11kVby400v, 20kVA three phase core type distribution transformer.

Sylhet Engineering College
Afﬁliated with SUST
Topic
Designing Of 20kVA, 11000/400 V 3 phase
delta-star core type distribution transformer
Submitted to:
Janibul Alam Soeb
Lecturer,
Dept. of EEE
Sylhet Engineering College
Submitted by:
Anamul Hasan (2016338532)
Bijoy Talukdar (2016338533)
Ahmed Sakhawat (2016338545)

[Designing of three phase distribution transformer]
Abstract
This note discusses the design of three-phase distribution transformers by establishing the procedure
for designing a transformer. It deals with rating of 20 kVA, 11000/400 V, 50Hz three phase distribution
transformer . It reviews the basic parts of a transformer such as core, windings, transformer oil and cool-
ing components etc. Then it contains the parameters for calculating to design the desired transformer.
The design explains about the efﬁciency of the transformer by taking losses (Core loss and copper ) under
consideration. It also provides a brief discussion about the test of transformer parts. A section concludes
about the cooling system and protection applied for the transformer.
1

Contents
1 Introduction 5
2 History 5
3 Transformer types 6
3.1 Transformers Based on Voltage Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1 Step-Up Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2 Step-Down Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Transformer Based on the Core Medium Used . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1 Air Core Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.2 Iron Core Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Transformers Based on Winding Arrangement . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1 Autotransformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Transformers Based on Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4.1 Power Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4.2 Distribution Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5 Instrument Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5.1 Current Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5.2 Potential Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6 Audio Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.7 Polyphase Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Different types of winding 11
4.1 Cylindrical Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Helical Windings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3 Multi-layer Helical Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.4 Crossover Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5 Disc and Continuous Disc Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.6 Sandwich Type Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5 Insulating material use in transformer 15
5.1 Insulating Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2 Insulating paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3 Pressboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.4 Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.5 Insulated copper conductor for winding . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.6 Insulating tape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6 Working principle 17
7 Transformer tests 18
7.1 Open Circuit Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2 Short Circuit or Impedance Test on Transformer . . . . . . . . . . . . . . . . . . . . . 19
2

8 Cooling methods 20
8.1 Air Natural . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2 Air Blast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.3 Oil Natural Air Natural (ONAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.4 Oil Natural Air Forced (ONAF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.5 Oil Forced Air Forced (OFAF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.6 Oil Forced Water Forced (OFWF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9 Losses of transformer 22
9.1 Core Losses or Iron Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1.1 Hysteresis loss in transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1.2 Eddy current loss in transformer . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.2 Copper Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10 Efﬁciency of transformer 23
11 Three phase transformer connections 23
11.1 Star-Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.2 Delta-Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.3 Star-Delta or Wye-Delta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.4 Delta-Star or Delta-Wye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.5 Open Delta (V-V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.6 Scott (T-T) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
12 Design difference between core type and shell type three phase transformer 26
12.1 Core type transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.2 Shell Type transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13 Transformer tank 28
14 Transformer bushings 28
15 Breather and Conservator tank 29
15.1 Breather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
15.2 Conservator tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
16 Temperature indicator in Transformer 30
16.1 Liquid temperature indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
16.2 Oil temperature indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
16.3 Winding temperature indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
17 Transformer fault conditions 32
17.1 Earth faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
17.2 Core faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
17.3 Interturn faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
17.4 Phase-to-phase faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
17.5 Tank faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
17.6 External factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3

18 Important notation used in transformer design 33
19 Design details 34
19.1 Core design: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
19.2 Window dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
19.3 Yoke Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
19.4 Overall dimensions of frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
19.5 L.V. winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
19.6 H.V. winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
19.7 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
19.8 Leakage reactance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
19.9 Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
19.10Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
19.10.1 Copper Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
19.10.2 Core Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
19.11Efﬁciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
19.12No load current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
19.13Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
19.14kVA rating calculation and veriﬁcation . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
20 Conclusion 43
4

1 Introduction
A transformer is a motionless electrical device that delegate electrical energy between two or more
circuits. A asymmetrical current in one coil of the transformer produces a varying magnetic flux,
which, in turn, induces a varying electromotive force (emf) or "voltage" across a second coil wound
around the same core. Electric power can be shifted between the two coils, except a metallic con-
nection within the two circuits. Faraday’s law of induction discovered in 1831 stated the induced
voltage upshot in any secondary coil due to changing magnetic flux laceration it. Transformers are
used to increment or decrements the alternating voltages in electric power applications. Since the
discovery of the first constant-potential transformer in 1885, transformers have become requisite
for the transmission, distribution, and utilization of alternating current electrical energy.
A wide range of transformer designs is come together in electronic and electric power applica-
tions. Transformers range in province from RF transformers less than a cubic centimeter in vol-
ume to units interconnecting the power grid weighing hundreds of tons.
Three phase transformers are used to step-up or step-down the high voltages in different stages of
power transmission system. The power generated at different generating stations is in three phase
nature and the voltages are in the range of 13.2KV or 22KV. In order to minimize the power loss to
the distribution end, the power is transmitted at any higher voltages like 132 or 400KV. Hence, for
transmission of the power at higher voltages, three phase step-up transformer is used to rise the
voltage. Also at the end of the transmission or distribution, these high voltages are stepped-down
to levels of 6600, 400, 230 volts, etc. For this, three phase step down transformer is used.
A three phase transformer can be constructed in two ways; a bank of three single phase transform-
ers or single unit of three phase transformer.
The former one is by fittingly connecting three single phase transformers having same ratings and
operating characteristics. In this case if the imperfection occurs in any one of the transformers, the
system still retained at reduced retention by other two transformers with open delta connection.
Hence, regularity of the supply is maintained by this type of connection. These are used in mines
because easier to transit individual single phase transformers. [1]
2 History
The electrical transformer deserves credit as one of the most important inventions of the industrial
age, which along with steam power, running water, gas lighting, includes the harnessing of elec-
tricity. In fact, the latter would not be accomplished without the transformer. At its essence, the
transformer lives up to its name by transforming (or converting) electrical energy from a higher
voltage to a lower one. There are hundreds of different transformer types designed to handle ex-
tremely high voltages and lower ones and everything in between. The complexity of transformers
run deep, with models designed to handle different electrical types (single or multiple phases) and
applications that include radio transmission. The voltage transformer and converter that we make
at ACUPWR are step up and step down types that match electrical appliances and devices with
their line voltage requirements, particularly when used in a country or region where the AC line
voltage is different. Electrical transformers trace their lineage to the English scientist and inventor
Michael Faraday and his discovery of the law of electro-magnetic induction. Also known as Fara-
day’s Law, the theory describes the phenomena of electrical voltage generated when a coil of wire
was wrapped around an iron core. That current would flow through the iron to an opposite side
(the iron was shaped not unlike a donut), and current with different voltage could be created using
wire that had more or fewer turns. Thus, the electricity was induced. American scientist Joseph
5

