformers
from the faults occurring inside the transformer. Short circuit
faults such as inter turn faults, incipient winding faults, and
core faults may occur due to the impulse breakdown of the
insulating oil or simply the transformer oil. Buchholz relay
will sense such faults and closes the alarm circuit.
Working principle
Buchholz relay relies on the fact that an electrical fault inside the
transformer tank is accompanied by the generation of gas and if
the fault is high enough it will be accompanied by a surge of oil
from the tank to the conservator
Whenever a fault occurs inside the transformer, the oil in the
transformer tank gets overheated and gases are generated. The
generation of the gases depends mainly on the intensity of fault
produced. The heat generated during the fault will be high
enough to decompose the transformer oil and the gases produced
can be used to detect the winding faults. This is the basic principle
behind the working of the Buchholz relay. Neha Gethe 55

Construction
Buchholz relay can be used in the transformers
having the conservators only. It is placed in the pipe
connecting the conservator and the transformer tank.
It consists of an oil filled chamber. Two hinged floats,
one at the top of the chamber and the other at the
bottom of the chamber which accompanies a mercury
switch each is present in the oil filled chamber. The
mercury switch on the upper float is connected to an
external alarm circuit and the mercury switch on the
lower is connected to an external trip circuit.
Operation
a. Operation of the Buchholz relay is very simple. Whenever any minor fault occurs
inside the transformer heat is produced by the fault currents. The transformer oil gets
decomposed and gas bubbles are produced. These gas bubbles moves towards the
conservator through the pipe line. These gas bubbles get collected in the relay chamber
and displaces oil equivalent to the volume of gas collected. The displacements of oil tilts
the hinged float at the top of the chamber thereby the mercury switch closes the contacts
of the alarm circuit.
Neha Gethe 56

b. The amount of gas collected can be viewed through the window provided
on the walls of the chamber. The samples of gas are taken and analyzed. The
amount of gas indicates the severity of and its color indicates the nature of
fault occurred. In case of minor faults the float at the bottom of the chamber
remains unaffected because the gases produced will not be sufficient to
operate it.
c. During the occurrence of severe faults such as phase to earth faults and
faults in tap changing gear, the amount of volume of gas evolves will be
large and the float at the bottom of the chamber is tilted and the trip circuit
is closed. This trip circuit will operate the circuit breaker and isolates the
transformer.
When does a buchholz relay operate?
1. Whenever gas bubbles are formed inside the transformer due to
severe fault.
2. Whenever the level of transformer oil falls.
3. Whenever transformer oil flows rapidly from the conservation tank
to the main or from the main tank to the conservation tank.
Advantages of Buchholz relay
Buchholz relay indicates inter turn faults and faults due to heating of core
and helps in the avoidance of severe faults.Nature and severity of fault can be
determined without dismantling the transformer by testing the air samples.
Neha Gethe 57

Limitation of Buchholz relay
It can sense the faults occurring below the oil level only. The relay is slow and has a minimum
operating range of 0.1second and an average operating range of 0.2 seconds.
Introduction of Insulating Oil
• Types
• Properties
• Parameters
• Electrical Parameter
• Dielectric Strength
• Specific Resistance
• Dielectric Dissipation Factor of Tan Delta
• Chemical Parameters
• Water Content
• Acidity
• Physical Parameters
• Inter Facial Tension
• Flash Point
• Pour Point
• Viscosity
Introduction of Insulating Oil
Insulating oil in an electrical power transformer is commonly known as transformer oil. It is normally
obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is also
known as mineral insulating oil. Transformer oil serves mainly two purposes one it is liquid insulation
in electrical power transformer and two it dissipates heat of the transformer e.i. acts as coolant. In addition
to these, this oil serves other two purposes, it helps to preserve the core and winding as these are fully
immersed inside oil and another important purpose of this oil is, it prevents direct contact of atmospheric
oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation.Neha Gethe 58

