CHAPTER -1

TRANSFORMER
1.1 INTRODUCTION
Power Transformer is a vital link in a power system which has made
possible the power generated at low voltages to be stepped up to extra high
voltages for transmission over long distances and then transformed to low
voltages for utilization at proper load centers. With this tool in hands it has
become possible to harness the energy resources at far off places from load
centers and connect the same through long extra high voltage transmission
lines working on high efficiencies. It may be said to be the simplest equipment
with no motive parts. Transformer works on the principle of electromagnetic
induction. By this principle, transformer transfers electric energy from one
circuit to another at the same frequency, usually with changed values of
voltage and current. It consists of two windings insulated from each other and
wound on a common core made up of magnetic material.

1.2 CONSTRUCTIONAL DETAILS

A transformer is a static device and its construction is simple as there are
no moving parts. The main components of a transformer are
i. The magnetic core
ii. Primary and secondary windings
iii. Insulation of windings
iv. Lead and tappings for coils with their supports, terminals and terminal
insulators.
v. Cooling arrangement
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1.2.1 MAGNETIC CORE

The transformer core is a closed magnetic circuit through the mutual

flux i.e, the flux which links with both the windings passes. Magnetic
circuit consists of an iron core. The core material and construction
should be such that both the magnetizing current and the core losses
is minimum. The transformer core is generally laminated in order to
reduce the eddy current losses and is made out of a good magnetic
material like silicon steel. The eddy current loss is proportional to the
square of the thickness of laminations. This apparently implies that
the thickness of the laminations should be extremely small in order to
reduce the eddy current losses to minimum. However there is a
practical limit beyond which the thickness of the laminations cannot
be decreased further on account of mechanical considerations. The
thickness of laminations or stampings varies from 0.33 mm to 0.5
mm. The thickness should not be reduced below 0.3 mm because in
that case, the laminations become mechanically weak and tend to
buckle. These laminations are made of the so called transformer
grade steel containing 3-5% silicon. The higher content of silicon
increases the resistivity of the core, thereby reducing the eddy current
core loss. High content silicon is a soft iron material having a narrow
hysteresis loop. This material has a high permeability and hence
magnetizing current is also small. The laminations are insulated from
each other by coating then with a thin coat of varnish. Various types
of stampings and laminations are employed in the construction of
transformers. The joints are staggered to avoid continuous gap
causing increase in magnetizing current. If the joints are not
staggered, the core will have less mechanical strength and during the
operation there would be undue humming noise. After arranging the
laminations they are bolted together.
The two types of transformer cores are:
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i.

Core type

ii.

Shell type

1.2.1.1 CORE TYPE TRANSFORMER

The magnetic core is built of laminations to form a rectangular frame and
the windings are arranged concentrically with each other around the legs or
limbs of the core. Here the windings surround a considerable part of core and
has only one magnetic path. It has two limbs for the two windings and is made
up of two L-type stampings. This arrangement results in a large separation
between the primary and the secondary windings and hence a large reactance
exists. The coils used usually are of cylindrical type and are usually wound.
The low voltage winding is wound on the core while the high voltage winding
is wound over the low voltage winding away from core in order to reduce the
amount of insulating materials required. For transformers of higher rating
stepped core with circular cylindrical coils are used. For transformers of
smaller rating, coils with rectangular cross section are used. Insulating
cylinders are used to separate windings from the core and from each other.
ADVANTAGES
i.

Core type transformers are much simpler in design and permit easier
assembly and insulation of windings.

ii.

Core type transformers are easier to dismantle for repair work.

