Approximately all the details about transformer, it's type and cooling methods of transformer.

1. Urooj Abid (syedauroojabid@gmail.com)
NED University Of Engineering & Technology
K-Electric Internship

2. “Transformer” is one of oldest
innovations in Electrical
Engineering. A Transformer is an
electrical device that can be used
to transfer the power from one
circuit and another circuit without
physical contact and without
changing its characteristics like
frequency, phase. It is an
essential device in every
electrical network circuitry. It
consists majorly two circuits,
namely primary circuit and
secondary circuit.

4. 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.
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.

5. Core Type Transformer
In this type of transformer, the windings
are given to the considerable part of the
circuit in the core type of the transformer.
The coils used are of form-wound and
cylindrical type on the core type. In core
type transformer, the coils are wounded in
helical layers with different layers
insulated from each other.
Shell Type Transformer
Shell type transformers are the most
popular and efficient type of transformers.
The shell type transformer has a double
magnetic circuit. The core has three limbs
and both the winding are placed on the
central limbs. The core encircles most
parts of the winding. Generally multi-layer
disc and sandwich coils are used in shell
type.

6. Single winding (auto
transformer)
An Auto Transformer is a
transformer with only one winding
wound on a laminated core. An
auto transformer is similar to a two
winding transformer but differ in
the way the primary and
secondary winding are
interrelated. A part of the winding
is common to both primary and
secondary sides. On load
condition, a part of the load
current is obtained directly from
the supply and the remaining part
is obtained by transformer action.
An Auto transformer works as
a voltage regulator.
Three winding (power
transformer)
Three Winding Transformer may be
built with a third winding, called the
tertiary, in addition to the primary
and secondary. This third winding is
knows as stabilizer winding. The
voltage ratings of all the three
windings of the transformer are
usually unequal. The primary
winding has the highest voltage
rating; the tertiary has the lowest
voltage rating, and the secondary
has the intermediate voltage rating.
Two winding
(ordinary transformer)
An Ordinary Transformer consists
of two windings called primary
winding and secondary winding.
These two windings are
magnetically coupled and
electrically isolated. But the
transformer in which apart of
windings is common to both
primary and secondary is called
Autotransformer. The
autotransformer is both the most
simple and the most fascinating of
the connections involving
two windings. It is used quite
extensively in bulk power
transmission systems because of
its ability to multiply the effective
KVA capacity of a transformer.
Autotransformers are also used on
radial distribution feeder circuits as
voltage regulators.

7. o It is used as a starter to give up to
50 to 60% of full voltage to the
stator of a squirrel cage induction
motor during starting.
o It is used to give a small boost to a
distribution cable, to correct the
voltage drop.
o It is also used as a voltage
regulator
o Used in power transmission and
distribution system and also in the
audio system and railways.
(Based on third winding)
o To supply the substation auxiliaries at
a voltage different from those of the
primary and secondary windings.
o Static capacitors or synchronous
condensers may be connected to the
tertiary winding for reactive power
injection into the system for voltage
control.
o A delta-connected tertiary reduces
the impedance offered to the zero
sequence currents thereby allowing a
larger earth-fault current to flow for
proper operation of protective
equipment. Further, it limits voltage
imbalance when the load is
unbalanced. It also permits the third
harmonic current to flow thereby
reducing third-harmonic voltages.
o Three windings may be used for
interconnecting three transmission
lines at different voltages.
o Tertiary can serve the purpose of
measuring voltage of an HV testing
transformer.
o Two-winding transformers are typically
used to step up or down voltage from
the transmission grid to the distribution
grid. When the high and low voltage is
greater than 2, it is more cost-effective
to use a two-winding transformer. Auto
transformers are preferred when the
ratio is less than 2.

8. Cylindrical type
Cylindrical winding is use in core type transformer
because these are low voltage windings used up to 6.6
kV for kVA up to 600-750, and current rating between 10
to 600 A. We often use cylindrical windings in its multi-
layer forms. We use rectangular conductors in two-
layered type because it is easy to secure the lead-out
ends. Oil ducts separate the layers of the windings this
arrangement facilitates the cooling through oil circulation
in the winding. In multi-layered cylindrical windings, we
use circular conductors, wound on vertical strips to
improve cooling conditions. The arrangement creates oil
ducts to facilitate better cooling. We use this types of
winding for high voltage ratings up to 33 kV, 800 kVA
and current ratings up to 80 A. The maximum diameter
we use for a bare conductor is 4 mm.
Disc type
Disc type windings are also use in core type
transformers called Disc-Helical Windings, the parallel
connected strips are placed side by side in a radial
direction to occupy total radial depth of 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 center outwards in the
radial direction. 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.