Types of Transformer Oil
Generally there are two types of transformer Oil used in transformer,
1. Paraffin based transformer oil
2. Naphtha based transformer oil
Naphtha oil is more easily oxidized than Paraffin oil. But oxidation product i.e. sludge in the naphtha oil
is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not precipitated in bottom of the
transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb
the transformer cooling system.
Paraffin oil although oxidation rate is lower than that of Naphtha oil but the oxidation product or
sludge is insoluble and precipitated at bottom of the tank and obstruct the transformer cooling system.
Although Paraffin based oil has above mentioned disadvantage but still in our country it is generally
used because of its easy availability. Another problem with paraffin based oil is its high pour point due
to the wax content, but this does not effect its use due to warm climate condition of India.
Properties of Transformer Insulating Oil
Some specific parameters of insulating oil should be considered to determined the serviceability of that
oil.
Parameters of Transformer Oil
 Electrical parameters :– Dielectric strength, specific resistance, dielectric dissipation factor.
 Chemical parameter :- Water content, acidity, sludge content.
 Physical parameters :- Inter facial tension, viscosity, flash point, pour point.
Neha Gethe 59

Electrical Parameter of Transformer Oil
Dielectric strength of transformer oil is also known
as breakdown voltage of transformer oil or BDV of transformer oil.
Break down voltage is measured by observing at what voltage, sparking
strants between two electrods immerged in the oil, separated by specific
gap. low value of BDV indicates presence of moisture content and
conducting substances in the oil.
For measuring BDV of transformer oil, portable BDV measuring kit is
generally available at site. In this kit, oil is kept in a pot in which one pair
of electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between
them.
Now slowly rising voltage is applied between the electrodes. Rate of rise of voltage is generally controlled
at 2 KV/s and observe the voltage at which sparking starts between the electrodes. That means at
which voltage dielectric strength of transformer oil between the electrodes has been broken down.
1. Generally this measurement is taken 3 to 6 times in same
sample of oil and the average value of these reading is
taken. BDV is important and popular test of transformer
oil, as it is primary indication of health of oil and it can be
easily carried out at site.
2. Dry and clean oil gives BDV results, better than the oil
with moisture content and other conducting impurities.
Minimum breakdown voltage of transformer oil or dielectric strength of transformer
oil at which this oil can safely be used in transformer, is considered as 30 KV.Neha Gethe 60

Specific Resistance of Transformer Oil
This is another important property of transformer oil. This is measure of
DC resistancebetween two opposite sides of one cm3 block of oil. Its unit is taken as ohm-
cm at specific temperature. With increase in temperature the resistivity of oil decreases
rapidly. Just after charging a transformer after long shut down, the temperature of the oil
will be at ambient temperature and during full load the temperature will be very high and
may go up to 90°C at over load condition. So resistivity of the insulating oil must be high at
room temperature and also it should have good value at high temperature as well.
That is why specific resistance or resistivity of transformer oil should be measured at 27°C
as well as 90°C.Minimum standard specific resistance of transformer oil at 90°C is 35 ×
1012 ohm–cm and at 27°C it is 1500 × 1012 ohm–cm.
Dielectric Dissipation Factor of Tan Delta of Transformer Oil
Dielectric dissipation factor is also known as loss factor or tan delta of transformer oil.
When a insulating materials is placed between live part and grounded part of an electrical
equipment, leakage current will flow. As insulating material is dielectric in nature
thecurrent through the insulation ideally leads the voltage by 90o.
Here voltage means the instantaneous voltage between live part and ground of the
equipment. But in reality no insulating materials are perfect dielectric in nature.
Hence current through the insulator will lead the voltage with an angle little bit shorter than
90°. Tangent of the angle by which it is short of 90° is called dielectric dissipation factor or
simply tan delta of transformer oil. Neha Gethe 61

