DONE BY MY COLLEAGUES IN COLLEGE BSEE-5

1. Prepared by:
DIANA ROSE L. MUYNA

2. Transformer
A transformer can be defined as a static
device which helps in the
transformation of electric power in one
circuit to electric power of the same
frequency in another circuit. The
voltage can be raised or lowered in a
circuit, but with a proportional increase
or decrease in the current ratings.

3. 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.

5. A transformer carries the
operations shown below:
1. Transfer of electric power from one circuit to
another.
2. Transfer of electric power without any change in
frequency.
3. Transfer with the principle of electromagnetic
induction.
4. The two electrical circuits are linked by mutual
induction.

6. Transformer Construction
For the simple construction of a transformer, you
must need two coils having mutual inductance
and a laminated steel core. The two coils are
insulated from each other and from the steel
core. The device will also need some suitable
container for the assembled core and windings,
a medium with which the core and its windings
from its container can be insulated.
In order to insulate and to bring out the
terminals of the winding from the tank, apt
bushings that are made from either porcelain
or capacitor type must be used.

7. In all transformers that are used commercially, the
core is made out of transformer sheet steel
laminations assembled to provide a continuous
magnetic path with minimum of air-gap included.
The steel should have high permeability and low
hysteresis loss. For this to happen, the steel should
be made of high silicon content and must also be
heat treated. By effectively laminating the core, the
eddy-current losses can be reduced. The
lamination can be done with the help of a light coat
of core plate varnish or lay an oxide layer on the
surface. For a frequency of 50 Hertz, the thickness
of the lamination varies from 0.35mm to 0.5mm for
a frequency of 25 Hertz.

8. GENERAL CLASSIFICATION
Type of magnetic circuit
Number of phases
Arrangement of windings
Methods of cooling
Type of service
Special features of construction

9. Types of Magnetic Circuit
“Laminated’’, or “Punched” type
- type involves stacking individually punched
sheet-steel laminations

10. “Ribbon-wound” type
- made up of continuous strips steel wound into a
tight coil.

11. Number of Phases
A three-phase transformer generally has the
three magnetic circuits that are interlaced to
give a uniform distribution of the dielectric
flux between the high and low voltage
windings. The exception to this rule is a
three-phase shell type transformer. In the
shell type of construction, even though the
three cores are together, they are non-
interlaced.

12. The three-limb core-type three-phase
transformer is the most common method of
three-phase transformer construction
allowing the phases to be magnetically
linked. Flux of each limb uses the other two
limbs for its return path with the three
magnetic flux's in the core generated by the
line voltages differing in time-phase by 120
degrees. Thus the flux in the core remains
nearly sinusoidal, producing a sinusoidal
secondary supply voltage.

13. The shell-type five-limb type three-phase
transformer construction is heavier and
more expensive to build than the core-type.
Five-limb cores are generally used for very
large power transformers as they can be
made with reduced height. A shell-type
transformers core materials, electrical
windings, steel enclosure and cooling are
much the same as for the larger single-phase
types.

14. Arrangement of Windings
a. Shell-type Transformers
In shell-type transformers the core surrounds a
considerable portion of the windings. The comparison is
shown in the figure below.

15. The coils are form-wound but are multi layer
disc type usually wound in the form of
pancakes. Paper is used to insulate the
different layers of the multi-layer discs. The
whole winding consists of discs stacked with
insulation spaces between the coils. These
insulation spaces form the horizontal
cooling and insulating ducts. Such a
transformer may have the shape of a simple
rectangle or may also have a distributed
form.

16. Both designs are shown in the figure below:

17. A strong rigid mechanical bracing must
be given to the cores and coils of the
transformers. This will help in
minimizing the movement of the device
and also prevents the device from
getting any insulation damage. A
transformer with good bracing will not
produce any humming noise during its
working and will also reduce vibration.

18. Core- Type Transformers
In core-type transformer, the windings
are given to a considerable part of the core.
The coils used for this transformer are form-
wound and are of cylindrical type. Such a
type of transformer can be applicable for
small sized and large sized transformers. In
the small sized type, the core will be
rectangular in shape and the coils used are
cylindrical. The figure below shows the large
sized type.

19. You can see that the round or cylindrical coils are
wound in such a way as to fit over a cruciform
core section. In the case of circular cylindrical
coils, they have a fair advantage of having good
mechanical strength. The cylindrical coils will
have different layers and each layer will be
insulated from the other with the help of
materials like paper, cloth, micarta board and so
on. The general arrangement of the core-type
transformer with respect to the core is shown
below. Both low-voltage (LV) and high voltage
(HV) windings are shown.

20. The low voltage windings
are placed nearer to the core
as it is the easiest to insulate.
The effective core area of the
transformer can be reduced
with the use of laminations
and insulation.