Henry is also credited with inventing the concept of electromagnetic induction. But, like many
life-altering, revolutionary inventions, credit for the electrical transformer does not belong to one
particular person. Rather, using Faraday’s Law as their guiding mantra, a succession of inven-
tors made inroads toward what became the ﬁrst truly usable, commercial transformer—usable,
at least, in a way that revolutionized people’s lives. In 1836, Rev. Nicholas Callan developed
an induction coil transformer that helped him develop a high-voltage battery (capable of power-
ing a machine that could lift 2 tons) that was mass-produced in London. Other names factor in,
each doing a bit more to apply Faraday’s law and magnetic induction coils. In 1876, a Russian,
Pavel Yablochov, invented a lighting system based on the inductance coil. Lucien Gaulard and
John Gibbs, from France and England respectively, devised a transformer and secondary genera-
tor in England that revolutionized AC (alternating current) power. In 1884, three physicists from
Austria-Hungary–Otto Blathy, Miksa Déri, Karol Zipernowski, pioneered the transformer designs
that are still used today. ZBD, as the trio is known, also created the world’s ﬁrst power station
using AC generators. Thomas Edison, purchased ZBD’s innovations to help create power utilities
and electrical grids in cities. Meanwhile, Edison’s rival, American inventor George Westinghouse,
purchased the rights to Gaulard’s invention. In 1886, William Stanley created a practical AC trans-
former based on Gaulard’s invention. Teaming up with Westinghouse, Stanley, at Westinghouse’s
behest, relocated to Great Barrington, Massachusetts to create an electrical grid using AC. [2]
Figure 1: First transformer by Otto Blathy, Miksa Déri, Karol Zipernowski
Stanley’s innovation of creating power distribution in Great Barrington was a revolutionary
development leading to Westinghouse’s preferred AC power being the standard in the United
States to supply electricity in cities, winning out over Thomas Edison and his preferred choice of
DC (direct current). Of course, everybody wins out in the end, especially when they use ACUPWR
transformers for converting line voltages.
3 Transformer types
There are several transformer types used in the electrical power system for different purposes,
like in power generation, distribution and transmission and utilization of electrical power. The
6

transformers are classified based on voltage levels, Core medium used, winding arrangements,
use and installation place, etc. Here we discuss different types of transformers are the step up
and step down Transformer, Distribution Transformer, Potential Transformer, Power Transformer,
1-phase and 3-phase transformer, Auto transformer, etc. [3]
3.1 Transformers Based on Voltage Levels
These are the most commonly used transformer types for all the applications. Depends upon the
voltage ratios from primary to secondary windings, the transformers are classified as step-up and
step-down transformers.
3.1.1 Step-Up Transformer
Figure 2: Step up transformer
As the name states that, the secondary voltage is stepped up with a ratio compared to primary
voltage. This can be achieved by increasing the number of windings in the secondary than the
primary windings as shown in the figure. In power plant, this transformer is used as connecting
transformer of the generator to the grid. [3]
3.1.2 Step-Down Transformer
Figure 3: Step down transformer
It used to step down the voltage level from lower to higher level at secondary side as shown
below so that it is called as a step-down transformer. The winding turns more on the primary side
than the secondary side.
In distribution networks, the step-down transformer is commonly used to convert the high
grid voltage to low voltage that can be used for home appliances. [3]
7

3.2 Transformer Based on the Core Medium Used
Based on the medium placed between the primary and secondary winding the transformers are
classified as Air core and Iron core. [3]
3.2.1 Air Core Transformer
Both the primary and secondary windings are wound on a non-magnetic strip where the flux
linkage between primary and secondary windings is through the air. Compared to iron core the
mutual inductance is less in air core, i.e. the reluctance offered to the generated flux is high in the
air medium. But the hysteresis and eddy current losses are completely eliminated in air-core type
transformer. [3]
Figure 4: Air core transformer
3.2.2 Iron Core Transformer
Figure 5: Iron core transformer
Both the primary and secondary windings are wound on multiple iron plate bunch which
provide a perfect linkage path to the generated flux. It offers less reluctance to the linkage flux
due to the conductive and magnetic property of the iron. These are widely used transformers in
which the efficiency is high compared to the air core type transformer. [3]
3.3 Transformers Based on Winding Arrangement
3.3.1 Autotransformer
Standard transformers have primary and secondary windings placed in two different directions,
but in autotransformer windings, the primary and the secondary windings are connected to each
other in series both physically and magnetically as shown in the figure below. On a single common
8

Figure 6: Autotransformer
coil which forms both primary and secondary winding in which voltage is varied according to the
position of secondary tapping on the body of the coil windings. [3]
3.4 Transformers Based on Usage
According to the necessity, these are classiﬁed as the power transformer, distribution transformer
measuring transformer, and protection transformer.
3.4.1 Power Transformer
Figure 7: Power transformer
The power transformers are big in size. They are suitable for high voltage (greater than 33KV)
power transfer applications. It used in power generation stations and Transmission substation. It
has high insulation level. [3]
3.4.2 Distribution Transformer
A distribution transformer or service transformer is a transformer that provides the ﬁnal voltage
transformation in the electric power distribution system, stepping down the voltage used in the
distribution lines to the level used by the customer. [4]
9

Figure 8: Distribution transformer
Basically, it is used for the distribution of electrical energy at low voltage is less than 33KV in
industrial purpose and 440v-220v in domestic purpose. It works at low efficiency at 50-70 percent,
Small size, Easy installation, Low magnetic losses It is not always fully loaded [3]
3.5 Instrument Transformer
They are generally known as an isolation transformer. Instrument transformer is an electrical
device used to transform current as well as voltage level. The instrument transformer is further
divided into two types Current Transformer (CT), Potential Transformer (PT). The current and
potential transformer is explained below in detail
3.5.1 Current Transformer
A current transformer is an instrument transformer, used along with measuring or protective
devices, in which the secondary current is proportional to the primary current (under normal
conditions of operation) and differs from it by an angle that is approximately zero. [5]
Current transformers perform the following functions:
• Current transformers supply the protective relays with currents of magnitude proportional
to those of power circuit but sufficiently reduced in magnitude.
• The measuring devices cannot be directly connected to the high magnitude supplies. Hence
current transformers are used to supply those devices with currents of magnitude propor-
tional to those of power.
• A current transformer also isolates the measuring instruments from high voltage circuits.
3.5.2 Potential Transformer
Potential transformer is a voltage step-down transformer which reduces the voltage of a high
voltage circuit to a lower level for the purpose of measurement. These are connected across or
parallel to the line which is to be monitored.
The basic principle of operation and construction of this transformer is similar to the standard
power transformer. In common, the potential transformers are abbreviated as PT. [6]
The potential transformer is made with high-quality core operating at low flux density so that
the magnetising current is small. The terminal of the transformer should be designed so that the
variation of the voltage ratio with load is minimum and the phase shift between the input and
output voltage is also minimum. [7]
10

Figure 9: Potential transformers
3.6 Audio Transformer
Audio Transformers are designed for use in audio amplifier applications for coupling and impedance
matching of amplifiers and speakers. [8]
Audio Transformers perform several functions: [9]
• They can step up (increase) or step down (decrease) a signal voltage
• They can increase or decrease the impedance of circuit
• Convert the circuit from unbalanced to balanced and from balanced to unbalanced
• Block DC current in a circuit while allowing the AC current flow
• They electrically isolate one audio device from another
3.7 Polyphase Transformers
This type of transformer is commonly associated with three phase electric power, which is a com-
mon method of transmitting large amounts of high voltage power, such as the national power
grid.
4 Different types of winding
4.1 Cylindrical Windings
Figure 10: Cylindrical windings
11

