designing and testing_2
TRANSCRIPT
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
13
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