1.2.1.2 SHELL TYPE TRANSFORMER

Here the core surrounds the considerable part of windings. The two
windings are carried by central limb. The core is made up of E and I
stampings and three limbs. It has two parallel paths for magnetic flux. The
central limb carries total mutual flux while the side limbs forming a part of a
parallel magnetic circuit carry half the total flux. Consequently, the crosssectional area of the central limb is twice that of each of the side limbs. Both
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high voltage and low voltage windings are divided into number of coils. The
coils used are of multilayer disc type and are former wound in the form of
pancakes. Each layer is insulated from each other by paper.
ADVANTAGE
i. It is possible to reduce the leakage reactance of shell type transformers
to any desired value.
ii. In shell type transformer, the core is exposed and therefore cooling is
better in core than in windings.
1.2.2 WINDINGS
There are two windings in a transformer. They are primary and secondary
windings. Generally the windings are made of copper. The windings used in
the transformers are of different types and employ different arrangements for
coils.
Shell type transformers use sandwich type of winding with coils shaped as
pancakes. In this type of winding both low voltage and high voltage windings
are split up into a number of coils. Each high voltage coil lies between two
voltage coils. The two low voltage coils at the ends have half the turns of a
normal low voltage coil and therefore these coils are called half coils. The
subdivision of low and high voltage windings into a number of coils gives a
better coupling between the two windings and therefore results in lower
leakage flux thereby reducing the leakage reactance. The leakage flux and
leakage reactance of the windings depend upon the number of sections in
which the windings are divided; the larger the number of coils, the lower is the
leakage reactance. Therefore, the advantage of sandwich coil is that with their
use the leakage reactance of the transformer can be controlled to any desired
value with a suitable division of windings.
The copper strips are made of electrolytic grade copper wire bars with
high conductivity and are annelid. Sharp edges are normally avoided and
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normal sharp is given at the corners. Since the transformer windings require to
with stand different high and power frequency voltage hence it is required that
the surface of these conductors are smooth. High air permeability paper is
used for covering. All the layers except the outermost are built wound. The
outermost layer is overlap wound.
1.2.3 LEAKAGE FLUX AND LEAKAGE REACTANCE
In an ideal two-winding transformer excited by the primary winding, all of
the magnetizing flux is within the core and both the primary and secondary
windings are linked by the same flux. Consider the following ideal
transformer under no load

The magnetic flux is denoted by the dotted lines inside the core. For this
particular transformer, it takes four lines of flux in the core in the upward
direction to induce a voltage equal to the voltage applied across the primary
winding. The spaces between the two windings and between the windings and
the core are shown greatly exaggerated. The magnetizing current is assumed
to be negligible compared to the load currents. The situation in a real
transformer is somewhat different than described above. The main difference
is that all of the magnetic flux is not contained in the core. This is because the
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load currents in the primary and secondary windings are considerably greater
than magnetizing current, so we cannot ignore the magnetic fields induced by
these currents in the spaces surrounding the winding conductors.
1.2.4 INSULATION
Paper is still used as the basic conductor insulation. Vegetable fibers are
fitted to form a sheet of paper. The fiber mainly consists of cellulose,
molecular formulae being (C6H10O5)n. The paper for insulation is prepared by a
complex chemical process. Enamel insulation is used as the inter-turn
insulation for low voltage transformers. For power transformer enameled
copper with paper insulation is also used.
1.2.5 TERMINALS AND LEAD
The connections to the windings are of insulated copper rods or bars. The
shape and size of lead is important in high voltage transformers owing to
dielectric stress and corona which are caused at bends and corners. Sharp
edges and corners should be avoided.
1.2.6 COOLING ARRANGEMENT
The transformer is a static device which converts energy at one voltage
level to another voltage level. During this process of energy transformer,
losses occur in the windings and core of the transformer. These losses appear
as heat. The heat developed in the transformers is dissipated to the
surroundings. The coolants used in transformers are:
1. air
2. oil
The transformers using air as the coolant are called dry type transformers
while transformers which use oil as the coolant are called oil immersed
transformers.
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There are a number of methods of cooling of transformers. The choice of