Dielectric Dissipation Factor of Tan Delta of Transformer Oil
Dielectric dissipation factor is also known as loss factor or tan delta of transformer oil. When a insulating
materials is placed between live part and grounded part of an electrical equipment, leakage current will
flow. As insulating material is dielectric in nature thecurrent through the insulation ideally leads
the voltage by 90o. Here voltage means the instantaneous voltage between live part and ground of the
equipment. But in reality no insulating materials are perfect dielectric in nature. Hence current through the
insulator will lead the voltage with an angle little bit shorter than 90°. Tangent of the angle by which it is
short of 90° is called dielectric dissipation factor or simply tan delta of transformer oil.
More clearly, the leakage current through an insulation does have two
component one is capacitive or reactive and other one is resistive or active.
Again it is clear from above diagram, value of ′δ′ which is also known as
loss angle, is smaller, means resistive component of thecurrent IR is smaller
which indicates high resistive property of the insulating material. High
resistive insulation is good insulator.
Hence it is desirable to have loss angle as small as possible. So we should
try to keep the value of tanδ as small as possible. High value of this tanδ is
an indication of presence of contaminants in transformer oil.
Hence there is a clear relationship between tanδ and resistivity of insulating oil. If resistivity of the
insulating oil is decreased, the value of tan-delta increases and vice verse. So both resistivity test and tan
delta test of transformer oil are not normally required for same piece of insulator or insulating oil.
In one sentence it can be said that, tanδ is measure of imperfection of dielectric nature of insulation
materials like oil. Neha Gethe 62

Chemical Parameters of Transformer Oil
 Water Content in Transformer Oil
Moisture or water content in transformer oil is highly undesirable as it affects adversely the dielectric
properties of oil. The water content in oil also affects the paper insulation of the core and winding of
transformer. Paper is highly hygroscopic in nature. Paper absorbs maximum amount of water from oil
which affects paper insulation property as well as reduced its life. But in loaded transformer, oil becomes
hotter, hence the solubility of water in oil increases as a result the paper releases water and increase
the water content in transformer oil. Thus the temperature of the oil at the time of taking sample for test is
very important. During oxidation acid are formed in the oil the acids give rise the solubility of water in the
oil. Acid coupled with water further decompose the oil forming more acid and water. This rate of
degradation of oil increases. The water content in oil is measured as pm(parts per million unit).
Water content in oil is allowed up to 50 ppm as recommended by IS–335(1993). The accurate measurement
of water content at such low levels requires very sophisticated instrument like Coulometric Karl Fisher
Titrator .
 Acidity of Transformer Oil
Acidity of transformer oil, is harmful property. If oil becomes acidic, water content in the oil becomes more
soluble to the the oil. Acidity of oil deteriorates the insulation property of paper insulation of winding.
Acidity accelerates thee oxidation process in the oil. Acid also includes rusting of iron in presence of
moisture.
The acidity of transformer oil is measure of its acidic constituents of contaminants. Acidity of oil is express
in mg of KOH required to neutralize the acid present in a gram of oil. This is also known as neutralization
number. Neha Gethe 63

Physical Parameters of Transformer Oil
 Inter Facial Tension of Transformer Oil
Inter facial tension between the water and oil interface is the way to measure molecular attractive force
between water and oil. It is measured in Dyne/cm or mili-Newton/meter. Inter facial tension is exactly
useful for determining the presence of polar contaminants and oil decay products. Good new oil generally
exhibits high inter facial tension. oil oxidation contaminants lower the IFT.
 Flash Point of Transformer Oil
Flash point of transformer oil is the temperature at which oil gives enough vapors to produce a
flammable mixture with air. This mixture gives momentary flash on application of flame under standard
condition. Flash point is important because it specifies the chances of fire hazard in the transformer. So it
is desirable to have very high flash point of transformer oil. In general it is more than 140°(>10°).
 Pour Point of Transformer Oil
It is the minimum temperature at which oil just start to flow under standard test condition. Pour point of
transformer oil is an important property mainly at the places where climate is extremely cold. If the oil
temperature falls bellow the pour point, transformer oil stops convection flowing and obstruct cooling in
transformer. Paraffin based oil has higher value of pour point, compared to Naphtha based oil, but in
India like country, it does not effect the use of Paraffin oil due tits warm climate condition. Pour Point of
transformer oil mainly depends upon wax content in the oil.
 Viscosity of Transformer Oil
In few wards, viscosity of transformer oil can be said that viscosity is the resistance of flow, at normal
condition. Obviously resistance to flow of transformer oil means obstruction of convection circulation of
oil inside the transformer. A good oil should have low viscosity so that it offers less resistance to the
convectional flow of oil thereby not affecting the cooling of transformer. Low viscosity of transformer
oil is essential, but it is equally important that, the viscosity of oil should increase as less as possible with
decrease in temperature. Every liquid becomes more viscous if temperature decreases.Neha Gethe 64

