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 arrangement1

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:

i. Core type

2

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 cross-

sectional area of the central limb is twice that of each of the side limbs. Both

high voltage and low voltage windings are divided into number of coils. The

3

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

normal sharp is given at the corners. Since the transformer windings require to

4

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

load currents in the primary and secondary windings are considerably greater

5

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.

6

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.

7

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.

Φ is the flux in the core.

N1 is the number of turns in the primary.

N2 is the number of turns in the secondary.

8

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-Delta9

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.

10

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 11

losses increase temperatures. Therefore, it is highly desirable to reduce the

load 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 cross-sectional 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 eddy-

current 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 12

constant. Doubling the number of turns requires about four times the volume

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 difficult than

with laminated steel cores. With grain-oriented steel, the direction of the core

flux must be kept more or less parallel to the grain of the steel by mitering

the corners of the laminations where the flux changes direction by 90°. Since

the flux 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 film applied to the surface. The oxide

film is extremely thin and it is more like high resistance film than true

electrical insulation; but since the potential differences between adjacent

laminations is quite small, the resistance of the oxide film 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

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1.6 MAGNETIZING REACTANCE

For an ideal transformer, the magnetizing current is assumed to be

negligible. For a real transformer, some magnetizing current must flow when

voltage is applied to the winding in order to establish a flux in the core. The

voltage induced in the winding by the flux 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 Xm, 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 flux density is

kept well below the point of saturation, Xm 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 flux 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 flux path

can drastically increase the core’s reluctance and decrease Xm. A proper core

design must therefore eliminate all air gaps in the flux path. Alternate layers

of core steel are stacked so that flux is diverted around the gaps where

laminations butt together. Since any flux that is diverted must flow between

the laminations through their surfaces, it is vital that these surfaces lie

perfectly flat 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 14

these layers have essentially the same permeability as air and since the flux

that is diverted from the air gaps must then travel through these oxide layers,

the core’s 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 65°C. For older transformers, the rated average

winding temperature rise was 55°C. Advances in insulating materials

allowed a 10°C 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 defined 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.

15

CHAPTER -2

DESIGN OF TRANSFORMER

2.1 DESIGN OF CORE

The net cross sectional area is obtained from the dimensions of various

packets and an allowance is made for the space lost between the laminations.

This allowance is known as the stacking factor and for sheet steel of 0.28

mm thickness with a coating of insulation it becomes 0.96. Area is also

ducted for oil ducts. The ratio of net cross sectional area to the gross area of

the core circle is known as utilisation factor (UF). UF increases if the number

of core steps increase. Usually optimum number of steps is 6 for smaller

transformer and 15 for large transformers. Improvement in UF increases the

core area and hence increases volts/turns for any particular core diameter and

specified flux density. This results in the reduction in winding forms and

reduction of copper. Thus core area optimization results in better economy.

The core section for core type of transformers may be rectangular, square or

stepped. Shell type transformers use cores with rectangular cross section.

2.1.1 RECTANGULAR CORE

For core type distribution transformers and small power transformers for

moderate and low voltage, the rectangular shaped core section may be used.

The ratio depth to width of core varies from 1.4 to 2. Rectangular shaped

coils are used for rectangular core. For a shell type transformer width of

central limb is 2 to 3 times the depth of core.

2.2SELECTION OF CORE AREA AND TYPE OF CORE

Selection of type of core depends upon the rating, operation duty and

transport limitations. For large three phase transformers, five limbed core is 16

recommended to overcome problem of higher height of the core. For single

phase transformers, one centre-wound limb with two return limbs is a

common configuration. In case of very large transformer cores have two

wound limbs with two return limbs.

2.3 EFFECT OF VARIATION OF LEG LENGTH

The maximum leg length of the transformer core being governed

primarily by the maximum transport height, for larger rating transformer

maximum value of the leg length gives overall economy since the core

weight and no load loss both decrease. For lower rating transformer, shorter

leg length offers better design in terms of economy.