These windings are layered type and uses rectangular or round conductor shown in Fig. (A)
and (b). The conductors are wound on ﬂat sides shown in Fig. (c) And wound on the rib side in
Fig. (d). [10]
4.2 Helical Windings
Figure 11: Single and Double helical winding
We use helical windings low voltage, high capacity transformers, where current is higher, at
the same time windings turns are lesser. The output of transformer varies from 160 - 1000 kVA
from 0.23-15 kV. To secure adequate mechanical strength the cross-sectional area of the strip not
made less than 75-100 mm square. The maximum number of strips used in parallel to make up
a conductor is 16. There are three types: Single Helical Winding, Double Helical Winding, Disc-
Helical Winding
Single Helical Windings consist of winding in an axial direction along a screw line with an inclina-
tion. There is only one layer of turns in each winding. The advantage of Double Helical Winding
is that it reduces eddy current loss in conductors. This is on account of the reduced number of
parallel conductors situated in the radial direction. [10]
Figure 12: Disk Helical windings
In Disc-Helical Windings, the parallel connected strips are placed side by side in a radial di-
rection to occupy total radial depth of winding.
12

4.3 Multi-layer Helical Winding
We use it commonly for high voltage ratings for 110 kV and above. These types of winding consist
of several cylindrical layers concentrically wound and connected in series. We make the outer
layers shorter than inner layers to distribute capacitance uniformly. These windings primarily
improve the surge behavior of transformers. [10]
Figure 13: Multilayer Helical windings
4.4 Crossover Winding
We use these windings for high voltage windings of small transformers. The conductors are paper
covered round wires or strips. The windings are divided into a number of coils in order to reduce
voltage between adjacent layers. These coils are axially separated by a distance of 0.5 to 1 mm.
The voltages between adjacent coils should not be more than 800 to 1000 V. The inside end of a
Figure 14: Crossover windings
coil is connected to the output side end of the adjacent one as shown in above ﬁgure. The actual
axial length of each coil is about 50 mm while the spacing between two coils is about 6 mm to
accommodate blocks of insulating material. The width of the coil is 25 to 50 mm. The crossover
winding has a higher strength than cylindrical winding under normal condition. However, the
crossover has lover impulse strength than the cylindrical one. This type also consumes more
labour cost. [10]
13

4.5 Disc and Continuous Disc Winding
Primarily used for a high capacity transformer. The winding consist of a number of flat coils or
discs in series or parallel. The coils are formed with rectangular strips wound spirally from the
centre outwards in the radial direction as shown in the figure below.
Figure 15: Disc and Continuous Disc Winding
The conductors can be a single strip or multiple strips in a parallel wound on the flat side. This
makes robust construction for this type of windings. Discs are separated from each other with
press-board sectors attached to vertical strips. The vertical and horizontal spacers provide radial
and axial ducts for free circulation of oil which comes in contact with every turn. The area of the
conductor varies from 4 to 50 mm square and limits for current are 12 - 600 A. The minimum width
of oil duct is 6 mm for 35 kV. The advantage of the disc and continuous windings is their greater
mechanical axial strength and cheapness. [10]
4.6 Sandwich Type Winding
Figure 16: Sandwich Type Winding
Allow easy control over the reactance the nearer two coils are together on the same magnetic
axis, the greater is the proportion of mutual flux and the less is the leakage flux. Leakage can be
reduced by subdividing the low and high voltages sections. The end low voltages sections contain
half the turns of the normal low voltage sections called half coils. In order to balance the magneto
motive forces of adjacent sections, each normal section whether high or low voltage carries the
same number of ampere-turns. The higher the degree of subdivision, the smaller is the reactance.
[10]
14

5 Insulating material use in transformer
5.1 Insulating Oil
Insulation oil plays a very important in transformer insulation system. In low voltage transformer,
for example, transformers used in the range of 12-1000V or low power rating transformers there
is no need of insulating oil in such transformers. Heat dissipation is very low in low voltage
transformers. In 11KV transformers, insulating oil has the important rule of acting as an electrical
insulation as well as a coolant to dissipate heat losses. Transformer oil is basically obtained by
fractional distillation and subsequent treatment of crude petroleum. So Transformer oil act as
liquid dielectric and coolant and it is placed in a tank in which core of the transformer is placed.
[11]
Figure 17: Insulating oil
5.2 Insulating paper
Figure 18: Insulating paper
Paper is a fabric made from vegetables fibers which are felted to form a web or sheet. The
fibrous raw materials are obtained from plants including cotton, hemp, manila, straw, and conif-
erous trees. It attains a very high value of electric strength when emerged in oil under vacuum.
”Craft insulating paper of medium air permeability” is used in layer winding insulation, con-
denser core of oil impregnated bushing. Craft insulating paper of high air permeability” is used
in covering over rectangular copper conductor and continuously transposed copper conductor.
“Crepe Kraft paper” is used in covering over flexible copper cable insulation of winding lead.
“Press paper” is used as backing paper for axial cooling duct. [11]
15

5.3 Pressboard
Pressboard is a widely used insulating material for making a variety of components used in elec-
trical, mechanical and thermal design of transformers. Pressboard id also made from vegetable
ﬁbers, whose cells contains much cellulose. The most difﬁcult practical insulation in power trans-
formers occur at the end of the windings and the lead outs from the windings. Pressboard molded
components can be made to any required shape. Angle rings and caps are the widely used mould-
ings. There are many kind of pressboards use in high voltage transformers but “soft pressboard
–laminated” is used in 11KV transformers as a block washer, terminal gear cleat and support and
spacer etc. “Pressboard moulding from wet sheet or wet wood pulp” is used in angle ring, cap,
sector, snouts, square tube, lead out, for insulating ends of winding, insulation between windings
and numerous other applications. [11]
Figure 19: Insulating pressboard
5.4 Wood
Wood based laminates are manufactured from selected veneers obtained from various timbers.
The veneers are dried and partially or fully impregnated with natural phenomenon. The areas
which required higher mechanical and lower electric strength, densities laminated wood is used
for making a variety of insulation components like coil clamping ring, cleat, support, core and
yoke etc. [11]
Figure 20: Insulating wood
5.5 Insulated copper conductor for winding
Different type of insulated copper conductor windings are used in power transformers for exam-
ple paper covered rectangular copper conductor, twin paper covered rectangular copper conduc-
16

tor bunched together, paper covered continuously transposed copper conductor, twin transposed
copper conductor bunched together, twin rectangular copper bunched together and provided with
a common paper strip between the two conductors and epoxy coated continuously transposed
conductor. These are used to the winding space factor and mechanical strength of windings.”
paper covered rectangular copper conductor” is used for making different kind of windings. “Pa-
per covered standard copper cable” is used for making lead and terminal. “Crepe paper covered
flexible copper cable” is used for making lead and terminal required to be bent to a small radius.
“PVC insulated copper cable-single and multi-core” is used to control wiring in marshalling box,
nitrogen, sealing system. [11]
5.6 Insulating tape
Insulating tape is used for various taping purposes .For example cotton tape, cotton newer tape,
glass woven tape, woven tape and phenol laminated paper base sheet. These tapes are used
in taping, banding, core bolt insulation, places where required high strength and in banding of
transformer cores. [11]
6 Working principle
The main principle of operation of a transformer is mutual inductance between two circuits which
is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically
separate and inductive, but are magnetically linked through a path of reluctance. The working
principle of the transformer can be understood from the figure below. [12]
As shown above the electrical transformer has primary and secondary windings. The core lam-
Figure 21: Primary and Secondary windings of core type transformer
inations are joined in the form of strips in between the strips you can see that there are some
narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘im-
bricated’. Both the coils have high mutual inductance. A mutual electro-motive force is induced in
the transformer from the alternating flux that is set up in the laminated core, due to the coil that is
connected to a source of alternating voltage. Most of the alternating flux developed by this coil is
17

