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25-05-2012, 03:08 PM
Post: #1
transformer PPT
transformer



.ppt  condition_monitoring_transformers.ppt (Size: 1.02 MB / Downloads: 435)

Working principle of a transformer


A transformer is static (or stationary) piece of apparatus which:
1.Transfers electric power from one circuit to another.
2.It does so without a change in frequency.
3.The principle is based on mutual induction between two circuits linked by a common magnetic flux.


Basic parts of a transformer


Basically a transformer consists o f a :
1.A primary coil or winding.
2.A secondary coil or winding.
3.A core that supports the coils or the windings


Transformer construction


Main constructional elements of Transformers are -

A) Magnetic circuit Core & clamping structure

B) Electric Circuit Winding,Insulation, Bracing devices.

C) Terminals Tapping, Tapping switches, Terminal
Insulator, Leads , Bushings

D) Tank Oil, Cooling devices, conservator, piping,
Breather

E) Protective Circuit Buchholz relay, WTI, OTI, Oil surge relay
& Monitoring pressure relief device, MOG


Transformer construction


A)Magnetic Circuit:
The core provides closed path for flux. It is made up of CRGO insulated laminations. (CRGO has iron loss of about 1.3 W / Kg at 1.6 Tesla )

B)Electric Circuit:
Winding, insulation & bracing are constructional parts of electrical circuit of transformers. This is the most vulnerable part of transformer because of direct association with power system. Must be designed to withstand voltage stress resulting from system fault, transient over voltage and thermal stresses (lightening or switching surges)
02-07-2012, 12:15 PM
Post: #2
RE: transformer PPT
Transformers


.pdf  Transformers.pdf (Size: 2.08 MB / Downloads: 224)
Introduction

Michael Faraday propounded the principle of electro-magnetic induction in 1831.
It states that a voltage appears across the terminals of an electric coil when the flux linked
with the same changes. The magnitude of the induced voltage is proportional to the rate of
change of the flux linkages. This finding forms the basis for many magneto electric machines.
The earliest use of this phenomenon was in the development of induction coils. These coils
were used to generate high voltage pulses to ignite the explosive charges in the mines. As
the d.c. power system was in use at that time, very little of transformer principle was made
use of. In the d.c. supply system the generating station and the load center have to be
necessarily close to each other due to the requirement of economic transmission of power.
Also the d.c. generators cannot be scaled up due to the limitations of the commutator.

Basic Principles

As mentioned earlier the transformer is a static device working on the principle of
Faraday’s law of induction. Faraday’s law states that a voltage appears across the terminals
of an electric coil when the flux linkages associated with the same changes. This emf is
proportional to the rate of change of flux linkages. Putting mathematically.

Constructional features

Transformers used in practice are of extremely large variety depending upon the
end use. In addition to the transformers used in power systems, in power transmission and
distribution, a large number of special transformers are in use in applications like electronic
supplies, rectification, furnaces, traction etc. Here the focus is on power transformers only.
The principle of operation of these transformers also is the same but the user requirements
differ. Power transformers of smaller sizes could be air cooled while the larger ones are
oil cooled. These machines are highly material intensive equipments and are designed to
match the applications for best operating conditions. Hence they are ‘tailor made’ to a
job. This brings in a very large variety in their constructional features. Here more common
constructional aspects alone are discussed.

Ideal Transformer

Earlier it is seen that a voltage is induced in a coil when the flux linkage associated
with the same changed. If one can generate a time varying magnetic field any coil placed in
the field of influence linking the same experiences an induced emf. A time varying field can
be created by passing an alternating current through an electric coil. This is called mutual
induction. The medium can even be air. Such an arrangement is called air cored transformer.
Indeed such arrangements are used in very high frequency transformers. Even though the
principle of transformer action is not changed, the medium has considerable influence on the
working of such devices.

