Standard Accessories on an Oil Filled Transformer: Liquid level gauge Liquid temperature gauge Pressure-vacuum gauge Pressure relief device Winding temperature gauge Transformer cooling fans High voltage bushings Low voltage bushings De-energized tap-changer Buchholz relay galvanized radiators Breather Lifting lugs Jacking facilities Multi-ratio, current transformers (CTs) for relaying service Surge arresters Grounding devices, such as reactors and resistors De-energized tap changer (DETC) Load tap changers Conservator oil preservation systems Various monitoring devices for gas, oil and winding temperatures; power factor; moisture; etc. Most customer-specified accessories

1. TRANSFORMER
ACCESSORIES
FOR TESTING
AND
PROTECTION
By
Durgada Sagar
Electrical Engineering Student
Reva University
Bangalore

2. Standard Accessories on a Oil Filled
Transformer
Liquid level gauge
Liquid temperature gauge
Pressure-vacuum gauge
Pressure relief device
Winding temperature gauge
Transformer cooling fans
High voltage bushings
Low voltage bushings
De-energized tap-changer
Buchholz relay
galvanized radiators
Breather
Lifting lugs
Jacking facilities

3. Multi-ratio, current
transformers (CTs) for relaying
service
Surge arresters
Grounding devices, such as
reactors and resistors
De-energized tap
changer (DETC)
Load tap changers
Conservator oil preservation
systems
Various monitoring devices for
gas, oil and winding
temperatures; power factor;
moisture; etc.
Most customer-specified
accessories

4. Liquid Level Gauge
• Provides continuous indication of
the liquid level inside of a
transformer’s main tank, conservator
tank or load tap changer
compartment.
• Basically, it consists of a float-arm
inside the tank, an indicating pointer.
• Float travel ranges from min to max
markings for various tank
configurations

5. Liquid Temperature Gauge
• known as electromechanical
thermometers or OTIs are
essential devices on oil-filled
transformers, providing accurate
temperature measurement of the
oil within.
• The temperature gauge is furnished
to indicate the top liquid
temperature in the tank in degrees
Centigrade.
• The thermometer can be furnished
with two SPDT contacts for a high
temperature alarm, for energizing a
fan circuit or for a low temperature
alarm.

6. Pressure-Vacuum Gauge
• Designed to balance pressure and vacuum conditions
within a sealed vessel by releasing or admitting air and gas.
• High pressures indicate an overload or trouble inside the
tank and should be investigated immediately. A sustained
zero pressure reading indicates a leak or a defective gauge.
• If internal pressure reaches the pre-determined setting,
valve will exhaust gas from the tank. If vacuum inside
vessel becomes too great, the valve will admit air into the
tank
• Note: Typically the transformer is filled with oil and
pressurized with nitrogen to a pressure of 0.2 atm (3 Psi)
[Std Atm = 1atm = 14.69595 psi]
psi = Pound-Force per square inch
1 pound = 0.45 kg

7. Pressure Relief Device
• The PRD is used for the relief of excessive
pressure buildup in large volumes of gas or
fluid in transformers. Its patented dual
gasket system provides quick response time
and automatically reseals after pressure has
subsided.
• Should the tank pressure increase above
that for which the device is set, the gas
pressure will lift the diaphragm and let the
gas escape quickly. Immediately after the
pressure returns to normal, the diaphragm
will reset and reseal the transformer.

8. Winding Temperature Gauge
• All large transformers have an oil or a winding
temperature indicating device of some type, and
most have temperature recorders as well. Indication
may be for top-oil temperature or hotspot
temperature. Additional temperature-sensing
equipment may be installed to provide alarm and
control signals needed to activate automatic cooling
systems.
• Indicator operation may be:
• Purely mechanical (tank-mounted temperature gauge
or thermometer),
• Purely electrical (SCADA transducer and some chart
recorders), or
• A combination of the two (chart recorder and
multipoint temp recorder).

9. Transformer Cooling Fans
• In order to increase the transformer
load without overheating the windings, a set
of fans can be furnished as an optional item
.Fan control consists of a contact on either
the liquid temperature gauge or the
winding temperature gauge (when furnished),
and “Manual-Auto” control switch .
• Recommended Maximum Temperature
settings are;
A. Oil Temperature Indicator: 85°C
B. Winding Temperature Indicator:
105°C

10. High-Voltage Bushings
• Solid porcelain bushings consist of high-grade porcelain
cylinders that conductors pass through. Outside surfaces
have a series of skirts to increase the leakage path distance to
the grounded metal case.
• High-voltage bushings are generally oil-filled condenser type.
• Condenser types have a central conductor wound with
alternating layers of paper insulation and tin foil and filled
with insulating oil.
• A good indication of insulation deterioration is a slowly
rising power factor. The most common cause of failure is
moisture entrance through the top bushing seal.
• Care must be taken in handling the bushing to avoid cracking
the porcelain or damaging its surface.

11. Tap-Changer
Do not operate the tap-changer while the transformer is
energized! Doing so can result in severe personal injury and
equipment damage.
• The tap-changer provides a
means of changing the voltage
ratio of a de-energized
transformer without breaking
the transformer seal.
• It is operated by means of
a rotatable handle located on
the side of the transformer.
• The tap-changer is normally
provided with five or
seven positions.

12. Buchholz’s Relay
• Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer.
• Internal faults such as an insulation faults between turns, breakdown of core of transformer, core heating energize
the alarm circuit and one can predict the type of fault in the transformer
• More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing
equipment energize the trip circuit and immediately isolate the transformer from the rest of power system

13. Silica gel Breather
• The function of breather in
transformer is to filter out the
moisture from air. Breather
consist of silica gel which absorbs
the moisture from air.
• The main reason to remove
moisture from air is because the
moisture degrades the dielectric
strength of the transformer oil.

14. Surge Arrestors
• Lightning arresters are
protective devices for limiting
surge voltages due to lightning
strikes or equipment faults or
other events, to prevent
damage to equipment and
disruption of service. Also
called surge arresters.
• Many surge divertors are essentially non-linear resistors which when subjected to high transient overvoltage have
their resistance go to zero, thereby creating a short circuit and blowing a fuse.
• In this way it protects against damage due to serious overvoltage and also disconnects the equipment from the
circuit.

15. Current Transformer
• A current
transformer (CT) is a type
of transformer that is used
to reduce or multiply
an alternating current (AC).
It produces a current in its
secondary which is
proportional to the current
in its primary.
The current transformer is used to reduce the currents in power systems, producing proportional
currents in the unit’s secondary windings that remain isolated from the main circuit.
The replica current is then used as an input to a protective relay, which can then automatically
isolate part of the power circuit should it experience a fault.A CT for operation
on a 110 kV grid

16. References
◦ electrical-engineering-portal.com
◦ Researchgate.net
◦ Library.e.abb.com (ABB Group)
◦ Wikipedia.com
◦ insulect.com
◦ Testguy.net
◦ www.qualitrolcorp.com
◦ Spxtransformersolutions.com
◦ Azom.com
◦ Sadtem.com