linked with the other coil and thus produces the mutual induced electro-motive force. The so pro-
duced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic
Induction as
e = M
dI
dt
If the second coil circuit is closed, a current flows in it and thus electrical energy is transferred
magnetically from the first to the second coil. The alternating current supply is given to the first
coil and hence it can be called as the primary winding. The energy is drawn out from the second
coil and thus can be called as the secondary winding. In short, a transformer carries the operations
shown below:
• Transfer of electric power from one circuit to another.
• Transfer of electric power without any change in frequency.
• Transfer with the principle of electromagnetic induction.
• The two electrical circuits are linked by mutual induction.
7 Transformer tests
For confirming the specifications and performances of an electrical power transformer it has to
go through numbers of testing procedures. Some tests are done at manufacturer premises before
delivering the transformer. Mainly two types of transformer testing are done at manufacturer
premises- type test of transformer and routine test of transformer. In addition to that some trans-
former tests are also carried out at the consumer site before commissioning and also periodically
in regular and emergency basis through out its service life.
7.1 Open Circuit Test
Open circuit test or no load test on a transformer is performed to determine ’no load loss (core
loss)’ and ’no load current I0’. The circuit diagram for open circuit test is shown in the figure
below. Usually high voltage (HV) winding is kept open and the low voltage (LV) winding is
Figure 22: Circuit Diagram for open circuit test
connected to its normal supply. A wattmeter (W), ammeter (A) and voltmeter (V) are connected
to the LV winding as shown in the figure. Now, applied voltage is slowly increased from zero to
18

normal rated value of the LV side with the help of a variac. When the applied voltage reaches to
the rated value of the LV winding, readings from all the three instruments are taken.
The ammeter reading gives the no load current Io. As Io itself is very small, the voltage drops
due to this current can be neglected.
The input power is indicated by the wattmeter (W). And as the other side of transformer is
open circuited, there is no output power. Hence, this input power only consists of core losses and
copper losses. As described above, no-load current is so small that these copper losses can be
neglected. Hence, now the input power is almost equal to the core losses. Thus, the wattmeter
reading gives the core losses of the transformer. Sometimes, a high resistance voltmeter is con-
nected across the HV winding. Though, a voltmeter is connected, HV winding can be treated as
open circuit as the current through the voltmeter is negligibly small. This helps in to ﬁnd voltage
transformation ratio (K).
The two components of no load current can be given as, Iµ = Iosinφo and Iw = Iocosφo.
cosφo(no load power factor)= W
(V1×Io)
(These values are referring to LV side of the transformer.) Hence, it is seen that open circuit
test gives core losses of transformer and shunt parameters of the equivalent circuit.
7.2 Short Circuit or Impedance Test on Transformer
Figure 23: Circuit Diagram for short circuit test
The connection diagram for short circuit test or impedance test on transformer is as shown
in the ﬁgure below. The LV side of transformer is short circuited and wattmeter (W), voltmere
(V) and ammeter (A) are connected on the HV side of the transformer. Voltage is applied to the
HV side and increased from the zero until the ammeter reading equals the rated current. All the
readings are taken at this rated current.
The ammeter reading gives primary equivalent of full load current (Isc). The voltage applied
for full load current is very small as compared to rated voltage. Hence, core loss due to small
applied voltage can be neglected. Thus, the wattmeter reading can be taken as copper loss in the
transformer.
Therefore, W = I2
scReq....... (where Req is the equivalent resistance of transformer) Zeq = Vsc
Isc
.
Therefore, equivalent reactance of transformer can be calculated from the formula
Z2
eq = R2
eq + X2
eq
. These, values are referred to the HV side of the transformer. Hence, it is seen that the short circuit
test gives copper losses of transformer and approximate equivalent resistance and reactance of the
19

transformer.
8 Cooling methods
No transformer is truly an ideal transformer’ and hence each will incur some losses, most of
which get converted into heat. If this heat is not dissipated properly, the excess temperature in
transformer may cause serious problems like insulation failure. It is obvious that transformer
needs a cooling system. [13]
8.1 Air Natural
This method of transformer cooling is generally used in small transformers (upto 3 MVA). In this
method the transformer is allowed to cool by natural air flow surrounding it. [13]
8.2 Air Blast
For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this
method, air is forced on the core and windings with the help of fans or blowers. The air supply
must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method
can be used for transformers upto 15 MVA. [13]
8.3 Oil Natural Air Natural (ONAN)
Figure 24: ONAN cooling method
This method is used for oil immersed transformers. In this method, the heat generated in the
core and winding is transferred to the oil. According to the principle of convection, the heated
oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled
oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air
flow around the transformer. In this way, the oil in transformer keeps circulating due to natural
20

convection and dissipating heat in atmosphere due to natural conduction. This method can be
used for transformers upto about 30 MVA. [13]
8.4 Oil Natural Air Forced (ONAF)
Figure 25: ONAF cooling method
The heat dissipation can be improved further by applying forced air on the dissipating surface.
Forced air provides faster heat dissipation than natural air ﬂow.
In this method, fans are mounted near the radiator and may be provided with an automatic
starting arrangement, which turns on when temperature increases beyond certain value. This
transformer cooling method is generally used for large transformers upto about 60 MVA. [13]
8.5 Oil Forced Air Forced (OFAF)
Figure 26: OFAF cooling method
21

In this method, oil is circulated with the help of a pump. The oil circulation is forced through
the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of
fans. The heat exchangers may be mounted separately from the transformer tank and connected
through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher
rating transformers at substations or power stations. [13]
8.6 Oil Forced Water Forced (OFWF)
Figure 27: OFWF cooling method
This method is similar to OFAF method, but here forced water flow is used to dissipate hear
from the heat exchangers. The oil is forced to flow through the heat exchanger with the help of
a pump, where the heat is dissipated in the water which is also forced to flow. The heated water
is taken away to cool in separate coolers. This type of cooling is used in very large transformers
having rating of several hundreds MVA. [13]
9 Losses of transformer
In any electrical machine, ’loss’ can be defined as the difference between input power and output
power. An electrical transformer is an static device, hence mechanical losses (like windage or
friction losses) are absent in it. A transformer only consists of electrical losses (iron losses and
copper losses). Transformer losses are similar to losses in a DC machine, except that transformers
do not have mechanical losses. Losses in transformer are explained below-
9.1 Core Losses or Iron Losses
Eddy current loss and hysteresis loss depend upon the magnetic properties of the material used
for the construction of core. Hence these losses are also known as core losses or iron losses.
9.1.1 Hysteresis loss in transformer
Hysteresis loss is due to reversal of magnetization in the transformer core. This loss depends upon
the volume and grade of the iron, frequency of magnetic reversals and value of flux density. It can
22