Practical Transformer

An ideal transformer is useful in understanding the working of a transformer. But it
cannot be used for the computation of the performance of a practical transformer due to the
non-ideal nature of the practical transformer. In a working transformer the performance as-
pects like magnetizing current, losses, voltage regulation, efficiency etc are important. Hence
the effects of the non-idealization like finite permeability, saturation, hysteresis and winding
resistances have to be added to an ideal transformer to make it a practical transformer.
Conversely, if these effects are removed from a working transformer what is left behind is an
ideal transformer.

Testing of Transformers

The structure of the circuit equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by solving that circuit for any load
conditions. The equivalent circuit parameters are available to the designer of the transformers
from the various expressions that he uses for designing the transformers. But for a user
these are not available most of the times. Also when a transformer is rewound with different
primary and secondary windings the equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily depended upon. From the analysis of
the equivalent circuit one can determine the electrical parameters. But if the temperature
rise of the transformer is required, then test method is the most dependable one. There are
several tests that can be done on the transformer; however a few common ones are discussed
here.
09-08-2012, 04:13 PM
Post: #3
RE: transformer PPT
Transformers



.docx  1Transformers.docx (Size: 21.13 KB / Downloads: 59)

They are used to change the amplitude of high frequency pulses by the turns ratio and to provide isolation betweencircuits.Its main parts are:-

Core:

Manufactured from laminations of CRGO(ColdRolled Grain Oriented)Silicon Steel. It gives very low specific loss at operating flux densities.
Windings:-They are made from paper insulated copperconductors which are transposed at regularintervalsthroughout the winding. It provides equal flux linkage andcurrent distribution between strands.
Bushing:-
They use a maximum amount of conductormaterial to provide the required shape and physicalstrength, and use a minimum amount of insulatingmaterial to provide only the insulating function.
Pressure Relief Valve:-
Transformers are provided with pressure relief valve which operates in case of suddenpressure formation inside the transformer.
Cooling System:-
Transformers are provided withcooling bank. These are provided with fans and pumps.These fans and pumps automatically operate, dependingupon the settings of winding temperature .

Protective Relays .

Current Transformers

A current transformer (CT) is used for the measurement of electric currents.
When current in a circuit is too high to directly apply to measuring instruments, a currenttransformer produces a reduced currentaccurately proportional to the current in thecircuit, which can be connected to measuringand recording instruments.

Voltage Transformers

Voltage transformers (VT) are another type of instrumenttransformer, used for metering and protection in high- voltage circuits.
They are designed to present negligible load to the supply being measured and to have a precise voltage ratio toaccurately step down high voltages so that metering andprotective relay equipment can be operated at a lowerpotential.
V Ts are typically used for higher voltages (for example,765 k V
for power transmission), or where isolation isdesired between the meter and the measured circuit.

Relays

A relay is an electrically operated switch.
These are used to protect electrical circuits from overloador faults.
Relays find applications where it is necessary to control acircuit by a low-power signal, or where several circuitsmust be controlled by one signal.

Operation

When an electric current is passed through the coil, theresulting magnetic field attracts the armature.
The consequent movement of the movable contacts eithermakes or breaks a connection with a fixed contact.
If the set of contacts were closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts wereopen.
W hen the current to the coil is switched off, the armature isreturned by a force, approximately half as strong as themagnetic force, to its relaxed position.
This force is provided by a spring.

Circuit Breaker

A circuit breaker is an automatically-operated electricalswitch.
It is designed to protect an electrical circuit from damagecaused by overload or short circuit.
Its basic function is to detect a fault condition, interruptthe continuity, and immediately discontinue electricalflow.

Operation of a C.B.

Circuit breakers for large currents or high voltages areusually arranged with pilot devices.
It senses a fault current and operate the trip openingmechanism.
The trip solenoid that releases the latch is usually energized by a separate battery.
Once a fault is detected, contacts within the circuit breakeropen to interrupt the circuit.
Some mechanically-stored energy (using something suchas springs or compressed air) contained within the breakeris used to separate the contacts.
Circuit breakers have solenoids to trip the mechanism, andelectric motors to restore energy to the springs.
The circuit breaker contacts carry the load current without excessive heating, and also withstand the heat of the arc produced when interrupting the circuit.
Contacts are made of copper or copper alloys, silveralloys, and other materials.
When a current is interrupted, an arc is generated.
This arc is contained, cooled, and extinguished in acontrolled way, so that the gap between the contacts canagain withstand the voltage in the circuit.
Once the fault condition has been cleared, the contacts areagain closed to restore power to the interrupted circuit.
10-08-2012, 10:44 AM
Post: #4
RE: transformer PPT
Transformers


