Transformer

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EEE-233
Electrical Machines-1
Lecture 2-4
Construction

2. 2
TRANSFORMER CONSTRUCTION

3. 3
TRANSFORMER CONSTRUCTION
• The construction of transformers varies greatly, depending on their
applications, winding voltage and current ratings and operating frequencies.
i. The core is made of silicon steel having low hysteresis loss and high
permeability.
ii. Core is laminated and electrically separated by a thin coating of insulating
material in order to reduce eddy current loss and the no-load current.
iii. Instead of placing primary on one limb and secondary on the other one-half
of each winding is wound on one limb to ensure tight coupling between the two
windings and reduce flux leakage.
iv. The winding resistances R1 and R2 are minimized to reduce I2R loss, reduce
winding temperature and to ensure high efficiency.

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TRANSFORMER CONSTRUCTION
 CORE CONSTRUCTION
• The iron core is made of
thin laminated silicon steel
(2-3 % silicon)
• Pre-cut insulated sheets
are cut or pressed in form
and placed on the top of
each other.
• The sheets overlap each
others to avoid (reduce) air
gaps.

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Circular core :
 If the area Ai is 10cm2,
 The diameter of the core
=3.56cm and
 Circumference
= πx3.56=11.2 cm
Square core :
 For Ai = 10cm2,
 Side of the square
= 3.16 cm
 Perimeter
= 4x3.16=12.64cm
Rectangular core:
 For Ai = 10 cm2 ,
 Sides of the rectangle
to be 10cm and 1.0cm
 Perimeter =
(10+1)2=22cm
• Which type of core is preferable for transformer design? And why?
 CHOICE OF CORE SECTIONS
TRANSFORMER CONSTRUCTION

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Another reason….
 Very high value of mechanical forces tries to deform the shape of the square or
rectangular coil (the mechanical forces try to deform to a circular shape) and hence
damage the coil and insulation.
 Since this is not so in case of circular coils, circular coils are preferable to square or
rectangular coils.
TRANSFORMER CONSTRUCTION
 CHOICE OF CORE SECTIONS

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TRANSFORMER CONSTRUCTION
 Thus a circular core and a circular coil is preferable.
 Since the core has to be of laminated type, circular core is not practicable as it required more
number of different size laminations and poses the problem of securing them together is in
position.
 However, a circular core can be approximated to a stepped core having different number of
steps.
 In practice the core is built of 0.35 mm thin strips arrange in a number of steps.
 By increasing the no of steps, the area of the circumscribing circle is more effectively utilized.
Laminated
circular core Three stepped core Four stepped core
Square core
 CHOICE OF CORE SECTIONS

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TRANSFORMER CONSTRUCTION
• In rectangular and square cores the diameter of the circumscribing circle is larger
than the diameter of stepped cores of same area of cross-section.
• When stepped cores are used the length of mean turn of winding is reduced with
consequent reduction in both cost of copper and copper loss.
• However with larger number of steps a large number of different sizes of
laminations have to be used. This results in higher labor charges for shearing and
assembling different types of laminations.
 CHOICE OF CORE SECTIONS

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TRANSFORMER CONSTRUCTION
 TRANSFORMER WINDING CONSTRUCTION
• The winding is made of copper or aluminum conductor, insulated with paper or
synthetic insulating material.
• The windings are manufactured in several layers, and insulation is placed
between windings.
• The windings are usually arranged concentrically around the core leg.

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TRANSFORMER CONSTRUCTION
 TRANSFORMER WINDING CONSTRUCTION
• Concentric coils are typically wound over cylinders with spacers attached so as
to form a duct between the conductors and the cylinder.
• The flow of liquid through the windings is based on natural convection, and is
controlled by using the barriers placed within the winding.

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TRANSFORMER CONSTRUCTION
 TRANSFORMER WINDING CONSTRUCTION
• The primary and secondary windings
in a physical transformer are wrapped
one on top of the other with the low-
voltage winding innermost. Such an
arrangement serves two purposes:
1. It simplifies the problem of
insulating the high-voltage winding
from the core.
2. It results in much less leakage flux
than would be the case if the two
windings were separated by a
distance on the core.

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TRANSFORMER CONSTRUCTION
 ACCESSORY EQUIPMENT
• The dried and treated transformer is placed in a
steel tank.
• The tank is filled, under vacuum, with heated
transformer oil.
• The end of the windings are connected to bushings.
• Large bushings connect the windings to the
electrical system.
• The transformer is equipped with cooling radiators
which are cooled by forced ventilation
• Cooling fans are installed under the radiators.
• The oil is circulated by pumps and forced through
the radiators.
• The oil temperature, Pressure are monitored to
predict transformer performance.

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TRANSFORMER CONSTRUCTION
 ACCESSORY EQUIPMENT

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TRANSFORMER CONSTRUCTION
 ACCESSORY EQUIPMENT
• Liquid-Level Indicator: A liquid-level indicator is a standard feature on liquid-filled
transformer tanks, since the liquid medium is critical for cooling and insulation.
• Pressure-Relief Devices: Pressure-relief devices are mounted on transformer
tanks to relieve excess internal pressures that might build up during operating
conditions.
• Liquid-Temperature Indicator: Liquid-temperature indicators measure the
temperature of the internal liquid.
• Winding-Temperature Indicator: A winding-temperature simulation method is used
to approximate the hottest spot in the winding.