be given by, Steinmetz formula:
Wh = µBmax1.6fV(watts)
where, µ = Steinmetz hysteresis constant V=volume of the core
9.1.2 Eddy current loss in transformer
In transformer, AC current is supplied to the primary winding which sets up alternating mag-
netizing flux. When this flux links with secondary winding, it produces induced emf in it. But
some part of this flux also gets linked with other conducting parts like steel core or iron body or
the transformer, which will result in induced emf in those parts, causing small circulating current
in them. This current is called as eddy current. Due to these eddy currents, some energy will be
dissipated in the form of heat.
9.2 Copper Loss
Copper loss is due to ohmic resistance of the transformer windings. Copper loss for the primary
winding is I2
1 R1 and for secondary winding is I2
2 R2. Where, I1 and I2 are current in primary and
secondary winding respectively, R1 and R2 are the resistances of primary and secondary winding
respectively. It is clear that Cu loss is proportional to square of the current, and current depends
on the load. Hence copper loss in transformer varies with the load.
10 Efficiency of transformer
Just like any other electrical machine, efficiency of a transformer can be defined as the output
power divided by the input power. That efficiency =
Output
Input + losses
.
Transformers are the most highly efficient electrical devices. Most of the transformers have full
load efficiency between 95% to 98.5%. As a transformer being highly efficient, output and input
are having nearly same value, and hence it is impractical to measure the efficiency of transformer
by using output / input. A better method to find efficiency of a transformer is using, efficiency =
(input - losses) / input = 1 - (losses / input).
11 Three phase transformer connections
Windings of a three phase transformer can be connected in various configurations as (i) star-star,
(ii) delta-delta, (iii) star-delta, (iv) delta-star, (v) open delta and (vi) Scott connection. These con-
figurations are explained below. [14]
11.1 Star-Star
Star-star connection is generally used for small, high-voltage transformers. Because of star con-
nection, number of required turns/phase is reduced (as phase voltage in star connection is 1√
3
times of line voltage only). Thus, the amount of insulation required is also reduced.
The ratio of line voltages on the primary side and the secondary side is equal to the transformation
ratio of the transformers.
23

Figure 28: Star-Star connection
Line voltages on both sides are in phase with each other. This connection can be used only if the
connected load is balanced. [14]
11.2 Delta-Delta
This connection is generally used for large, low-voltage transformers. Number of required phase/turns
is relatively greater than that for star-star connection. The ratio of line voltages on the primary and
the secondary side is equal to the transformation ratio of the transformers. This connection can
be used even for unbalanced loading. Another advantage of this type of connection is that even
if one transformer is disabled, system can continue to operate in open delta connection but with
reduced available capacity. [14]
Figure 29: Delta-Delta connection
11.3 Star-Delta or Wye-Delta
The primary winding is star star (Y) connected with grounded neutral and the secondary winding
is delta connected. This connection is mainly used in step down transformer at the substation end
of the transmission line. The ratio of secondary to primary line voltage is 1√
3
times the transfor-
mation ratio. There is 30o shift between the primary and secondary line voltages. [14]
24

Figure 30: Star-Delta Connection of three phase transformer
11.4 Delta-Star or Delta-Wye
Figure 31: Delta-Star connection of three phase transformer
The primary winding is connected in delta and the secondary winding is connected in star with
neutral grounded. Thus it can be used to provide 3-phase 4-wire service. This type of connection
is mainly used in step-up transformer at the beginning of transmission line. The ratio of secondary
to primary line voltage is
√
3 times the transformation ratio. There is 30o shift between the primary
and secondary line voltages. [14]
11.5 Open Delta (V-V)
Figure 32: V-V connection of three phase transformer
25

Two transformers are used and primary and secondary connections are made as shown in the
figure below. Open delta connection can be used when one of the transformers in delta-delta bank
is disabled and the service is to be continued until the faulty transformer is repaired or replaced.
It can also be used for small three phase loads where installation of full three transformer bank is
un-necessary. The total load carrying capacity of open delta connection is 57.7percent than that
would be for delta-delta connection. [14]
11.6 Scott (T-T)
Figure 33: T-T connection of three phase transformer
Two transformers are used in this type of connection. One of the transformers has center
taps on both primary and secondary windings (which is called as main transformer). The other
transformer is called as teaser transformer. Scott connection can also be used for three phase to
two phase conversion. The connection is made as shown in the figure. [14]
12 Design difference between core type and shell type three phase trans-
former
12.1 Core type transformer
Figure 34: Core type three phase transformer
• Limbs are surrounded by the windings.
• No separate flux return path is essential.
26

• All limb carries equal flux.
• Laminated core is build to form rectangular frame.
• Winding has poor mechanical strength because they are not supported or braced.
• Beyond one level it is not possible to reduce leakage because high voltage and low voltage
winding cannot be subdivided to great extent.
• Limbs are surrounded by the windings so cooling is better in winding then limb.
• Permits easier assemble of parts and insulation of winding.
• Easy to dismantle for for maintanance or repair.
• Much simpler in design. [15], [16]
12.2 Shell Type transformer
Figure 35: Shell type three phase transformer
• Windings are surrounded by the limbs.
• Separate flux return paths is essential.
• Central limb carries whole flux and side limb carries half of the total flux.
• Lamnated core is built to form rectangular frame.
• Winding has excessive mechanical strength because they are supported or braced.
• It is possible to reduce leakage because high voltage and low voltage winding can be subdi-
vided by using sandwitch coil.
• Windings are surrounded by the limbs so cooling is better in core than winding.
• Great difficulty to assemble parts and insulation of winding.
• More complex in design. [15], [16]
27

13 Transformer tank
Tank bodies of most of the transformers are made from rolled steel plates which are fabricated to
form the container. Small tanks are welded from steel plates while larger ones are assembled from
boiler plates. The tanks are providing with lifting lugs. Small transformers have cooling tubes.
Such transformers have plain tanks with provision for pipe and valves to direct and control the
oil flow.
While designing tanks for transformers, a large number of factors have to be considered. These
factors include keeping the weight, stray load losses and cost a minimum, and it is obvious that
these are requirements contradictory.
The tank should be strong enough to withstand stresses produced by jacking and lifting. The size
of the tank be large enough to assume cores, windings, internal connections and also must give
the essential clearance between the windings and the walls. Alluminium is increasingly being
Figure 36: Tanks of transformer
used for transformer tanks as a means of reducing weight. The use of alluminium in place of steel
reduces the stray magnetic fields and consequently the stray load loss. However, aluminium tanks
are costlier. Also the use of aluminium necessitates special lifting arrangements in order prevent
stressing of tank. However, usually aluminium tanks are made of cast aluminium parts mounded
on a shallow mild steel tray. The mild steel tray is arranged to carry the main lifting and jacking
members.
Where mild steel tanks are used for units with high leakage flux, electromagnetic screens or
shuntsare used to reduce eddy current losses.
14 Transformer bushings
A transformer bushing is an insulating structure that facilitates the passage of an energized,
current-carrying conductor through the grounded tank of the transformer. The conductor may
be built in to the bushing, i.e., a bottom-connected bushing, or the bushing may be built with the
provision for a separate conductor to be drawn through its centre, a.k.a., a draw-lead or draw-rod
bushing.
The two principal types of bushing construction are solid or bulk type and capacitance-graded
(sometimes called condenser type). The bushings used for the low voltage winding(s) of a trans-
former are often solid type with a porcelain or epoxy insulator. Capacitance-graded bushings,
designed for higher voltage ratings, are used for a transformer’s high voltage winding.
28