Neha Gethe 65
Classification of Electrical Insulation
Class of Insulating Maximum Permissible
Temp
Insulation material
Class o or y 90oC Impregnated cellulose, cotton,silk, wood,
paper,pressboard
CLASS A 105oC Impregnated cellulose,cotton,silk,phenoli
resin, Mineral oil
CLASS B 120oC Cellulose Triacetate
CLASS C 130oC Mica, Glass fiber, asbestos with organic
binder
CLASS D 155oC As in class B with suitable binder
CLASS E 180oC As in class B with silicone binder
CLASS F >180oC Mica, porcelain,Glass Quartz and similar
inorganic materials

Insulating materials used in transformers
Insulation is one of the most important constituent of a transformer. The durability and stability of the
transformer depends upon the proper utilization of insulating materials in it. In transformers mainly
three insulating materials are used.
 Transformer oil
 Insulating paper
 Press board
Of the three, the major insulating material used is transformer oil.
 Transformer oil
As said earlier transformer oil is the major insulating material used
in transformer. It is one of the important factors that determine the
life and satisfactory operation of the transformer.
The transformer performs the following two functions.
1. It provides insulation in combination with the insulating
materials used in the conductors and coils.
2. It also acts as a coolant to extract heat from the core and the
windings.
Transformer makes use of hydrocarbon mineral oil. It mainly
consists of four generic classes of organic compounds. They are
aromatics, paraffins, napthenes and olefines. Transformer oil will
provide better insulation when aromatics, paraffins, napthenes and
olefines are present in it at a right proportion.
Neha Gethe 66

The transformer is affected by its operating conditions.
The presence of moisture or suspended particles in
transformer oil affects its dielectric property. Hence
transformer oil it should be tested periodically. If the
oil is containing moisture or suspended particles it
should be filtered or replaced by fresh oil.
Property
Recommended
value
Permittivity 2.2
Thermal conductivity 0.12 W/m deg C
Specific Heat 2.06 kJ/kg deg C
Co-efficient of
Expansion
0.00078/ deg C
Mean density factor 0.00065/ deg C
 Insulating paper
Insulating paper is made from the vegetable fibers. These fibers mainly consist of
cellulose. The main properties of these papers are listed below:---
 Grammage
Grammage is the ratio of mass to the area. It influences most of the
electrical and mechanical properties. The recommended value for 125
micrometer thick paper is 100 gm/mt2.The maximum allowable
variation is 5%.
 Density
The paper recommended in transformer use can have a density range
of 0.6 to 1.3 gm/c.c.
 Moisture content
Moisture content reduces the dielectric property of the paper. Since
paper is hygroscopic, moisture ingression takes place with usage. The
maximum allowable moisture content is 8%.
Neha Gethe 67

 Oil and water absorption
The dielectric property of paper increased when impregnated in oil
under vacuum and decreases with water content. Even though the
water content is not recommended the maximum permitted water
content is 10%.
 Air permeability
Permeability is the rate at which the air can pass through it. The
dielectric strength of paper is inversely proportional to the air
permeability. The recommended value of air permeability is 0.2-0.5.
 Tensile strength and elongation
Paper must be able to withstand the tension during wrapping.
Recommended value of tension is 78 - 85 N-mt/gm in wrapping
direction and 25 - 30 N-mt/gm.
The electrical parameters of
insulating paper are indicated below.
Property
Recommend
ed Value
Break down
voltage
7- 7.5
kV/mm
(min) at 90
deg. C
Dissipation
factor
0.003 (max)
Conductivity
10 ns/mt
(max)
 Press board
Press board is also made up of vegetable fibers and contains cellulose. Solid press board unto 6
mm to 8mm thick is ordinarily made. Since the most difficult insulation problem in HT
transformer occur at the ends of the windings and lead outs from the windings hence moulded
pressboards are widely used in these parts for insulation. Synthetic resin bonded paper based
laminates are used in voltage stressed zones. The important parameters considered are density,
tensile strength, elongation, conductivity, oil absorption, moisture content, compressibility etc.
Neha Gethe 68

Neha Gethe 69

Neha Gethe 70
 Transformer is a vital link in a AC power system which has made possible power
generated at low voltages to be stepped up to extra high voltages for transmission over
long distance and then transformed to low voltages for utilization at proper load
centre.