methods depend upon the size, type of application and the type of conditions
obtaining at the site where the transformer is installed. The cooling methods
used for dry type transformers are:
1. Air Natural
2. Air Blast
In our project we are using air natural as the cooling arrangement.
AIR NATURAL
This method uses the ambient air as the cooling medium. The natural
circulation of surrounding air is utilized to carry away the heat generated by
natural convection.
1.3 WORKING PRINCIPLE OF TRANSFORMER
When primary winding is connected to an alternating current source, an
exciting current flows through the winding. As the current is alternating, it
will produce an alternating flux in the core which will be linked by both the
primary and secondary windings. The induced emf in the primary winding is
almost equal to the applied voltage and will oppose the applied voltage. The
emf induced in the secondary winding can be utilized to deliver power to any
load connected across the secondary. Thus power is transferred from the
primary to the secondary circuit by electromagnetic induction. The flux in the
core will alternate at the same frequency of the supply voltage. The frequency
of induced emf in the secondary is the same as that of the supply voltage. The
magnitude of the emf induced in the secondary winding will depend upon its
number of turns.

Where
V1 is the applied primary voltage.
V2 is the secondary voltage.
E1 is the emf produced in the primary side.
E2 is the emf induced in the secondary side.
is the flux in the core.
N1 is the number of turns in the primary.
N2 is the number of turns in the secondary.

1.4 CLASSIFICATION OF TRANSFORMER

Generally, transformers are classified on the basis of
DUTY THEY PERFORM
1. Power transformer-for transmission and distribution purposes
2. Current transformer-instrument transformers
3. Potential transformer-instrument transformer
CONSTRUCTION
1. Core type transformer
2. Shell type transformer
3. Berry type transformer
VOLTAGE OUTPUT
1. Step-up transformers----transformer which raise the voltage.
2. Step-down transformers--transformer which lower the voltage.
3. Autotransformer(Variable from 0 to rated value)
APPLICATION
1. Welding transformer
2. Furnace transformer
1.4.5 COOLING
1. Duct type transformer(Air natural or air blast)
2. Oil immersed
a) Self cooled
b) Forced air cooled
c) Water cooled
d) Forced oil cooled
1.4.6 INPUT SUPPLY
1. Single phase transformer
2. Three phase transformer
a. Star-Star
b. Star-Delta
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c. Delta-Delta
d. Delta-Star
e. Open Delta
f. Scott Connection
1.5 TRANSFORMER IMPEDANCE AND LOSSES
Voltages and currents are strictly transformed according to the turns
ratio and the power output from the transformer is equal to the power input
to the transformer. The conditions expressed by the ideal transformer laws
are approached, but never realized in physical transformers. Transformed
voltages and currents are always less than the values predicted by the turns
ratio because of losses.
1.5.1 CONDUCTOR LOSSES
When an alternating magnetic field is applied to any conductor, eddy
currents are induced around the paths surrounding the lines of magnetic flux
that penetrate the conductor. These currents generate local I 2R losses even if
the conductor itself is not carrying any net electrical current. Large amounts
of leakage flux can occur when a transformer is heavily loaded. The
magnetic fields associated with leakage flux not only penetrate the winding
conductors themselves, but can involve other metallic parts as well. The eddy
currents that are induced by these fields are proportional to the leakage flux,
which in turn is proportional to the load currents. Therefore, the square of
eddy currents and the eddy-current losses are both proportional to the square
of the load current. These eddy losses are externally manifested by a
component that increases the effective resistance of the conductors, even if
the eddy losses occur in metallic parts that are electrically isolated from the
conductors. Let this eddy-loss component of the conductor resistance be
denoted Re.
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When an AC current flows in a conductor, the magnetic fields within the