Figure 37: Transformer bushing
Unlike a solid type construction, in a capacitance-graded transformer bushing, conducting lay-
ers are inserted at predetermined radial intervals within the insulation that separates the centre
conductor from the insulator (housing) of the bushing. These multiple conductive inserts create
capacitive elements linking the centre conductor of the bushing to ground. Their purpose is to con-
trol the voltage field around the center conductor so that the voltage distributes more uniformly
across the surrounding insulation system in the bushing.
In solid type bushings, electrical grade mineral oil is often used between the conductor and the
insulator, which may be contained within the bushing or shared with the transformer. Typical in-
sulation used in a capacitance-graded bushing is oil-impregnated paper (OIP), resin-impregnated
paper (RIP), and resin bonded paper (RBP). Capacitance-graded bushings also use mineral oil,
usually contained within the bushing.
Transformer bushing failures are often credited as one of the top causes of transformer failures
so the condition of the bushings is of high interest to transformer asset owners. Typical bush-
ing failure modes include moisture ingress, electrical flashover, lightning strike, short-circuited
capacitance-graded layer(s), bushing misapplication, corrosive sulphur, broken connection be-
tween ground sleeve and flange, and a broken tap connection. [17]
15 Breather and Conservator tank
15.1 Breather
When the temperature changes occur in transformer insulating oil, the oil expands or contracts
and there an exchange of air also occurs when transformer is fully loaded. When transformer gets
cooled, the oil level goes down and air gets absorbed within. This process is called breathing and
the apparatus that pass through the air is called breather. Actually, silica gel breathers controls
the level of moisture, entering electrical equipment during the change in volume of the cooling
medium and/or airspace caused by temperature increasing. [18]
29

Figure 38: Breather of transformer
15.2 Conservator tank
This is a cylindrical tank mounted on supporting structure on the roof of the transformer’s main
tank. When transformer is loaded, the temperature of oil increases and consequently the volume
of oil in the transformer gets increased. Again; when ambient temperature is increased, the vol-
ume of oil is also increased. The conservator tank of a transformer provides adequate space for
expansion of oil. Conservator tank of transformer also acts as a reservoir of oil. [18]
Figure 39: Conservator of transformer
16 Temperature indicator in Transformer
16.1 Liquid temperature indicators
Figure 40: Liquid temperature indicators
30

Temperature indicators are designed to measure the temperature of the insulating liquid inside
power transformer and distribution transformer tanks. [19]
16.2 Oil temperature indicators
Figure 41: Oil temperature indicators
This device measures top oil temperature with the help of sensing bulb immersed in the pocket
by using liquid expansion in the bulb through a capillary line to operating mechanism. A link and
lever mechanism ampliﬁes this movement to the disc carrying pointer and mercury switches.
When volume of the liquid in operating mechanism changes, the bellow attached to end of cap-
illary tube expands and contracts. This movement of bellow is transmitted to the pointer in tem-
perature indicator of transformer through a lever linkage mechanism. [20]
16.3 Winding temperature indicators
Figure 42: Winding temperature indicators
The Winding is the component with highest temperature within the transformer and, above
all, the one subject to the fastest temperature increase as the load increases. Thus to have total
control of the temperature parameter within transformer, the temperature of the winding as well
as top oil, must be measured. An indirect system is used to measure winding temperature, since it
is dangerous to place a sensor close to winding due to the high voltage. The indirect measurement
is done by means of a Built-in Thermal Image.
31

Winding Temperature Indicator is equipped with a specially designed Heater which is placed
around the operating bellows through which passes a current proportional to the current pass-
ing through the transformer winding subject to a given load. Winding Temperature is measured
by connecting the CT Secondary of the Transformer through a shunt resistor inside the Winding
Temperature Indicator to the Heater Coil around the operating Bellows. It is possible to adjust
gradient by means of Shunt Resistor.
In this way the value of the winding temperature indicated by the instrument will be equal to
the one planned by the transformer manufacturer for a given transformer load. [21]
17 Transformer fault conditions
A number of transformer fault conditions can arise practically in any time following some special
situations. These include the following 5 most common internal faults and few external:
1. Earth faults
2. Core faults
3. Interturn faults
4. Phase-to-phase faults
5. Tank faults
17.1 Earth faults
Earth Fault occurs whenever a live wire is connected to ground because of some problem in the
electrical system.
Like any other electrical equipment, the live wire in a transformer is protected or insulated from
the ground by insulation. The insulating media is mainly paper insulation. Because of manu-
facturing defect of insulating paper or because of ageing often this insulating paper becomes less
effective in providing insulation. When such situation occurs, the insulating paper raptures and
winding get connected to the ground and we observe earth fault in a transformer. Sometimes,
the connecting cables, moving parts of tap changer etc. also come in contact of the tank or core
making earth fault. [22]
17.2 Core faults
Core faults due to insulation breakdown can permit sufficient eddy-current to flow to cause over-
heating, which may reach a magnitude sufficient to damage the winding. [23]
17.3 Interturn faults
These are the faults which occurs internal to the transformer which may seriously damage the
insulation of the transformer and causes break down in transformer. So the transformer should be
immediately protected from these faults. [24]
32

17.4 Phase-to-phase faults
There is some phenomenon happen when phase to phase fault occur in transformer. During this
fault, the overall voltage ratio is not the same as the transformer’s turn ratio. Furthermore, an
unbalanced fault on one side of the transformer present differently on the other side of the trans-
former. [25]
17.5 Tank faults
Tank faults resulting in loss of oil reduce winding insulation as well as producing abnormal tem-
perature rises. So it is very important to take the insulation oil for the transformer that should be
able to control the parts of transformer over temparature. [23]
17.6 External factors
In addition to fault conditions within the transformer, abnormal conditions due to external factors
result in stresses on the transformer. [23] These conditions include:
• Overloading
• System faults
• Over voltages
• Under-frequency operation
18 Important notation used in transformer design
• φm = main ﬂux, Wb
• Bm = maximum ﬂux density, Wbm−2
• δ = current density, Am−2
• Agc = gross core area, m2
• Ai = net core area, m2
• Ac = area of copper in the window, m2
• Aw = window area, m2
• D = distance between core center, m
• d = diameter of circumscribing circle, m
• Kw = window space factor
• f = frequency, Hz
• Et = emf per turn, V
• Tp = number of turns in primary
33

• Ts = number of turns in secondary
• Ip = current in primary winding, A
• Is = current in secondary winding, A
• Vp = terminal voltage in primary winding, V
• Vs = terminal voltage in secondary winding, V
• ap = area of conductors of primary winding, m2
• as = area of conductors of secondary winding, m2
• li = mean length of ﬂux path in iron, m
• Lmt = length of mean per turn of windings, m
• Gi = weight of active iron, kg
• Gc = weight of copper, kg
• gi = weight per m3 of iron, kg
• gc = weight per m3 of copper, kg
• pi = loss in iron per kg, W
• pc = loss in copper per kg, W [26]
19 Design details
19.1 Core design:
The value of K is 0.45 for distribution transformer.
Voltage per turn, Et = K
√
Q
= 0.45 ×
√
20
= 2.012V
So, Flux in the core, φm =
Et
4.44 × f
=
2.012
4.44 × 50
= 0.009063 Wb.
Hot rolled silicon steel 92 is used. The value of ﬂux density Bm is assumed as 1.0Wbm−2.
Net iron area, Ai =
φm
Bm
=
0.009063
1.0
= 9.063 × 103mm2
34

Using a cruciform core, Ai = 0.56d2
Diameter of circumscribing circle,
d = 9.063×103
0.56 mm2
=127.22 mm
Using ﬁg 43;
Widths of laminations,
Figure 43: Square cruciform core and 2 stepped cruciform core
a = 0.85 × d
= 0.85 × 127.22
=108.137 mm
b = 0.53 × d
= 0.53 × 127.22
=67.426 mm
Nearest standard dimension are, a=110mm, b=70mm
19.2 Window dimensions
The window space factor,
Kw =
8
30 + kV
=
8
30 + 11
0.195121
≈ 0.19
The current density in the winding is taken as 2.3A/mm2.
Output of transformer,
Q = 3.33f BmKwδAw Ai × 10−3
Aw =
20
3.33 × 50 × 1 × 0.19 × 2.3 × 106 × 0.009063 × 10−3
35

