A study of Transformer - e world, electrical basics, electrical world, hazards in transformer, ilektech, transformer, transformer parts, transformer protection

1. Transformer
A transformer is a magnetically coupled device in which magnetic field
produced by time-varying current induction voltage in another. An electric
transformer changes ac electric power at one voltage to another. The
transformer two coils, these are electrically separated but linked through a
common magnetic current circuit. AC current in one coil produce a flux that
links with other winding and induces voltage in that.

2. STANDARD TRANSFORMER ACCESSORIES
1) Thermometer Pockets
This pocket is provided to measure temperature of the top oil in tank with
mercury in glass type thermometer. It is essential to fill the pocket with
transformer oil before inserting the thermometer, to have uniform and
correct reading
2) Buchholz relay
A Buchholz relay is a safety device mounted on some oil-filled
power transformers and reactors, equipped with an external overhead oil
reservoir called a "conservator". The Buchholz relay is used as a protective
device sensitive to the effects of dielectric failure inside the equipment. A
generic designation for this type of device is "gas detector relay
3) Silica gel breather
During the breathing process, the incoming air may consist of moisture and
dirt which should be removed in order to prevent any damage. Hence the air
is made to pass through the silica gel breather, which will absorb the
moisture in the air and ensures that only dry air enters in to the transformer.
Silica gel in the breather will be blue when installed and they turn to pink
colour when they absorb moisture which indicates the crystals should be
replaced. These breathers also have an oil cup fitted with, so that the dust
particles get settled in the cup.
4) Air release plug
Air release plug is normally provided on the tank cover for transformer with
conservator. Space is provided in the plug which allows air to be escaped
without removing the plug fully from the seat. Plug should be unscrewed till
air comes out from cross hole and as soon as oil flows out it should be closed.
Air release plugs are also provided on radiator headers and outdoor bushings.
5) Winding temperature Indicator
The windings temperature indicator indicates ‘Hot spot’ temperature of the
winding. This is a ‘Thermal Image type’ indicator. This is basically an oil
temperature indicator with a heater responsible to raise the temperature
equal to the ‘’Hot spot’’ gradient between winding and oil over the oil
temperature. Heater coil is fed with a current proportional to the windings
current through a current transformer mounted on the winding under
measurement it has maximum pointer and re setting device and two sets of
contacts for alarm and trip.
6) Oil Temperature Indicator
Oil temperature indicator provides local temperature of top oil. Instruments
are provided with temperature sensing bulb, temperature recording dial
with the pointer and maximum reading pointer and resetting device.
Electrical contacts are provided to give alarm or trip at a required setting (on
capillary tube type thermometer).
7) Conservator tank
It is an Expansion Vessel. It maintains oil in the Transformer above a
Minimum Level. It has a Magnetic Oil Level Gage. It can give an alarm if the

3. oil level falls below the limit. A portion of the Tank is separated for use with
OLTC. This usually has oil level indicators. Main Conservator Tank can have a
Bellow. It has an oil filling provision. It has an oil drain valve. Provision is
there for connecting a breather
8) Off Load Tap Changer (OLTC)
To change the turn’s ratio on the source winding, a switch is operated by a
hand wheel on the exterior of the tank. The hand wheel is used to operate a
switch within the tank via an exterior operating rod and interior insulated
operating rods. The switch takes the form of fixed terminals or contacts
arranged in a circle. Turning the hand wheel moves the contact or finger
around the center of the circle to complete the circuit and give the desired
ratio. This is known as changing tap positions and is performed with the
transformer off potential since these switches cannot open a circuit carrying
current.
9) Under Load Tap Changer (ULTC)
To respond to changing voltage levels on the load side of the transformer is
accomplished by adjusting the transformers turns ratios. The under load tap
changer switch is designed to change the tapped windings while carrying load
current. It is normally operated by a motor and can be operated by hand. The
tap changer can be located electrically in the low voltage winding or
electrically in the neutral end of the high voltage winding. The motor and
control cabinet for the tap changer is located on the side of the transformer
10) Bushings
The electrical power circuits must be insulated where they enter the tank. A
bushing provides an insulated oil-tight and weather-tight entrance for the
conductor into the transformer. It is usually composed of an outer porcelain
body, and at higher voltages, additional insulation in the form of oil and
wound paper is used within the porcelain column
11) Transformer Cooling
In smaller size, liquid-filled types of transformers, natural convection carries
the heat of the insulating medium to the walls of the tank. As the size of the
transformer increases so does the heat generated, additional means of
cooling the transformer are required - Radiator, Fans and Pumps
12) Explosion Vent
When an electrical fault occurs under oil, very high pressures are possible.
These pressures could readily burst the sheet steel tank if some means were
not used to guard against this. The explosion vent consists of a large diameter
pipe (4 inches or larger) extending slightly above the conservator tank of the
transformer and curved in the direction of the ground. A diaphragm is fitted
at the end of the pipe; it will rupture at a relatively low pressure to release
the forces from within the transformer to atmosphere. On some
transformers, a second diaphragm is located at the bottom of the pipe where
the explosion vents connects to the transformer tank. This prevents oil from
entering the explosion vent except under fault pressure

4. Losses in Transformer
The distribution transformers are98% efficient. The efficiency of a transformer
depends as follows
1) Copper loss (winding loss)
The copper loss is due to current flowing through the windings. When current
flow the windings causes resistive heating. (Frequencies, skin effect and
proximity effects are also creates additional losses)
2) Core loss (Iron loss)
They are of two types
a) Hysteresis loss
Each time the magnetic field is reversed, a small amount of
energy is lost due to hysteresis within the core. For a given core
material, the transformer losses are proportional to the
frequency, and is a function of the peak flux density to which it is
subjected.
b) Eddy current loss
Eddy currents (also called Foucault currents) are loops of
electrical current induced within conductors by a changing
magnetic field in the conductor, due to Faraday's law of
induction. Eddy currents flow in closed loops within conductors,
in planes perpendicular to the magnetic field

5. Hazards Associated with a Transformer
A transformer is connected to an electrical source and current flows through the
transformer. There is some hidden danger in transformer
Electrical contact – resulting in electric shock, burns
 Equipment failure resulting explosions and fire
 Flashover due to insulation break down
 Contact
 Component failure
Mechanical failure – Components with moving gears and parts
Oil Leaks - Oil/Liquid filled transformers provide insulation and cooling
Transformer protection
To eliminate and control electrical hazards the transformer must be isolated and de-
energized for work on or near it. In addition, work methods are used to control hazards
• Lightning Arrestors on HV & LV for Surge Protection
• HV / LV Over Current protection
• Earth Fault Protection
• HG Fuse Protection for Small Capacity Transformers.
• horn Gap fuse, used in 11kV & 33kV circuits to disconnect in case of fault
• Normally each Power Transformers will have a LV circuit breaker. For a group of
transformers up to 5 MVA in a substation a group control circuit breaker is provided.
Each transformer of 8 MVA and above will have a circuit breaker on the HV side