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TRANSFORMER CONSTRUCTION
 ACCESSORY EQUIPMENT
• Sudden-Pressure Relay: A sudden- (or rapid-) pressure relay is intended to
indicate a quick increase in internal pressure that can occur when there is an internal
fault.
• Desiccant (Dehydrating) Breathers: Desiccant breathers use a material such as
silica gel to allow air to enter and exit the tank, removing moisture as the air passes
through.
• Liquid-Preservation Systems: Liquid-temperature indicators measure the
temperature of the internal liquid.

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TRANSFORMER CONSTRUCTION
 LIQUID-PRESERVATION SYSTEMS
• Preservation systems isolates the
transformer’s internal environment from the
external environment.
• Conservator system involve the use of a
separate auxiliary tank.
• The auxiliary tank is allowed to “breathe,”
usually through a dehydrating breather
• Buchholz Relay: Its purpose is to detect
faults that may occur in the transformer
resulting in generation of gases.

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 TYPES OF TRANSFORMERS
• The manner in which the primary and secondary are wound on the core,
transformers are of two types
a) Core-type and
b) Shell-type
• Depending on the application, transformers can be classified as
a) Power transformers.
b) Distribution transformers and
TRANSFORMER CONSTRUCTION

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TRANSFORMER CONSTRUCTION
 CORE TYPE TRANSFORMER
• In a core-type transformer, there is a single path for the magnetic circuit.
• Each limb carries one half of primary and secondary windings. The two
windings are closely coupled together to reduce the leakage reactance.

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TRANSFORMER CONSTRUCTION
 SHELL TYPE TRANSFORMER
• In shell type transformers the windings are put around the central limb and
the flux path is completed through two side limbs.
• The central limb carries total mutual flux while the side limbs forming a part
of a parallel magnetic circuit carry half the total flux.
• The cross sectional area of the central limb is twice that of each side limbs.

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TRANSFORMER CONSTRUCTION
 COMPARISON OF CORE & SHELL TYPE TRANSFORMERS
Sr.
No
Core Type Transformer Shell Type Transformer
1. The core has only one window. The core has two windows.
2. Winding encircles the core. Core encircles the windings.
3. Cylindrical windings are used. Sandwich type windings are used.
4. Easy to repair. It is not so easy to repair.
5. Better cooling since more
surface is exposed to the
atmosphere.
Cooling is not very effective.

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TRANSFORMER CONSTRUCTION
 CORE AND SHELL TYPE TRANSFORMER

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TRANSFORMER CONSTRUCTION
 POWER TRANSFORMER
• The transformers used in sub-stations and generating stations are called
power transformers. They have ratings above 200kVA. Usually a substation will
have number of transformers working in parallel.
• During heavy load periods all the transformers are put in operation and
during light load periods some transformers are disconnected.
• Therefore the power transformers should be designed to have maximum
efficiency at or near full load.
• Power transformers are designed to have considerably greater leakage
reactance than that is permissible in distribution transformers in order to limit
the fault current.
• In the case of power transformers inherent voltage regulation is less
important than the current limiting effect of higher leakage reactance.

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TRANSFORMER CONSTRUCTION
 DISTRIBUTION TRANSFORMER
• Transformers up to 200kVA those are used to step down distribution voltage
to a standard service voltage or from transmission voltage to distribution
voltage are known as distribution transformers.
• They are kept in operation all the 24 hours a day whether they are carrying
any load or not.
• The load on the distribution transformer varies from time to time and the
transformer will be on no-load most of the time.
• Hence, in distribution transformer the copper loss (which depends on load)
will be more when compared to core loss (which occurs as long as transformer
is in operation).
• Therefore, distribution transformers are designed with less iron loss and
designed to have the maximum efficiency at a load much lesser than full load.
• Also it should have good regulation to maintain the variation of supply
voltage within limits and so it is designed with small value of leakage
reactance.

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 POWER AND DISTRIBUTION TRANSFORMERS
TRANSFORMER CONSTRUCTION

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TRANSFORMER CONSTRUCTION
 COOLING OF TRANSFORMERS
• Heat is produced in a transformer by the iron losses in the core and Cu-loss (I2R
loss) in the windings. To prevent undue temperature rise, this heat is removed by
cooling.
• The heat dissipation in transformer occurs by Conduction, Convection and
Radiation.
• The paths of heat flow in transformer are the following
1. From internal most heated spots of a given part (of core or winding) to their
outer surface in contact with the oil.
2. From the outer surface of a transformer part to the oil that cools it.
3. From the oil to the walls of a cooler, eg. Wall of tank.
4. From the walls of the cooler to the cooling medium air or water.
• In the path 1 mentioned above heat is transferred by conduction. In the path 2
and 3 mentioned above heat is transferred by convection of the oil. In path 4 the
heat is dissipated by both convection and radiation.

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TRANSFORMER CONSTRUCTION
 COOLING OF TRANSFORMERS
• In small transformers (< 50 kVA), natural air cooling is sufficient
• Medium size power or distribution transformers are cooled by housing them in
tanks filled with oil.
• For large transformers, external radiators are added to increase the cooling
surface of the oil filled tank.
• The oil circulates around the transformer and moves through the radiators
where the heat is released to surrounding air.
• Sometimes cooling fans blow air over the radiators to accelerate the cooling
process.

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TRANSFORMER CONSTRUCTION
 COOLING OF TRANSFORMERS

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TRANSFORMER CONSTRUCTION
 COOLING OF TRANSFORMERS

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TRANSFORMER CONSTRUCTION
 COOLING OF TRANSFORMERS

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