= 0.03033mm2
= 30.33 × 103mm2
Taking the ratio height to width of window =2.5m
So, Hw × Ww = 30.33 × 103 = 2.5W2
w
Ww = 30.33×103
2.5
=110.145 mm
Now, Hw = 2.5 × Ww
275.3625 mm
So, width of window 110 mm and height 275mm
Area of window provided, Aw = (275 × 110)mm2
= 30.250 × 103mm2
= 0.030250m2
Distance between adjacent limb, D = d + Ww
= 127.22 + 110.145mm
=237.365 mm
19.3 Yoke Design
The area of yoke is taken as 1.2 times that of limb.
So, ﬂux density in yoke =
1
1.2
= 0.833Wbm2
Net area of yoke = 1.2 × 9.063 × 103
= 10.875 × 103mm2
Gross area of yoke =
10.875 × 103
0.9
= 12.084 × 103mm2
Taking the section of the yoke as rectangular,
Depth of yoke Dy ≈ a ≈ 108.137mm
Height of yoke Hy =
12.084 × 103
108.137
≈ 112mm
19.4 Overall dimensions of frame
Reference ﬁgure 44, Height of frame, H = Hw + 2Hy
= (275.3625 + 2 × 112)mm
=499.3625 mm
Width of frame, W = 2D+a
=2(237.365)+108.137
=582.137 mm
Depth of yoke, Dy = a = 108.137mm
36

Figure 44: Three phase core type transformer
19.5 L.V. winding
Secondary line voltage = 400 V; Connection = Star
Secondary Phase voltage, Vs =
400
√
3
= 230.94 V
Number of turns per phase, Ts =
Vs
Et
=
230.94
2.012
=114
Secondary phase current, Is =
20 × 1000
3 × 230.94
= 28.86 A
A current density of 2.3A/mm2 is used;
Area of secondary conductor as =
28.86
2.3
= 12.55mm2
Using a bare conductor of 8.0 × 2.6mm
Area of bare conductor 20.8mm2
Current density in secondary winding, δs =
28.86
20.8
= 1.3875A/m2
The conductors are paper covered. The increase in dimensions on account of paper covering is 0.5
mm.
Dimensions of insulated conductor = 8.5 × 3.1mm2
Using three layers for the windings.
Helical winding is used. Therefore space has to be provided for (38+1) = 39 turns along the axial
depth.
Axial depth of low voltage winding
Lcs = 39 × axial depth of conductor
= 39 × 8.5
=331.5mm
37

Using 0.5 mm pressboard cylinders is between layers.
Radial depth of l.v. winding,
bs = number of layers × radial depth of conductor+insulation between layers
= 3 × 3.1 + 2 × 0.5
= 10.3mm
Figure, 45 shows a cross section through l.v. coil,
Figure 45: L.V. winding all dimensions
Diameter of circumscribing circle, d=127.48 mm.
Using pressboard wraps 1.5 mm thick as insulation between l.v. winding and core.
Inside diameter of l.v. winding, 127.22 + 2 × 1.5 =130.22mm
Outside diameter of l.v. winding, 130.22 + 2 × 10.3 = 150.82mm
19.6 H.V. winding
Primary line voltage = 11000 volt, Connection=Delta
Primary phase voltage Vp = 11000V
Number of turns per phase, Tp =
11000 × 114
230.94
=5429
As plus minus 5% tappings are to be provided, therefore the number of turns is increased to
Tp = 1.05 × 5429
= 5700
Using 8 coil, Voltage per coil is = 11000/8
= 1375
Turns per coil,
5700
8
=712
Using 7 normal coils of 753 turns and one reinforced coil of 424 turns.
Total H.V. turns provided, Tp = 7 × 753 + 424 = 5700
Taking 24 layers per coil, Turns per layer is, 753/24 =31
Maximum voltage between layers, 2 × 31 × 2.012 = 125V. which is bellow the allowable limit.
High voltage winding phase current, Ip =
20 × 1000
3 × 11000
38

=0.606 A
As the current is bellow 20A, cross over coils are used for high voltage windings.
Taking current density 2.4Amm−1
Area of h.v. conductor, ap =
Ip
2.4
=
0.606
2.4
= 0.253mm2
Diameter of bare conductor, 4×0.253
π
=0.568 mm
Using paper cover conductors. The nearest standard conductor size has,
bare diameter has=0.568 mm, insulated diameter = 0.735 mm with ﬁne covering.
Modiﬁed area of conductor ap = π
4 (0.568)2
= 0.253mm2
Actual value of current density used δp = 0.606/0.253 = 2.395Amm−1
Axial depth of one coil = 28 × 0.735
= 20.58mm
The spacers used between adjacent coils are 5mm in height.
Axial length of h.v. winding:
Lcp = number of coils ×axial depth of each coil+depth of spacers
= 8 × 20.58 + 8 × 5
=204.64 mm
The height of window is 275.3625mm and therefore, the space left between winding and window
is 275.3625-204.64=70.7225 mm. This is occupied by insulation and axial bracing of the coil.
The insulation used between layers is 0.3mm thick paper.
So, radial depth of h.v. coil, bp = 24 × 0.735 + 23 × 0.3
=24.54 mm
From equation, Insulation thickness=5+0.9kV mm, the thickness between h.v. and l.v. winding
is = 5 + 0.9kV = 5 + 0.9 × 11 = 15mm. This includes the width of oil ducts also.
The insulation between h.v. and l.v. winding is a 5mm thick bakelized paper cylinder. The h.v.
winding is wound on a former 5mm thick and the duct is 5mm wide, making the total insulation
between h.v. and l.v. winding is 15mm.
So, inside diameter of h.v. winding = outside diameter l.v. winding +2× thickness of insula-
tion
= 150.82 + 2 × 15
=180.82 mm
Outside diameter of h.v. winding = 180.82 + 2 × 24.54 = 229.9 mm
Clearance between winding of two adjacent limbs=237.365-229.9=7.465
19.7 Resistance
Mean diameter of primary =
180.82 + 229.9
2
=205.36 mm
39

Length of mean turn of primary winding, Lmtp = π × 205.36 × 10−3 = 0.645 mm
Resistance of primary winding at 75oC, rp =
TpρLmip
ap
=
5700 × 0.021 × 0.645
0.253
= 305.164Ω
Mean diameter of secondary winding =
130.22 + 150.82
2
=140.52 mm
Length of mean turn of secondary winding, Lmts = π × 139.34 × 10−3 = 0.4414m
Resistance of secondary winding at 75oC, rs =
114 × 0.021 × 0.4414
20.8
= 0.0508Ω
So, total resistance referred to primary side, Rp = 305.164 + (5429
114 )2 × 0.0508Ω
= 420.375Ω
P.U. resistance of transformer, r =
IpRp
Vp
=
0.606 × 420.375
11000
=0.0232
19.8 Leakage reactance
Mean diameter of windings =
130.22 + 229.9
2
=180.06 mm
Length of mean turn, Lmt = π × 180.06 × 10−3 = 0.5656m
Height of winding, Lc =
Lcp + Lcs
2
=
204.64 + 331.5
2
=268.07 mm
Leakage reactance of transformer referred to primary side, Xp = 2π f µoT2
p
Lmt
Lc
(a +
bp+bs
3 )
= 2π × 50 × 4π × 10−7 × (5429)2 × ( 0.5656
0.26807 ) × (15 + 24.54+10.3
3 ) × 10−3
= 653.37Ω
P.U. leakage reactance, x =
0.606 × 653.37
11000
=0.036
P.U. impedence s = (0.0232)2 + (0.036)2
=0.04283
19.9 Regulation
P.U. regulation = rcosφ + xsinφ
Per unit regulation at unity power factor, = r = 0.0232, at zero p.f. lagging = 2
m0.036, at0.8p.f.lagging
= 0.0232 × 0.8 + 0.036 × 0.6
= 0.04016
40