21. Methods of Cooling
ONAN Cooling of Transformer
This is the simplest transformer cooling
system. The full form of ONAN is "Oil Natural Air
Natural". Here natural convectional flow of hot oil
is utilized for cooling. In convectional circulation
of oil, the hot oil flows to the upper portion of the
transformer tank and the vacant place is occupied
by cold oil. This hot oil which comes to upper side,
will dissipate heat in the atmosphere by natural
conduction, convection & radiation in air and will
become cold. In this way the oil in the transformer
tank continually circulate when the transformer
put into load.

22. As the rate of dissipation of heat in air
depends upon dissipating surface of the oil
tank, it is essential to increase the effective
surface area of the tank. So additional
dissipating surface in the form of tubes or
radiators connected to the transformer tank.
This is known as radiator of transformer or
radiator bank of transformer. We have
shown below a simplest form on Natural
Cooling or ONAN Cooling arrangement of an
earthing transformer below.

24. ONAF Cooling of Transformer
Heat dissipation can obviously be increased, if
dissipating surface is increased but it can be make
further faster by applying forced air flow on that
dissipating surface. Fans blowing air on cooling
surface is employed. Forced air takes away the heat
from the surface of radiator and provides better
cooling than natural air. The full form of ONAF is
"Oil Natural Air Forced". As the heat dissipation
rate is faster and more in ONAF transformer
cooling method than ONAN cooling
system, electrical power transformer can be put
into more load without crossing the permissible
temperature limits.

26. OFAF Cooling of Transformer
In Oil Forced Air Natural cooling system
of transformer, the heat dissipation is
accelerated by using forced air on the
dissipating surface but circulation of the hot
oil in transformer tank is natural
convectional flow. The heat dissipation rate
can be still increased further if this oil
circulation is accelerated by applying some
force. In OFAF cooling system the oil is
forced to circulate within the closed loop of
transformer tank by means of oil pumps.

27. OFAF means "Oil Forced Air Forced" cooling
methods of transformer. The main advantage of
this system is that it is compact system and for
same cooling capacity OFAF occupies much less
space than farmer two systems of transformer
cooling. Actually in Oil Natural cooling system,
the heat comes out from conducting part of the
transformer is displaced from its position, in
slower rate due to convectional flow of oil but
in forced oil cooling system the heat is
displaced from its origin as soon as it comes out
in the oil, hence rate of cooling becomes faster.

29. OFWF Cooling of Transformer
We know that ambient temperature of
water is much less than the atmospheric air
in same weather condition. So water may be
used as better heat exchanger media than
air. In OFWF cooling system of transformer,
the hot oil is sent to a oil to water heat
exchanger by means of oil pump and there
the oil is cooled by applying sowers of cold
water on the heat exchanger's oil pipes.
OFWF means "Oil Forced Water Forced"
cooling in transformer.

30. ODAF Cooling of Transformer
ODAF or Oil Directed Air Forced Cooling of
Transformer can be considered as the improved
version of OFAF. Here forced circulation of oil directed
to flow through predetermined paths in transformer
winding. The cool oil entering the transformer tank
from cooler or radiator is passed through the winding
where gaps for oil flow or pre-decided oil flowing paths
between insulated conductor are provided for ensuring
faster rate of heat transfer. ODAF or Oil Directed Air
Forced Cooling of Transformer is generally used in very
high rating transformer.

31. ODWF Cooling of Transformer
ODAF or Oil Directed Water Forced
Cooling of Transformer is just like ODAF
only difference is that here the hot oil is
cooled in cooler by means of forced water
instead of air. Both of these transformer
cooling methods are called Forced Directed
Oil Cooling of transformer

32. Instrument Transformer
- may be classified as metering and relay transformers,
and may be ether current or potential transformers.
- are used for two reasons: (1) to protect station
operators from contact with high-voltage circuits and,
(2) to permit the use of instruments with a reasonable
amount of insulation and a reasonable current-
carrying capacity.
- the function of the instrument transformers is to
deliver to the instruments a current and voltage that
shall always be proportional to the primary current
and voltage and that does not exceed a safe potential
above ground.

33. a. Voltage transformers
- used with voltmeters, watt
meters, watt-hour meters,
power factor meters, frequency
meters, synchroscopes and
synchronizing apparatus,
protective and regulating
relays, and the no-voltage and
over-voltage trip coils of
automatic circuit breakers.

34. b. Current transformers
-used with ammeters, watt
meters, power-factor meters,
watt-hour meters, compensators,
protective and regulating relays,
and the trip coils of circuit
breakers.
One current transformer can be
used to operate several
instruments, provided that the
combined burden does not exceed
that for which the transformer is
designed and compensated.