2.4 CHOICE OF FLUX DENSITY

Design of magnetic circuit is one of the most essential components of

transformer design. Transformer core is made up of lamination steel sheets

and provides a comparatively low reluctance path to the magnetic flux with

consequent benefit of smaller magnetizing current, higher flux linkage and

high ratio of mutual to leakage flux resulting in reduction of stray loss. The

core design is governed by rating of the transformer, its operational condition

and transport limitations.

The value of flux density in the core determines the core area. Higher

values of flux density give a smaller core area and therefore there is a saving

cost of iron. Also with the reduction in core area the length of mean turn of

windings is also reduced. Thus there is a saving in conductor costs also. But

higher flux density, the iron losses become high resulting in considerable

temperature gradient across the core. High flux density necessitates a large

magnetizing current which contains objectionable harmonics. The value of

flux density to be chosen also depends upon the service conditions of the

17

transformer. The usual values of maximum flux density Bm for transformers

using hot rolled silicon steel are:

Distribution transformer : 1.1 to 1.35 wb/m2

Power transformer :1.25 to 1.45 wb/m2

Lower values should be used for small rating transformers.

For transformers using cold rolled grain oriented steel the following values

may be used:

For transformer upto 132KV : 1.55 wb/m2

For 275 KV transformer : 1.6 wb/m2

For 400 KV and generator transformers : 1.7 to 1.75 wb/m2

2.5 DESIGN OF WINDINGS

The area of conductors in primary and secondary winding is determined

after choosing suitable current density to be used in the winding. The

permissible current density in the winding is limited by local heating and

efficiency. The temperature rise in the windings may become excessive if

higher values of current density are chosen and this may cause injury to the

insulation. the choice of current density is important as the I2R losses and

hence the load at which the maximum efficiency occurs depends on it.

Therefore current density in a winding should be chosen to guarantee the

level of losses and cooling condition required. The level of iron and I2R

losses required is different in distribution and power transformers. Thus the

value of current density is different for different transformers.

For distribution ,small and medium ,self oil cooled type

upto 50KVA

δ=1.1 to 2.3A/mm2

For large power transformers ,self oil cooled type or oil. Blast

δ =2.2 to 3.2 A/mm2

18

For large power transformers with forced circulation of oil or with water

cooling coils

δ =5.4 to 6.2 A/mm2

2.6 DESIGN OF INSULTION

During the processes of power transfer from one circuit to another-

electrical, mechanical and thermal phenomena take place in a transformer.

The winding voltages produces an electrostatic field in the dielectric used

and therefore stress the insulation; the currents in the windings set up

magnetic fields which give rise to electromagnetic forces on the windings

and to mechanical stressing of insulation; finally the losses in the transformer

produce temperature rise which produces thermal stressing of insulation

Hence, the fundamental considerations in the design of insulation of

transformers may be described as those of arranging core, windings and

insulation to obtain satisfactory electrical, mechanical and thermal

characteristics during the steady state as well as transient conditions. The

three basic considerations in the design of insulations are:

2.6.1 ELECTRICAL CONSIDERATIONS

The basic insulation structure is primarily determined from consideration

of the magnitude and nature of voltages which appear between different parts

of the transformer i.e. voltages between individual turns, between coil or

layers, between winding and from windings to core and tank.

Tests like sustained frequency high voltage tests and impulse test are

applied to check the strength of insulation between the various parts with a

view to ensure that the transformer will have a reasonable life(average 20

years) and will be able to withstand damage under abnormal conditions

imposed by lightning, switching surges and other transient phenomena. The

electrical design should also take care of the eddy current losses in

conductors and leakage reactance of windings.19

2.6.2 EDDY CURRENT LOSS

The windings should be so designed that the stray load loss is small. The

stray load loss includes eddy current loss in conductors and connectors and

also in tank walls and clamping structure. The conductors should be split in

to small strips to reduce eddy current losses in conductor. The radial width of

strips should be small and they should be transposed.