19.10 Losses
19.10.1 Copper Loss
I2R loss at 75oC = 3I2
pRp = 3 × 0.6062 × 420.375 = 463.13W
Taking stray load loss 15 percent of above.
Total I2R loss including stray load loss, Pc = 1.15 × 463.13 = 532.5995W
19.10.2 Core Loss
Taking the density of laminations as 7.6 × 103kgm−3
weight of three limbs = 3 × 0.3 × 0.009063 × 7.6 × 103
= 61.99kg
The flux density in the limbs is 1Wbm−2 and corresponding to this density, specific core loss is
1.2W/kg for 92 grade
Figure 46: (1-Grade 74, 2-Grade 80, 3-Grade 86, 4-Grade 92) Loss curve of electrical sheet steel(non
oriented) 0.35 mm thick
So, Core loss in limbs = 61.99 × 1.2 = 74.388W
Weight of two yokes = 2 × 0.582137 × 0.010875 × 7.6 × 103
=96.23 kg
Corresponding to a flux density of 0.833Wb/m2 in the yoke, the specific core loss = 0.83W
So, core loss in yoke = 96.23 × 0.83 = 79.8709W
Total core losses, Pi = 74.388 + 79.8709 ≈ 154.2589W
19.11 Efficiency
Total losses at full load = 154.2589+532.5995 = 686.8584W
Efficiency at full load and unity p.f. =
20000
20000 + 686.8584
×100
=96.6797 %
For maximum efficiency x2Pc = Pi
41

So, x =
Pi
Pc
=
154.2589
532.5995
=0.5382
Thus maximum efficiency occurs at 53.82% of full load. This is a good figure for distribution trans-
former.
19.12 No load current
Corresponding to flux densities of 1Wb/m2 and 0.833Wb/m2 in core and yoke respectively atc =
120A/m and aty = 80A/m in figure 47
Figure 47: B-H curve of electrical steel(non oriented) Grades 203 and 190, Grades 92 and 86(cold
rolled non oriented transformer steel)
Total magnetizing mmf = 3 × 120 × 0.3 + 2 × 80 × 0.582137
=201.142 A
Magnetizing mmf per phase ATo =
201.142
3
= 67.047 A
Magnetizing current Im =
AT0
√
2Tp
=
67.047
√
2 × 5429
= 8.733 × 10−3A
Loss component of no load current Il = 154.2589/(3 × 11000)
= 4.6745 × 10−3A
No load current I0 = (8.733 × 10−3)2 + (4.6745 × 10−3)2
= 9.905 × 10−3
No load current as a percentage of full load current =
9.905 × 10−3
0.606
×100 =1.63 %
42

19.13 Tank
Height over yoke H=499.3625mm. Allowing 50 mm at the base and about 150 mm for oil. Height
of oil level=499.3625+50+150=699.3625mm.
Allowing another 200mm height for leads etc., height of tank Ht = 699.3625 + 200 = 899.3625mm.
The height of tank is taken as 0.95m or Ht = 0.95m
Assuming a clearance of 40mm along the width on each side.
Width of tank Wt = 2D + De + 2l = 2 × 237.365 + 229.9 + 2 × 40 = 784.63mm
The width of tank taken as Wt = 0.84m
The clearance along the length of the transformer is greater than that along the width. This is
because additional space is needed along the length to accommodate tappings etc. The clearance
used is approximately 50mm on each side.
Length of tank Lt = De + 2b = 229.9 + 2 × 50 = 329.9mm
The length of tank Lt is taken as 0.34m.
Total loss dissipating surface of tank St = 2(0.84 + 0.34) × 0.95 = 2.242m2
Total specific loss dissipation due to radiation and convection is 12.5W/m2 −0C
Temperature rise =
686.8584
2.242 × 12.5
= 24.51oC
This is below 350C and therefore plain tank is sufficient for cooling and no tubes are required.
19.14 kVA rating calculation and verification
We know, Q = 3.33f BmKwδAw Ai × 10−3
=3.33 × 50 × 1 × 0.19 × 2.3 × 106 × 30.33 × 10−3 × 0.009063 × 10−3
=20.00046372
≈ 20kVA
20 Conclusion
The efficiency of this Transformer is nearly 96.6797% which is maintained by the losses(core loss
and copper loss) of the transformer. If the losses of resistive and reactive part can be reduced
then the efficiency can be increased. In designing of transformer, all the parts must be taken
appropriately.
References
1. https://en.wikipedia.org/wiki/Transformer
2. https://acupwr.com/blogs/news/80121091-a-brief-history-of-transformers
3. https://www.elprocus.com/various-types-of-transformers-applications/
4. https://en.wikipedia.org/wiki/Distribution-transformer
5. https://owlcation.com/stem/What-is-a-Current-transformer-How-does-it-work
6. https://www.electronicshub.org/potential-transformers/
7. https://circuitglobe.com/potential-transformer-pt.html
8. https://www.electronics-tutorials.ws/transformer/audio-transformer.html
9. https://uk.rs-online.com/web/c/power-supplies-transformers/transformers/audio-transformers/
10. https://www.electrical4u.com/transformer-winding/
11. http://microcontrollerslab.com/insulation-materials-transformers/
43

12. http://www.circuitstoday.com/transformer
13. https://www.electricaleasy.com/2014/06/cooling-methods-of-transformer.html
14. https://www.electricaleasy.com/2014/05/three-phase-transformer-connections.html
15. https://circuitglobe.com/difference-between-core-type-and-shell-type-transformer.html
16. http://electricalacademia.com/electrical-comparisons/difference-between-core-type-and-shell-
type-transformer/
17. https://uk.megger.com/applications/transformers/bushings
18. https://www.electrical4u.com/transformer-accessories/
19. https://new.abb.com/products/transformers/transformer-components/measurement-and-
safety-devices/temperature-indicator/oil-temperature-indicators
20. https://www.electrical4u.com/oil-winding-and-remotetemperature-indicator-of-transformer/
21. http://www.scientiﬁccontrols.com/oil-winding-temperature-indicator.html
22. https://www.quora.com/Why-do-Earth-faults-occur-in-a-transformer-When-the-transformer-
neutral-is-grounded-this-fault-trips-the-HT-breaker-Are-there-any-solutions-to-get-rid-of-this
23. http://electrical-engineering-portal.com/5-transformer-fault-conditions
24. http://ecetutorials.com/transformer/faults-and-protections-in-the-transformer/
25. http://engineering.electrical-equipment.org/electrical-distribution/delta-wye-transformer-phase-
phase-fault.html
26. Electrical Machine Design by A.K. Sawhney
44

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