conductor form a series of concentric circles. The flux density B at any point
in the conductor is proportional to the total current enclosed by the magnetic
path divided by the length of the circular path. Moving away from the centre
of the conductor, the total current enclosed by the path tends to increase
faster than the length of the path. Therefore, the flux density increases near
the outer edges of the conductor. The direction of the magnetic field is
perpendicular to the direction of the current flow, and this forces current
toward the edge of the conductor and reduces the flux density near the centre
at the same time. The concentration of current toward the edge of a
conductor is called the skin effect, reducing the area of the conductor that
actually carries current and increasing the effective resistance of the
conductor. The skin effect is more pronounced for large-diameter conductors.
Let the skin-effect component of the conductor resistance be denoted Rs.
The total AC resistance of the conductor, including the eddy-loss
component and the skin-effect component is expressed by the following
equation:
RAC = RDC +Re + Rs
where
RAC is the AC resistance of the conductor
RDC is its DC resistance.
The conductor losses are equivalent to placing a lumped resistance in
series with the terminals of an ideal transformer. Conductor losses are
commonly referred to as load losses, because they result only from load
currents. Load losses are sometimes referred to as copper losses; however,
this is somewhat of a misnomer. Eddy-current losses in any metallic part that
is exposed to leakage flux will still show up as load losses. Load losses limit
the KVA capacity of a transformer because the heat generated by these losses
increase temperatures. Therefore, it is highly desirable to reduce the load
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losses as much as possible by reducing the AC resistance of the conductor.

Reducing RDC as well as Re and Rs can do this. Reducing RDC can be done by
shortening the conductor length and/or by increasing the conductor crosssectional area. Shortening the conductor length can only be achieved to a
point, and increasing the conductor cross-sectional area has the unfortunate
effect of increasing both the eddy-current losses and the skin effect losses.
These losses can be reduced by special conductor designs. Subdividing the
conductors into strands that are insulated from each other to break up the
eddy current paths can reduce eddy-current and skin effect losses. Generally,
the strands have a rectangular shape with the long dimension oriented in the
same direction as the leakage flux. By subdividing one large-area conductor
into a number of small-area conductors, the skin effect is substantially
reduced as well.
1.5.2 NO-LOAD LOSSES
Alternating magnetic flux produces both hysteresis losses and eddycurrent losses in the steel. Hysteresis losses depend on several factors
including the frequency, the peak flux density, the type of core steel used,
and the orientation of the flux with respect to the grain of the steel. All of
the above factors, except the frequency, are under the control of the
transformer designer. Core losses are sometimes referred to as iron losses
and are commonly referred to as no load losses, because core losses do not
require load currents. Decreasing the induced voltage per turn can reduce the
peak flux density. This obviously involves increasing the numbers of turns in
both the primary and secondary windings in order to maintain the same
transformer turns ratio. The disadvantage of adding more turns is that this
increases the length of conductor and increases the conductor resistance.
More cross- sectional area is required in order to keep the resistance
constant. Doubling the number of turns requires about four times the volume
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of copper. Another way of reducing core losses is to use various types of

low-loss core steels that are now available, including amorphous core
materials, which have extremely low losses and superior magnetic properties.
Unfortunately, amorphous core materials have ceramic-like properties, so
fabricating transformer cores with these materials is much more difcult than
with laminated steel cores. With grain-oriented steel, the direction of the core
ux must be kept more or less parallel to the grain of the steel by mitering
the corners of the laminations where the ux changes direction by 90. Since
the ux will cross the grain at about a 45 angle at the mitered edges, the
hysteresis losses will increase somewhat in these places .These additional
localized core losses must be factored into the calculation of the total core
losses. Building up the core with thin laminated strips controls eddy losses in
the core, each strip having an oxide lm applied to the surface. The oxide
lm is extremely thin and it is more like high resistance lm than true
electrical insulation; but since the potential differences between adjacent
laminations is quite small, the resistance of the oxide lm is very effective in
breaking up the eddy current paths. During the manufacture of the core, the
core cutting machine must not be allowed to get dull; otherwise, burrs will
form along the edges of the laminations. Burrs are imperfections that form
electrical bridges between the laminations and create paths for eddy currents
and increased losses. Some- times the eddy currents near a burr can be large
enough to cause localized overheating that can actually cause core damage.
Core losses are approximately proportional to the square of the excitation
voltage E applied to the transformer. Therefore, placing an equivalent linear
conductance Gm across the transformer terminals can approximate
transformer core losses. The core losses are expressed by
Wm = E2Gm
1.6 MAGNETIZING REACTANCE
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For an ideal transformer, the magnetizing current is assumed to be