2.6.3 LEAKAGE REACTANCE

A given arrangement of core and windings determines the leakage

reactance of the windings. The leakage reactance is adjusted by changing the

winding configuration and brought within desired limits.

2.7 MECHANICAL CONSIDERATIONS

The basic mechanical considerations in the design of insulation are of

two types:

1. The insulation must be capable of withstanding the mechanical

stresses imposed on it during the manufacturing processes.

2. The insulation must be able to withstand the mechanical stresses

which are developed in the winding due to electromagnetic

phenomenon. The electromagnetic forces and mechanical stresses

produced under fault conditions, particularly dead short circuit, the

electromagnetic forces may be increased several hundred times. The

insulation must be designed to withstand stresses produced under

abnormal conditions for a specified period of time.The mechanical

design of insulation should be such be such that hoop, bursting and

compressive stresses are minimized. Also there should be axial

balance between the windings and they should be adequately braced.

20

2.8 THERMAL CONSIDERATION

The thermal aspects of design of insulation are determined from the

considerations of insulation material used, selection of safe maximum

operating temperature and types of cooling method employed.The

transformer structure should be such that the losses developed in the core and

windings produces temperature rises in the various parts which nowhere

exceed the permissible limits both under normal and over load/fault

conditions and which, in the interest of economy, approach those limits as

nearly as possible.

21

CHAPTER -3

DESIGN CALCULATION

POWER RATING : 100VA

PRIMARY VOLTAGE : 230V

SECONDARY VOLTAGE : 115V

3.1 CORE DESIGN

According to standard specifications,

Turns per volt for 100VA power transformer Te = 4.6

Flux flowing through the core φm = 1/(4.44 × f ×Te)

φm = 1/(4.44 × 50 *×4.6)

φm = 0.979 × 10-3 wb

Assume,

Bm = 1.0 wb/m2 ,

Net iron core area Ai = φm/Bm

Ai = (0.979 × 10-3)/1

Ai = 0.979 × 10-3 mm

Gross core area Agi = Ai/stacking factor

Agi= (0.979 × 103)/0.9

Agi= 1.088 ×103 mm2

In our project rectangular core is used. So

Depth of the core=Width of the central limb

Width of the central limb A = √Agi

A = √(1.088 × 103)

A = 32.985 mm=1.2997"

22

3.2WINDING DESIGN

3.2.1 PRIMARY WINDING

Assume η = 90%

Primary winding current Ip = VA/(η ×Vp)

Ip = 100/(0.9 × 230)

Ip = 0.48 A

Assume current density to be 2.3 A/mm2

Area of the primary winding conductor ap = Ip /δ

ap = 0.48/2.3=0.209 mm2

ap = π ×r2=0.209 mm2

D = 0.516 mm

Nearest standard bare conductor diameter Dcond = 0.53 mm

Diameter of insulated conductor Dins = 0.602 mm

Space factor for primary winding Sf = 0.8 ×( Dcond × Dins)2

Sf = 0.8 × (1.06/1.155)2

Sf = 0.62

Area of primary conductor used ap = (π/4) × Dcond2

ap = (π/4) × 0.532

Number of primary winding turns Tp = VpTe

Tp = 230 × 4.6

= 1058 turns

Window space required by primary winding = (Tpap)/Sf

= (1058 × 0.221)/0.6

= 377.126 mm2

3.2.2 SECONDARY WINDING

Secondary winding current Is = VA/Vs

Is = 100/115=0.87A

Area of secondary winding conductor as = Is/δ23

as = 0.87/2.3=0.378 mm2

as = πr2=0.378 mm2

D = 0.695 mm

Nearest standard bare conductor diameter Dcond = 0.710 mm

Diameter of insulated conductor Dins = 0.791 mm

Space factor for primary winding Sf = 0.8 × (Dcond/Dins)2

Sf = 0.8×(0.710/0.791)2

Sf = 0.645

Area of secondary winding conductor as = (π/4) ×Dcond2

as = 0.396 mm2

Number of secondary winding turns Ts = 1.05VsTe

Ts = 1.05 × 115 × 4.6

Ts = 556 turns

Window space required by the secondary winding = (Tsas)/Sf

= ( 556×0.396)/0.645

= 341.358 mm2

3.3 STAMPING SIZE

Total window space required

Aw=1.2[space required for primary and secondary winding]