The purpose of transformer is to transfer electrical energy from system of one voltage
to system of another voltage.
 As the real interdependence of engineering theory and practice is now fully recognized, it
becomes almost essential to deal with theory briefly before discussing practice.
 A transformer consists essentially of a magnetic core built up of insulated silicon steel
lalminations upon which are wound two distinct set of coils suitably located with respect
to each other and termed the primary and secondary windings.
POWER TRANSFORMER-THEORY
 The primary winding is that winding to which supply voltage is applied irrespective of
whether it is the higher or lower voltage winding ; the other winding to which the load is
directly connected is termed the secondary winding.
 As the phenomenon of electromagnetic induction can only take place in static apparatus
when the magnetic flux is continually varying, it is clear that static transformers can only
be used in alternating current circuits.

Neha Gethe 71
 If an alternating e.m.f. is applied to the terminals of the primary winding of a
transformer with the secondary winding open circuited, a very small current
will flow in the primary circuit only, which serves to magnetize the core and
to supply the iron loss of the transformer.
 Thus an alternating magnetic flux is established in the core which induces an
e.m.f. in both primary and secondary windings.
 The magnetizing ampere-turns are given by the product of the magnetizing
current and the primary turns.
 The no-load current is given by the total no-load ampere-turns divided by the
primary turns.
 As primary and secondary windings are wound on the same core, and as the
magnetizing flux is common to the two windings, obviously the voltage
induced in a single turn of each winding will be the same, and the induced
voltages in the primary and secondary windings are therefore in direct
proportion to the number of turns in those windings.

Neha Gethe 72
The formula connecting induced voltage, flux and number of turns is
E = 4KfΦmNf
E = rms value of the induced e.m.f. in the winding considered
Kf = form factor of the e.m.f. wave(1.1 for sine wave)
f = frequency of the supply in hertz
Φm = total magnetic flux through the core( max value) in webers
N = number of turns in the winding considered
The above formula holds good for the voltage induced in either primary or secondary
windings depending on number of turns N in the winding.
As the power supply voltage wave is a sine wave the form factor Kf becomes 1.11 and the
equation simplifies to
E = 4.44ΦmNf
For practical designing the formula is re arranged as:
V/N = BmAf/22.51 X 104
The specified voltage of the winding divided by the volts per turn gives the actual turns
for the winding

Neha Gethe 73
 The losses which theoretically occur in an unloaded transformer are the iron losses, copper loss due to
the flow of no-load current in the primary winding, and dielectric loss.
 In practice only the iron losses are of importance in transformers, and these losses are the sum of the
hysteresis and eddy current losses which are constant for a given applied voltage and unaffected by
the load on transformer.
 The dielectric losses are also functions of the primary and secondary voltages but they vary slightly
with the temperature of the windings as affected by the load on transformer.
 When the secondary circuit is closed, a secondary current flows, the value of which is determined by
the magnitude of the secondary terminal voltage and the impedance of the load circuit.
 The m m f due to the secondary load current produces a certain load flux in the core which is in phase
with the secondary current.
 The secondary load current is immediately balanced by a primary load current of such a value that
the primary and secondary load ampere-turns are equal.
 The secondary load flux is similarly counteracted by a primary load flux which is in phase with the
primary load balancing current, and therefore in phase opposition to, and of the same magnitude as,
the secondary load flux.
 Therefore the core is left in its initial state of magnetization corresponding to open circuit conditions
and this explains why the iron loss is independent of the load.
 The voltage drops due to resistance and leakage reactance is manifested at the secondary terminals.