.pdf  ELECTRICALMACHINES-I.pdf (Size: 2.08 MB / Downloads: 64)

Introduction

Michael Faraday propounded the principle of electro-magnetic induction in 1831.
It states that a voltage appears across the terminals of an electric coil when the flux linked
with the same changes. The magnitude of the induced voltage is proportional to the rate of
change of the flux linkages. This finding forms the basis for many magneto electric machines.
The earliest use of this phenomenon was in the development of induction coils. These coils
were used to generate high voltage pulses to ignite the explosive charges in the mines. As
the d.c. power system was in use at that time, very little of transformer principle was made
use of. In the d.c. supply system the generating station and the load center have to be
necessarily close to each other due to the requirement of economic transmission of power.
Also the d.c. generators cannot be scaled up due to the limitations of the commutator. This
made the world look for other efficient methods for bulk power generation and transmis-
sion. During the second half of the 19th century the alternators, transformers and induction
motors were invented. These machines work on alternating power supply. The role of the
transformers became obvious. The transformer which consisted of two electric circuits linked
by a common magnetic circuit helped the voltage and current levels to be changed keeping
the power invariant.

Constructional features

Transformers used in practice are of extremely large variety depending upon the
end use. In addition to the transformers used in power systems, in power transmission and
distribution, a large number of special transformers are in use in applications like electronic
supplies, rectification, furnaces, traction etc. Here the focus is on power transformers only.
The principle of operation of these transformers also is the same but the user requirements
differ. Power transformers of smaller sizes could be air cooled while the larger ones are
oil cooled. These machines are highly material intensive equipments and are designed to
match the applications for best operating conditions. Hence they are ‘tailor made’ to a
job. This brings in a very large variety in their constructional features. Here more common
constructional aspects alone are discussed. These can be broadly divided into

1. Core construction
2. Winding arrangements
3. Cooling aspects
3.1 Core construction

Transformer core for the power frequency application is made of highly permeable
material. The high value of permeability helps to give a low reluctance for the path of
the flux and the flux lines mostly confine themselves to the iron. Relative permeability μr
well over 1000 are achieved by the present day materials. Silicon steel in the form of thin
laminations is used for the core material. Over the years progressively better magnetic prop-
erties are obtained by going in for Hot rolled non-oriented to Hot rolled grain oriented steel.

Insulation

The insulation used in the case of electrical conductors in a transformer is varnish
or enamel in dry type of transformers. In larger transformers to improve the heat transfer
characteristics the conductors are insulated using un-impregnated paper or cloth and the
whole core-winding assembly is immersed in a tank containing transformer oil. The trans-
former oil thus has dual role. It is an insulator and also a coolant. The porous insulation
around the conductor helps the oil to reach the conductor surface and extract the heat. The
conductor insulation may be called the minor insulation as the voltage required to be with-
stood is not high. The major insulation is between the windings. Annular bakelite cylinders
serve this purpose. Oil ducts are also used as part of insulation between windings. The oil
used in the transformer tank should be free from moisture or other contamination to be of
any use as an insulator.

Cooling of transformers

Scaling advantages make the design of larger and larger unit sizes of transformers
economically attractive. This can be explained as below. Consider a transformer of certain
rating designed with certain flux density and current density. If now the linear dimensions
are made larger by a factor of K keeping the current and flux densities the same the core and
conductor areas increase by a factor of K2. The losses in the machine, which are proportional
to the volume of the materials used, increase by a factor of K3.The rating of the machine
increases by a factor of K4.
27-01-2013, 01:17 PM
Post: #5
RE: transformer PPT
i want constructional features and working of transformer used in distribution station for ppt seminar topics
Rating transformer PPT Options
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