Aw=

1.2[377.126+341.358]

Aw=862.181 mm2=1.336 sq.inch

Width of central limb A=1.299 "

So we choose the standard model No:3

A=1.25"=1.25×25.4=31.75 mm

B=3.75"=3.75×25.4=95.25 mm

C=3.1/8"=3.125×25.4=79.375mm

D=5/8"=0.625×25.4=15.875 mm

E=5/8"=0.625×25.4=15.875 mm24

Remarks:4 holes 7/32” dia

The lamination model:

Thickness of the core = Agi/A=(1.088×103)/32.985

= 32.985 mm=1.837"

Window width Ww= (B-A-2D)/2

= 15.875 mm

Height of the window Hw = C-2E

= 47.625 mm

Area of the window provided Aw = Ww × Hw

Aw = 1.3 sq.inch

This is more than the required window space.

25

CHAPTER – 4

TESTING OF TRANSFORMERS

1. Open circuit test (or) No load test

2. Short circuit test (or) Impedance test

By using these two tests we can find,

1. Circuit constants(R0,X0,R01,X01,R02 and X02)

2. Core loss and full load copper loss

3. Predetermine the efficiency and voltage regulation at any load

These tests are convenient to perform and very economical because they

provide the required information without actually loading the transformer.

Other two tests are

i. Load test

ii. Sumpner’s test

4.1 OPEN CIRCUIT TEST

The open circuit tes.t is useful to find

i. No-load loss (or) core loss

ii. No load current i0 which is helpful in finding out R0 and X0

26

The connections are made as shown in the circuit diagram. One

winding of the transformer is left open and the other winding is connected to

the supply of normal voltage and frequency. The applied voltage v1 is

measured by a voltmeter, the no load current I0 by an ammeter and no load

input power W0 by a wattmeter.

As the normal rated voltage is applied to the primary, normal iron loss

will occur in the transformer core .Hence wattmeter will read the iron loss

and small copper loss in the primary.as the no load current i0 is small copper

loss is negligible in primary and nil in secondary winding. Hence wattmeter

reading gives the iron losses in the transformer and it is same at all loads

Iron losses Pi = Wattmeter reading=W0

No-load current = Ammeter reading =I0

Applied voltage = Voltmeter reading =V1

Input power W0 = V1I0 cosθ0

No-load power factor cosφ0 = W0/V1I0

Φ0 = cos-1(W0/V1I0)

No-load watt full component IW = I0 cosφ0=W0/V1

No load magnetizing component Iµ = I0 sinφ0=√(I20-I2

w)

No load resistance R0 = V1/Iw

27

= V12/W0

No load reactance X0 = V1/Iµ=V1/√(I20-I2

w)

Thus the open circuit test gives no load losses P1,Iw,Iµ,R0and X0

4.2 SHORT CIRCUIT TEST

Short circuit test is useful to find

i. Full-load copper loss

ii. Equivalent resistance and reactance referred to metering side

In this test, the secondary winding is short circuited by a thick conductor

and variable low voltage is applied to the primary winding. the input voltage

is gradually raised with the help of a variac till Isc full load current flows in

the primary winding. There is no output from the transformer under short

circuit conditions. Therefore input power is all loss and this load is almost

entirely copper loss. Since applied voltage is very small and therefore iron

losses are so small that these can be neglected and so the reading of the

wattmeter gives total copper loss at full load.