Neha Gethe 74
The output of a power transformer is expressed in kilo volt ampere (kVA) which is the product of the rated
secondary(no-load) voltage E2 and the rated output current I2 and multiplied by the appropriate phase
factor for poly phase circuits.
 Single phase transformers
kVA = E2I2 x 10-3
 Three phase transformers
kVA = E2I2 x 10-3 x 1.73
The relationship between phase and line currents and voltages for star or delta configuration:
Three phase star connection
Phase current = line current I = kVA x 1000/(E x 1.73)
Phase voltage = E/1.73
Three phase delta connection
Phase current = I/1.73 = kVA x 1000/(E x 3)
Phase voltage = line voltage = E
E and I = line voltage and current respectively
The approximate percentage voltage regulation for a current loading of a times the rated current at a power
factor of cos ǿ2 is given by :
% regulation = a(VR cos ǿ2 + Vx sin ǿ2 ) + a2/200 (Vx cos ǿ2 -VRsin ǿ2 )2
Where,
VR = percentage resistance at full load
= copper loss x 100/rated kVA
Vx = percentage reactance voltage = √(Vz
2-VR
2)
Vz = percentage impedance voltage at full load

Neha Gethe 75
 Major materials like copper, cold-rolled grain oriented silicon steel, insulating oil,
pressboard, paper insulation and certain ferrous and non ferrous items are essential to
build a compact and trouble free transformer.
 Designing an insulation system for application in higher voltage class transformer is an art
and with the use of the best materials available to day, it is possible to economise on size
as well as produce a reliable piece of equipment.
 Keeping in view the transportability, operational limitations and guaranteed technical
performance of the transformer, particular type core construction is adopted.

 Winding and insulation arrangement is an important design aspect. Spiral , layer, helical
and disc windings are used, the disc type is commonly used in the latest design practices.
 The impulse voltage withstand behaviour of disc winding can be enhanced by
interleaving the disc winding.
 The initial voltage distribution at the line end should be taken care of for high voltage
windings.
 The transient voltage distribution and internal heat transfer in the windings have to be
taken care of.
 The insulation design becomes more complex as we move towards higher and higher
voltage class of transformer windings. Proper sizing and routing can be further examined
by detailed electrostatic field plots.

Neha Gethe 76
 To vary the voltage in a transformer, tap changers are used which have a different type of
regulating winding connection, viz. linear, reversing, course-fine etc.
 The electromagnetic forces during short circuits or line faults stress the winding as Radial
and Axial forces occur then. The stresses have to be calculated and dimensioning the
clamping structures are to be adequate to withstand these forces.
 The cooling of transformer becomes more relevant from the point of view of ageing of
insulation system and ensuring longer life due to less thermal degradation.
 Stresses developed in the main tank and other structures are to be taken care of.
 Proper selection of transformer auxiliaries is essential to ensure safe operation of the
transformer as it provides protection under fault conditions.
 Major auxiliaries are: gas operated bucholtz relay, temperature indicators, pressure relief
valve, bushings, cable box, oil preservation system etc.
 Cellulose insulation used in power transformers has approximately 6 to 10 % of
moisture by weight at ambient temperature, being a hygroscopic material. Vacuum
drying becomes very important to remove moisture from insulation.
 To ensure quality and conformation to design calculations, testing is an important
activity on the transformer. The basic testing requirement and testing code are set out in
national and international standards.

Neha Gethe 77
The transformer being a vital equipment , its protection is equally important. The transformer is protected
against short circuit, surges, over voltages, internal faults and earth faults.
The typical design procedure includes selection of core-size, winding conductor, reactance calculation,
and then realization of main guaranteed parameters like percentage impedance, no-load loss, load loss ,
estimation and control of stray losses, winding gradient and structural design
BY NEGLECTING MAINTENANCE INEVITABLE COSTLY FAILURES ARE THE RESULT