9. Power transformer
Power transformers are generally
used in transmission network for
stepping up or down the voltage
level. It operates mainly during high
or peak loads and has maximum
efficiency at or near full load.
Distribution Transformers​ steps
down the voltage for distribution
purpose to domestic or commercial
users. It has good voltage
regulation and operates 24 hrs. a
day with maximum efficiency at
50% of full load. Power Transformer
is generally rated at Higher
MVA(>200MVA)and it is used in
step-up and step down application
in Transmission
Instrument transformer
Instrument Transformers are used
in AC system for measurement of
electrical quantities i.e. voltage,
current, power, energy, power
factor, frequency. Instrument
transformers are also used with
protective relays for protection of
power system. Basic function of
Instrument transformers is to step
down the AC System voltage and
current. The voltage and current
level of power system is very high.
It is very difficult and costly to
design the measuring instruments
for measurement of such high level
voltage and current. Generally
measuring instruments are
designed for 5 A and 110 V. The
measurement of such very large
electrical quantities, can be made
possible by using the Instrument
transformers with these small rating
measuring instruments. Therefore
these instrument transformers are
very popular in modern power
system.
Distribution transformer
A distribution transformer is the type
of transformer that performs the
last voltage transformation in
a distribution grid. It converts the
voltage used in the transmission lines
to one suitable for household and
commercial use, typically down to
240 volts. These transformers
are pole-mounted if the transmission
lines run above ground and they
are pad-mounted, if the transmission
lines lie underground, as is the case in
many suburban areas. The pad-
mounted transformer is usually green
and visible on the front lawns of many
homes.

11. Air Natural (AN)
This method of transformer
cooling is generally used in
small transformers (up to 3
MVA). In this method the
transformer is allowed to cool
by natural air flow surrounding
it.
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.

12. Oil Natural Air Natural (ONAN)
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 convection
and dissipating heat in atmosphere due to natural
conduction. This method can be used for transformers
up to about 30 MVA.
Oil Natural Air Forced (ONAF)
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
up to about 60 MVA.

13. Oil Forced Air Forced (OFAF)
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.
Oil Forced Water Forced (OFWF)
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.

15. Percentage reactance of a transformer (or in
general, a circuit) is the percentage of phase
voltage drop when full load current flows through it,
i.e.
%X=(IX/V)*100.
Now Short Circuit Current is V/X
So short Circuit current is I*(100/%X).
Percentage reactance of a transformer (or in
general, a circuit) is the percentage of phase
voltage drop when full load current flows through it,
i.e. %X=(IX/V)*100. Now Short Circuit Current is
V/X So short Circuit current is I*(100/%X).

17. The ratio of the actual electrical power dissipated by an AC
circuit to the product of the r.m.s. values of current and
voltage. The difference between the two is caused by
reactance in the circuit and represents power that does no
useful work. In AC circuits, the power factor is the ratio of
the real power that is used to do work and the apparent
power that is supplied to the circuit.
The power factor can get values in the range from 0 to 1.
When all the power is reactive power with no real power
(usually inductive load) - the power factor is 0.
When all the power is real power with no reactive power
(resistive load) - the power factor is 1.

19. Active Power
The power which is
actually consumed
or utilized in an AC
Circuit is called True
power or Active
Power or real power.
It is measured in kilo
watt (kW) or MW. It
is the actual
outcomes of the
electrical system
which runs the
electric circuits or
load.
Reactive Power
The power which
flows back and froth
that mean it moves in
both the direction in
the circuit or react
upon itself, is called
Reactive Power. The
reactive power is
measured in kilo volt
ampere reactive
(kVAR) or MVAR.
Apparent Power
The product of root
mean square (RMS)
value of voltage and
current is known as
Apparent Power.
This power is
measured in kVA or
MVA.

20. Thank You