Full-load Cu loss Pcu = Wattmeter reading=Wsc

Applied voltage = Voltmeter reading=Vsc

Full load primary current=Ammeter reading=I1

Pcu = I12R1+ I1

2R2= I12R01

R01 = Pcu/I12

Where

R01 is the total resistance of transformer referred to primary

28

Total impedance referred to primary,Z01=Vsc/I1

Total leakage reactance referred to primary X01=√Z201-R2

01

Short circuit power factor cosφs =Pcu/VscI1

Thus short circuit test gives full load Cu loss,R01,X01 and cosφs

4.2.1 EFFECTS OF SHORT CIRCUITS ON TRANSFORMERS

Transformers are susceptible to damage by secondary short-circuit

currents having magnitudes that can be many times rated load current. The

damage results from the following effects:

i. The I2R losses in the winding conductors are increased by the square of the

current. This increases the temperature rise of the windings. Because

protective devices limit the duration of short circuits (as opposed to

overloads), the temperature rise of the winding can be calculated by dividing

the total energy released by the I2R losses by the thermal capacity of the

conductor.

ii. The short-circuit currents exclude flux in the core and increase stray flux

around the core. This stray flux induces currents in metallic parts other than

the winding conductors, which can be damaged thermally.

29

iii. A short circuit applied to the secondary circuit of an autotransformer can

substantially increase the voltage across the series winding and across the

common winding through induction. This not only presents the possibility of

damaging the winding insulation by overvoltage, but will also drive the core

into saturation and significantly increase core losses with potential dam-

aging effects from temperature.

iv. Bushings and tap changers have current ratings that are usually only

marginally greater than the rated load of the transformer. Since fault currents

are many times rated currents and these components have short thermal time

constants, they can be seriously overloaded and thermally damaged.

v. Stray flux in the vicinity of current-carrying conductors produces mechanical

forces on the conductors. When a short circuit is applied to a transformer,

there is a significant increase in stray flux, resulting in greater mechanical

forces on the windings, leads, bushings, and all other current-carrying

components. These components, especially the windings, must be braced to

withstand these forces.

A good transformer design must take all of the above effects into

account to minimize the risk of damage and assure a long service life.

4.2.2 EFFICIENCY FROM OC AND SC TEST

From the open circuit test, we can get core loss of the transformer and

from short circuit test, we can get full load copper loss. Now we can find the

full load efficiency of the transformer at any power factor without actually

loading the transformer.

Efficiency =Full load KVA p.f/(Full load KVA x p.f)+Pi+Pcu

For any load =(n x Full load KVA)x p.f/(n x Full load KVA) x p.f+pi+n2Pcu

30

4.3 LOAD TEST

Load test is helpful to determine the following

Efficiency of the transformer Regulation of the transformer

The connections are given as per the circuit diagram

The variac should be kept in minimum position while switching on and off the supply side DPSTS

At the time of switching on the supply there should not be any load connected

The transformer is excited to its rated voltage on no load. The meter

readings are observed at no load condition. The load is gradually increased

and meter readings are noted for each loading. The transformer is loaded till

it draws rated current from the supply .Note that the applied voltage to the

primary should be kept to its rated voltage on loading

Ws= Output power

Wp=Input power

Efficiency =(Ws/Wp)x100

31

%Regulation =((V2-V20)/V2)x100

V2-no load secondary rated terminal voltage

V20=secondary voltage on load

From these data the following characteristic curves are drawn.

4.4 VOLTAGE REGULATION OF A TRANSFORMER

All the electrical appliances are designed to operate satisfactorily at

constant voltage. Therefore, the transformers from which electric supply is

obtained must maintain their output voltage without variations the voltage in

a transformer on load varies and it is due to its leakage reactance. ”The

regulation of a transformer is defined as reduction in magnitude of the

terminal voltage due to load with respect to the no-load terminal voltage”.