STANDARDS
IS-2026-1977- Specification for Power Transformers
Part I - General
Part II - Temperature Rise
Part III - Insulation level & Dielectric tests
Part IV - Terminal markings, Tapings & Connections
IS:10028-1985 -Code of Practice For Selection ,Installation & Maintenance of Power
Transformer
Part I - Selection
Part II - Installation
Part III - Maintenance
IS:10561-1983 Application Guide for Power Transformers
IS:8468-1977 -Specification for on -Load Tap changer
IS:335-1972 -Specifications for new Insulating oils for Transformers &Switchgears
IS: 1866 -2000 - Specifications for used Insulating oils for Transformers
IS: 6600 -1972 - Guide for Loading of oil immersed Transformers
IS:11171-1985 -Specification for Dry type Power Transformer
IS:3347 Part IV/Sec1-1973 -Dimension for porcelain Transformer Bushings
IS:3347 Part IV/Sec2-1973 -Dimension for porcelain Transformer Bushings for use in
Normal & Lightly polluted Atmosphere
Neha Gethe 78

NAME PLATE DETAILS
The number of the standard IS : 2026 : 1962
 Manufacturers name and country of manufacture
 Manufacturer’s serial number
 Rated KVA
 Frequency
 Number of phases
 Rated voltage (HV/LV)
 Rated current (HV/LV)
 Percentage impedance
 Class of Insulation
 Winding connection and phase displacement symbols of vector diagrams i.e.vector
group
 Connection Diagram
 Type of cooling
 Weight of core and windings assembly in Kg.
 Total quantity and weight of insulating liquid in litre’s and Kgs.
 Total weight of transformer in Kgs.
 Temperature rise of oil for which the transformer is designed.
 Year of manufacture
 Customer’s reference (if given)
Neha Gethe 79

Neha Gethe 80

Neha Gethe 81

82
(Circuit breakers)CT
(Bus bar)
(Line switches)
CVT
(Line Bay)
Neha Gethe

 Instrument transformers are basically measuring and controlling elements in the power
network.
 AC range extension beyond the reasonable capability of indicating instruments is
accomplished with instrument transformers, since the use of heavy current shunts and
high voltage multipliers would be prohibitive both in cost and in power consumption.
 Instrument transformers are also used to isolate instruments from power lines and to
permit instrument circuits to be grounded.
 Current transformers and Voltage transformers are designated as instrument
transformers as their secondary load is made up of measuring , control and protective
instruments.
 The instrument transformers are used to transform the primary current or voltage to
values suitable for standard ratings of instruments, meters, relays and other measuring
or control devices
 The instrument transformers are needed to insulate measuring and control devices
connected in the secondary circuit from the primary-circuit operating voltages.
 To provide complete protection, the secondary circuit should be grounded at one point.
Metal cases should also be grounded.
Neha Gethe 83

 The current circuits of instruments and meters normally have very low impedance, and
current transformers must be designed for operation into such a low impedance
secondary burden.
 The insulation from the primary to secondary of the transformer must be adequate to
withstand line-to-ground voltage, since the connected instruments are usually at
ground potential.
 Normal design is for operation with a rated secondary current of 5 A, or 1 A in some
cases, and the input current may range upward to many thousand amperes.
 The potential circuits of instruments are of high impedance, and voltage transformers
are designed for operation into a high-impedance secondary burden.
 In the usual design, the rated secondary voltage is 110 V, and instrument transformers
have been built for rated primary voltages up to 350kV.
 With the development of higher transmission voltages and inter system ties at these
levels, the coupling-capacitor voltage transformer (CCVT) has come into use for
metering purposes to replace the conventional voltage transformer which, at these
voltages, is bulkier and more costly.
Neha Gethe 84

Magnetic circuit (core)
Instrument Transformer
Sensing function
U1
U2
Voltage sensor
Secondary
Windings
Primary
Windings
Neha Gethe 85

Magnetic circuit (core)
Instrument Transformer
Sensing function
U1
U2
Voltage sensor
Secondary
Windings
Primary
Windings
Neha Gethe 86

I2
Instrument Transformer
Sensing function
I2
Current sensor
Multiple secondary
I1
Neha Gethe 87

 Location of instrument transformers in power network
Neha Gethe 88

Neha Gethe 89

Neha Gethe 90

Current transformer – Theory
The primary winding is series connected in the line and serve the double purpose of;
1.Convenient measurement of large currents and
2.Insulation of instruments, meters, and relays from high-voltage circuits.
The current transformer has a high permeability core of relatively small cross section
operated normally at a very low flux density.
 A current transformer is intended to operate normally with the rated current of the
network flowing through the primary winding which is inserted in series in the network.
The secondary winding of the current transformer, connected to measuring instruments
and relays supplies a current which is proportional to and in phase with the current
circulating in the primary except for the difference due to current error and phase
displacement inherent in the design of the current transformer.
Neha Gethe 91