negligible. For a real transformer, some magnetizing current must ow when
voltage is applied to the winding in order to establish a ux in the core. The
voltage induced in the winding by the ux restrains the magnetizing current.
The magnetizing current is not really sinusoidal, but contains many odd
harmonics in addition to the fundamental frequency. If we neglect the
harmonics and concentrate on the fundamental frequency, the magnetizing
current in the winding lags the applied voltage by 90. In a two-winding
transformer, this is equivalent to placing a reactance X m, called the
magnetizing reactance, in parallel with the transformer terminals. The peak
value of the magnetizing current is determined from the B-H curve of the
core, which seen is very nonlinear. Therefore, the magnetizing reactance is
not a constant but is voltage dependent; however, if the peak ux density is
kept well below the point of saturation, X m can be approximated by a
constant reactance in most engineering calculations. It is generally desirable
to maximize Xm in order to minimize the magnetizing current. Inductance is
inversely proportional to the reluctance of the core along the ux path and
the reluctance of an air gap is several thousand times the reluctance of the
same distance through the steel. Therefore, even tiny air gaps in the ux path
can drastically increase the cores reluctance and decrease X m. A proper core
design must therefore eliminate all air gaps in the ux path. Alternate layers
of core steel are stacked so that ux is diverted around the gaps where
laminations butt together. Since any ux that is diverted must ow between
the laminations through their surfaces, it is vital that these surfaces lie
perfectly at against each other. All ripples or waves must be eliminated by
compressing the core laminations together tightly. This also points out why
the oxide layers on the lamination surfaces must be extremely thin: since
these layers have essentially the same permeability as air and since the ux
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that is diverted from the air gaps must then travel through these oxide layers,
the cores reluctance would greatly increase if these layers were not kept
extremely thin.
1.7 TEMPERATURE RISE AND THE THERMAL CAPABILITY
Transformer KVA ratings have been alluded to on a number of occasions
up to this point without explaining how the KVA rating is determined. The
KVA rating of a transformer is simply the steady-state KVA load applied to
the output of the transformer at the voltage rating of the output winding that
produces an average winding temperature rise (above the ambient
temperature) equal to 65C. For older transformers, the rated average
winding temperature rise was 55C. Advances in insulating materials
allowed a 10C increase in average temperature. Therefore, the winding
temperature is a function of load losses and no-load losses.
The thermal capability of a transformer is dened in a slightly different
way from the rated KVA. Thermal capability is the KVA load applied to the
output of a transformer that causes the hottest area in the windings, called the
winding hot spot, to reach some limiting temperature. The hot-spot
temperature determines the rate of loss of life of the transformer as a whole,
which is a cumulative effect. Therefore, the hot-spot temperature limit is
usually based on a loss-of-life criterion.

REFERENCES
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1. Gibbs J.B. Transformer Principles and Practice, McGraw Hill Book

Company, Newyork Second Edition, 1950.
2. Power Transformer Handbook, edited by Hochart, Bernard: English
Edition1987,Butterworths, Oxford.
3. Rao .S., Power Transformers and Special Transformers, Khanna Tech
Publications, Delhi,1991,Second Edition.
4. Jeszensky. S.,History of Transformers, IEEE Power Engineering
Review, December 1996, p 9-12.
5. Steed.K.C., Amorphour Core Transformers, Power Engineering
Journal, April 1994, vol8,No.2,p92.
6. Eddy Current Losses in Transformer Windings and Circuit Wiring,
Unitrode Seminar Manual SEM600,1998.
7. The

Complete

History

of

the

Transformers,Internet

article,www.xs4all.nl/-wjlbeek/history1.html.
8. Manual on Power Transformer (0-100MVA), Siemens.

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