% regulation=|V2 on no-load|-|V2 when loaded| |V2 on no-load|

4.5 RATING OF A TRANSFORMER

The name plate of a transformer specifies the rated voltages of primary

and secondary windings, the rated currents of primary and secondary

windings and the rated volt-amp. Meanings of these specifications are as

under:

1. While manufacturing a transformer, insulations are provided to

primary and secondary windings. These insulations are such that these

32

windings safely withstand certain voltages V1 and V2. If voltages

across these windings are more than V1 and V2, there may be a chance

of damage. Thus V1 and V2 are maximum safe voltages. These are the

rated voltages of the transformers.

2. Conductors used to manufacture primary and secondary windings have

certain cross-sectional areas. These areas usually decide their current

carrying capacities.

4.6 READING AND APPLYING NAMEPLATE INFORMATION

Every distribution and power transformer has a metal nameplate attached

to the tank that gives vital information on how the transformer is to be

connected and operated. The information is printed or stamped on the

nameplate so it is a permanent part of the transformer. A transformer’s

nameplate has been compared to a birth certificate because it contains so

many vital statistics that will follow it throughout its service life.

PROCEDURE

1. OPEN CIRCUIT TEST

1. Connections are made as per the circuit diagram.

2. Precautions:

i. At the time of starting transformer should be at no load

condition.

ii. Autotransformer should be kept in minimum output

position.

3. The DPST switch on the LV side was closed.

4. The Autotransformer was adjusted to energize the transformer with

rated Primary voltage on the LV side.

33

5. The voltmeter, Wattmeter and Ammeter readings were noted at no load

condition.

6. The Autotransformer was brought to its initial position.

7. The supply was switched off.

2.SHORT CIRCUIT TEST

1. Connections are made as per the circuit diagram.

2. Precautions:

i. Autotransformer should be kept in minimum output

position.

ii. The LV side should be shorted.

The DPST switch on the primary side was closed.

3. The Autotransformer was adjusted to energize the transformer with

rated Primary current on the HV side.

4. The voltmeter, Wattmeter and Ammeter readings were noted at no load

condition.

5. The Autotransformer was brought to its initial position.

6. The supply was switched off.

3.LOAD TEST

1. Connections are made as per the circuit diagram.

2. Precautions:

i. Keep the transformer in minimum output position.

ii. Avoid more than one connection at a terminal of a meter.

3. DPST switch is closed and the supply is switched on at the no load

condition.

4. The autotransformer is adjusted to set the rated primary voltage.

5. The no load condition the voltmeter, ammeter and wattmeter readings

are taken and tabulated.34

6. Now the load is increased in steps and the corresponding change in

voltmeter, ammeter and wattmeter readings are taken and tabulated.

7. The load is reduced in steps, set the Autotransformer in the initial

position and the supply is switched off.

8. The efficiency at various loads are calculated and the average is found

out.

TABULATION

1. OPEN CIRCUIT TEST

Multiplication factor=1

V0 in volts I0 in amps W0 in watts

115 0.05 8

2.SHORT CIRCUIT TEST

Multiplication factor=1

Vsc in volts Isc in amps Wsc in watts

31.2 0.48 16

35

3.PREDETERMINATION OF EFFICIENCY

PF=0.8 VA=100 WSC=16 W Wi=8 W

S.no Fraction

of load

X

O/P

power=capacit

y x fraction of

load x p.f

Losses

PL=W0+

X2Wsc

Input Power

Pi=P0+PL

Efficiency

η=

(P0/Pi)x100

1 0.25 20 9 29 68.97

2 0.5 40 12 52 76.92

3 0.75 60 17 77 77.92

4 1.0 80 24 104 76.92

PF=1 VA=100 WSC=16 W Wi=8 W

S.no Fraction

of load

X

O/P

power=capacit

y x fraction of

load x p.f

Losses

PL=W0+

X2Wsc

Input Power

Pi=P0+PL

Efficiency

η=

(P0/Pi)x100

1 0.25 25 9 34 73.53

2 0.5 50 12 62 80.65

3 0.75 75 17 92 81.52

4 1.0 100 24 124 80.65

36

4.LOAD TEST Multiplication factor=1

S.No

Primary

Voltage

V1

(volts)