92
 Current transformer – Theory
Es
IsXs
IsRs
ф
I
m
I0
IPKTIs
VS
Ie
IS
β
τ
α
VECTOR DIAGaRAM
KT-Turns ratio
Secondary turns/primary turns
IP-Primary current
IS-secondary current
I0-exciting current
Ф-main core flux
Im-magnetising component of I0
Ie-quadrature component of I0
τ-Angle of burden
Es-induced emf in secondary
Vs-secondary voltage
β-phase angle error
Neha Gethe

 The primary current does not depend on the secondary current and the voltage drop in
the primary winding is negligibly low due to very low impedance
 The secondary of current transformer is connected to a low impedance burden such as
relay coil, ammeter coil.
 The primary current Ip produces magnetic flux ø in the core which induces emf Es in the
secondary. Es is at 900 behind flux ø.
Secondary voltage Vs is given by;
Vs = Es – Is (Zs+Zb)
Zs – secondary impedance
Zb - burden
Primary current Ip is given by;
Ip – I0+KTIs
KT – Turns ratio – secondary turns to primary turns.
As part of the primary current is used for exciting the core creating the magnetic flux Ip/Is ≠
KT and hence,Ip = I0 + KTIs , this difference caused due to excitation current is called as
current error or current ratio error and is expressed as percentage in the magnitude of the
secondary current.
From the vector diagram it can be seen that, due to exciting current I0, the reversed secondary
current vector leads primary current vector by an angle β. This angle is called the phase angle
error of CT. Neha Gethe 93

INSTRUMENT TRANSFORMERS CT characteristics
Neha Gethe 94

Current Transformers Saturation curve
10 20 30 40
Induction
B [T]
Primary current
Ip/In
Protection CT
18500 gauss
Metering CT
8000 gauss
Neha Gethe 95

 TERMINAL MARKINGS OF CT
Neha Gethe 96

Neha Gethe 97

Polarity of CT
Polarity check is important to verify the correctness of terminal markings and the
protection scheme adopted.
Rated Burden of current transformers
 Burden of the current transformers is usually expressed as the apparent power in volt-
ampere (VA) absorbed at the rated secondary current.
 The Burden includes the impedance of all the meters/relays connected to the
secondary circuit of the particular core and also the lead resistance of the cables used
to connect the equipment to the secondary circuit of the core.
 The Burden can be measured by applying rated secondary current and measuring the
voltage at secondary circuit
Neha Gethe 98

Different cores of current transformers
Accuracy IS 2705
Metering core: for metering purpose specified accuracy is essential over a range of primary
current ranging between 5% to 125% of rated current. As per standards, the accuracy class
of the metering core is to be 0.2S, 0.2 or 0.5 class at specified burden.
Metering core:
Instrument Security Factor (ISF)
 ISF of CT is defined as the ratio of instrument limit primary current to the rated primary
current. The instrument limit primary current of metering CT is the value of primary
current beyond which CT core becomes saturated.
 ISF can be calculated by applying 50% of the rated current to the secondary core and
measuring the voltage across the secondary terminals.
 ISF = V measured /(Rated Burden + R circuit of CT secondary at 750C
Neha Gethe 99

Current transformers - Protection core:
 For protection purpose, actual transformation of secondary quantity is required over a
wide range from full rating up to short circuit level specified for the transformer.
 For protection scheme, different cores are required for different protections and
accuracy class of each core is to be specified. For backup protection 5P10, 5P15 or
5P20 class is required and for primary protection PS class is required.
 Accuracy class of each secondary core differs depending upon the type of application.
For metering core it should be 0.2S, 0.2 or 0.5, for over current and earth fault protection
5P10, 5P15 or 5P20 class and PS class for primary protection and Bus bar protection.
PS class core;
In this core “P” stands for Protection and “S” stands for Special Purpose.
Neha Gethe 100