35. c. Through-type Transformers
- this type have no primary winding but use the
current carried by the cable or busbar to energize the
core.
- usually regarded as suitable for instrument use if the
ratio is 500:5 amp., or larger

36. d. Bushing-type Transformer
- a special form of through-type transformer.
- made in form of a hollow cylinder, built up
of ring-shaped iron punchings on which the
secondary winding is wound.
- is mounted over the terminal bushing of a
circuit breaker to supply current for tripping
coil or tripping relay.

37. e. Metering Outfits
It is possible to combine the necessary
current- and voltage-transformer elements,
which are needed to measure the power
flowing over a three-phase line, all in one
tank, thereby simplifying the outside
connections and installation very much.

38. Autotransformer
- is built in the same general manner as any other
transformer, but it has only one winding.
- are used as motor starters, as balance coils systems at
different voltage.
-not adaptable to general distribution work, because
for this type of service it is generally desired to keep
the secondary and primary coils electrically insulated
from each other.

39. Constant-current Transformer
The constant-current transformer, usually
called a regulator, has a movable secondary
winding that automatically changes position
to provide constant current for any load
within its full-load rating. The balance point
between coil weight and magnetic force may
be adjusted to provide the desired output
current.

40. Induction-voltage Regulators
Induction regulators are nothing more than
constant-voltage transformers, one winding of
which can be moved with respect to the other,
thereby obtaining a variable secondary voltage.
They are used at the end of distribution lines to
maintain constant voltage. The primary, or
movable, coil is connected across the line, while the
secondary, or stationary, coil is connected in series
with the line.

41. There are two types of induction regulators:
a. Single-phase Regulators
b.Polyphase Regulators are wound
with polyphase windings on both the rotor
and stator in the same general manner as a
wound-rotor induction motor.

42. Conservator-type Transformer
Oil type Transformer had
come to existence since 1892 or
more than 100 years ago. In the
beginning, the Oil type
Transformers were "Open Tank"
type. It had air inlet and outlet
for the expansion of the oil
volume. The Oil volume goes up
and down according to the
Temperature of the Oil. We can
say that the Transformer is
"Breathing".

43. The Open Tank type Transformers were improved in
time. A small oil reserved tank is connected above the
Transformer Main Tank. This is called Conservator
type Transformer. In order to prevent the danger of
moisture and oxygen come in contact with the oil,
"Silica Gel", desiccant agents are connected to the
conservator tank.

44. Oil will be filled in the Main Tank and outflow
into the conservator tank. The oil level will not
be more than half of the conservator tank.
When Transformers are energized, the
temperature of the windings will increase. The
surrounding oil will be hot and oil volume will
increase according to the increasing
temperature. The oil will expand at the
maximum of 7% of the total oil volume in the
tank. The air in the conservator tank will be
pushed out to the atmosphere by the increasing
volume of oil.

45. When the transformer cooled down, the volume of oil
will decrease. The outside air will be sucked into the
conservator to balance the pressure. During the cool
down (by the decreasing of the load,
or the cooler ambient temperature, or by rain water),
the moisture in the air will enter into the conservator.
We need the Silica Gel to help prevent the moisture
enter into the transformer, but cannot prevent totally.
For Conservator Type Transformer, we recommend to
test the oil at least once a year. The best time to get the
oil sample for testing is after the Rainy Season.

46. In order to prevent moisture entering into the
transformer totally, a Rubber Bag (Rubber
Air Cell or Rubber Diaphragm) will be placed
in the conservator tank. This Rubber Bag
will act as a partition to prevent the oil come
in contact with the air, but it is still flexible
for the oil expansion. The Rubber Bag
Conservator system usually will be used in
large Power Transformer.

47. CENTRIFUGAL PURIFIER
Centrifugal force is defined as that force which
impels a thing (and any or all of its parts) outward
from a center of rotation. Every time you lean in as
you take a fast turn, you are counterbalancing
centrifugal force. How far in you lean is
determined by the amount of centrifugal force
exerted in the turn. Most people do it
automatically, for centrifugal force, along with
gravity, is the most prevalent physical force exerted
upon us and upon all matter.

48. The purpose of the centrifugal purifier(fig. 4-26)
in the JP-5 filling and transfer system is to
separate and remove water, solids, and
emulsions from JP-5 during transfer from
storage to service tanks. The disk bowl
centrifuge is a “constant efficiency” type of
separator; that is, it achieves the same degree of
efficiency at the end of a run as at the
beginning. The reason for the constant
efficiency is that accumulated solids are stowed
away from the separation zone. Separation
occurs within the disk spaces, and the
separated liquids are discharged from outlets
that are removed from interference of the
stowed solids.