Primary

Current

I1

(amps)

Input

Power

W1

(Watts)

Secondary

voltage

V2

(Volts)

Secondary

current

I2

(Amps)

Output

power

W2

(Watts)

%

efficiency

1 115 0.25 29 210 0.12 25.2 86.89

2 115 0.45 51 200 0.23 46 90.20

3 115 0.62 71 192 0.32 61.44 86.54

4 115 0.79 89 181 0.41 74.21 83.38

5 115 0.96 106 174 0.49 85.26 80.26

Average efficiency =85.454 %

Graph

37

CHAPTER-5

APPLICATIONS OF TRANSFORMER

Transformers are used in

As an instrument transformer for measuring current and measuring

voltage.

Electrical power engineering for transmission and distribution.

As a step down and step up transformer to get reduced or increased

output voltage.

Radio and TV circuits, telephone circuits, controls and instrumentation

circuits.

Furnaces and welding transformer.

Transformers are used in impedance matching.

Transformer can be used to prevent DC to pass from one circuit to

another.

Transformer can isolate two circuit electrically.

Transformer can act as a impedance transferring device.

38

CHAPTER- 6

LITERATURE SURVEY

*David.L.Harris ,P.E,Customer Technical Executive ,Waukesha

Electric Systems,”The Design and Performance of circular Disc, Helical

and layer windings for Power Transformer Applications” Minnesota

Power Systems Conference,November 3-5-2009,Earl Brown Heritage

Center,University of Minnesota.

This paper deals with the winding design and manufacturing practices for

power and distribution transformers which has focused in the differences

between the rectangular core and coils used in production of distribution

transformers and disc and helical windings and circular core common in

power transformers.

Rectangular core and coil designs are frequently used in distribution

transformer designs and offer advantages of reduction in direct labor and

material when compared to circular coils with disc and helical windings

usually wound with sheet conductors for the LV winding. The rectangular

core design reduces the core window and result in reduction of core losses

compared to circular core design.

The thermal design, through fault and short circuit withstand

considerations are dealt. Flux fields are depend on the balance of the ampere

turn distribution of the HV and LV windings. Radial forces are attempting to

crush the inner winding and elongate the outer winding whereas axial cause

inner winding to be vertically displaced from the outer winding.

39

“High Voltage Miniature Transformer Design”,Shelton Gunewardena

Mil-Spec Magnetics Inc. WendelE. Archer Robert 0. Sanchez Passive

Devices/Interconnects Department Sandia National Laboratories

This paper presents an example of optimization of a small flyback

transformer designed to charge energy storage capacitors up to 2 kV from a

low voltage source. The larger the stray capacitance, the smaller the output

voltage. The design control parameters were the material characteristics,

winding pattern, magnetic gap, and turns for primary and secondary. The

goal was to minimize the charging time and maximize the output voltage.

A bobbin was designed that allowed the secondary winding to be wound

first in two sections (a two section bobbin).The center flange of the bobbin

was specifically designed with a smaller diameter allowing for layer

insulation across the entire primary and the primary winding over the layer

insulation. This winding configuration significantly reduced the secondary

distributed capacitance and leakage inductance. It is apparent that there is a

degree of uniformity in the electrical characteristics of the entire lot of parts.

This paper has dealt with the problems encountered in designing and

building small transformers to achieve specific requirements, such as in High

Voltage Flyback transformers. They have attempted to describe the most

significant steps in achieving desired characteristics of this particular type of

transformer

40

CHAPTER -7

CONCLUSION

It is necessary to provide reliable and efficient power supply to the

consumers. In this project a power transformer of 100VA was designed

manually. The open circuit test, short circuit test and load test have been

conducted. By using open circuit test the core loss of the transformer has

been found and from short circuit test the copper loss of the transformer has

been found. The efficiency of the transformer has been found by using load

test and the average efficiency of the transformer was found to be 85.454%.

41

CHAPTER -8

REFERENCES

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.

42