The document provides instructions for safely installing a power transformer. Key points include: - Tools and materials used during installation must be clean and free of loose fibers to prevent contamination of transformer components. - All transformer parts must be thoroughly cleaned before assembly to remove rust and moisture. - Transformer oil must be filtered, cleaned, and dried to specifications before filling transformer components to ensure proper insulation. - Proper preparation of the installation site is required, including a sturdy foundation with drainage and adequate space around the transformer. - Throughout the installation process, special care must be taken to prevent moisture and air from contaminating transformer components and oil.

1. Installation of Power Transformers
A power transformer is the second most costly equipment installed in power
system. Hence, special care to be taken during installation of a new power
transformer in an electrical substation. During accessing the interior of a power
transformer, the worker should empty their pockets of all loose articles so that
there would not be any risk of falling anything inside the transformer from their
pocket.
The tools used during the work, should be securely tied with cotton tap so that
they can be recovered if accidentally fall into the transformer interior. Jute should
not be used as cleaning materials for a transformer. Loose fibers of the jute may be
remained even after completing the cleaning work, which may affect the
insulating properties of the equipment in future. Cleaning should be done by clean
cotton cloth without any suspended loose fibers. Generally, many parts of a power
transformer are transported from manufacturers separately.
All the parts or components dispatched separately should be properly cleaned
from inside and outside before being fitted to the transformer body. Special
attention, to be given if, interior surface of the any component is found rusty.
Transformer oil is also dispatched in separate sealed steel drums. This oil is highly
inflammable hence no flame should be brought nearer to the transformer oil.
Transformer oil and the cellulose-based insulating material used in transformer,
both absorb moisture easily hence, transformer oil and cellulose-based insulated
parts of the transformer should be handled very carefully. The main body of
power transformer can be supplied by the manufacturer without or with
insulating oil inside it. In both cases it is best practice to circulate dry and warm oil
through it until its temperature is 5oC to 10oC above ambient.
This should be done before allowing external access to the interior of the tank. The
warm and dry oil should be circulated in the transformer during all the time the
transformer remains open to atmosphere. The oil pump and all joints in the oil
pipe work should be properly air tight otherwise air will enter in the transformer
and get lodged in the vulnerable places such as winding.
Locations and Site Preparation for Installation of a Power Transformer
For installation of a power transformer, no special type of foundation is required.
The foundation should be strong enough to carry the heavy load of transformer
and it should be efficiently designed to prevent accumulation of water below the
transformer. It is essential to drain all the oil coming out from transformer during
any fire hazard and hence special oil drainage facilities should be provided in the
foundation design.

2. This last point is strongly recommended especially for large power transformer.
The foundation should be created in such a place where easy accessibility can be
provided to all indicating and protecting devises.
That means there should be sufficient place all around for proper viewing of
diaphragm plates, thermometers, valves, oil gauges, etc. Sufficient and minimum
standard electrical clearances of all live parts of the transformer from earth and
other live bodies are also to be provided. If the transformer is of Oil Natural Air
Natural (ONAN) type, there should be sufficient space available all around the
transformer for circulating the air naturally.
If the ONAN transformer is indoor type, the room must be well ventilated. In this
case the air inlets and outlets should be of sufficient size and number. Air inlets of
the room must be as nearer the floor as possible and air outlets must be as high as
the building will allow and this arrangement will facilitate sufficient and efficient
circulation of air during natural cooling of transformer. Sometime in case of indoor
transformer, exhaust fans are used for forced circulation of air in the room.
Where wheels or rollers are not fitted on the base of the transformer body, level
concrete plinth with bearing plates of sufficient size and strength can be adopted
for outdoor transformer. To prevent the formation of rust, it is essential to avoid
presence of air and water in the space between the plinth and the base of the
transformer by use of cretex or similar bituminous compounds.
Where rollers are fitted on the base, suitable rails or tracks should be fitted with
the foundation to place and lock the rollers on the rail. Locking of rollers on the
rail is essential because it prevents the accidental movement of transformer during
earth quakes or any similar hazard.
Preparation of Oil during Installation of Transformer
When transformer is dispatched without oil, required entire oil which is supplied
by the manufacturer in separate sealed steel drums. But when the transformer is
dispatched with oil in the main tank, the required oil for conservator tank,
radiators of transformer, and for other essential parts is dispatched in separate
sealed steel drums.
Although the oil supplied by the manufacturer are brand-new but still it is
required to be filtered, cleaned and dried before feeling in the transformer.
Filtering process must be continued until the transformer oil satisfies the
specifications indicated by manufacturer.
The parameters of the oil, to be checked during filter process are generally for
electrical strength, resistivity at 27oC, resistivity at 90oC, tan delta value at 90oC,

3. and moisture content in ppm. Oil is easily contaminated hence it is important,
when sampling the oil and filling the tank, to keep the oil free from contamination.
All the components used for handling the transformer oil should be cleaned and
flushed with fresh transformer oil before use. For collecting sample of the oil for
the testing purpose, glass containers with glass stopper is preferable over the
metallic container. Cleanliness of the container is essential because very small
amount of moisture can affect the test results. Never Use wax for sealing the oil
sample containers.
If possible, glass stopper is covered by a pack of silica gel tied in a piece of cloths.
Flexible steel hose pipe is preferable for filtering and handling of insulating oil.
Some kind of a specially made synthetic rubber pipe or PVC pipe can be used as
hose but ordinary rubber hose pipe must not be used in any case as the
transformer oil dissolves the sulphur from rubber and is thereby contaminated.
The steel hose used for this purpose must also be properly cleaned and free from
moisture and loose rust.
Oil Filling in Transformer Tank during Installation
The oil can only be filled in the transformer which satisfies the standard
specifications mentioned by the manufacturer of transformer. When the
transformer is supplied gas filled, the oil filling is done in the main tank under
vacuum.
The large transformers are generally designed to withstand the full vacuum for
long periods. During oil filling in the main tank of the transformer, it is preferable
to connect the inlet hose pipe at the bottom of the tank. After filling the oil up to
the top of the transformer core, maximum suitable vacuum is maintained above
the oil level in the tank to minimize the hazard of bubbles lodging in the
inaccessible corners of winding.
It should be noted that condenser bushing, diaphragm, Buchholz relay, tap
changer board, conservator tank, radiators units etc are not designed for
withstanding vacuum hence they should not be subjected to vacuum. That means
vacuum processes only to be done in main tank without connecting or with
blocking all the above-mentioned components of the transformer.
Drying out Power Transformer using Streamlined Filter Machine
The most common process of drying out a power transformer is by using a
streamlined filter machine connected across the inlet and outlet oil hose. The
vacuum chamber and oil heater incorporated in the filter machine are mainly
utilised for this purpose. Before using the filter machine it should be ensured that

4. the oil previously filled in the machine, must be drained out and the machine is
refilled with the transformer oil supplied by the manufacturer.
This oil first circulated inside the machine itself for several times and tested again
and again for achieving required result. If the required result is achieved then the
machine should be connected across inlets and outlet hose of the transformer and
the entire oil should be circulated through the transformer and filter machine
several times and the oil sample is to be tested during a specified interval normally
after each one hour.
The vacuum pump of the filter machine should have the capacity of creating
vacuum as high as possible, but not less than 710 mm of mercury. Where possible,
an additional vacuum pump can be connected at the top cover of the tank to keep
the oil in the tank under vacuum consistently. This additional vacuum pump may
accelerate the drying out process of the transformer.
Although, the oil was filled in the transformer under vacuum from the bottom of
the tank, but during drying out process oil is drawn out from the bottom and fed
into the top of the transformer tank. This helps to remove any settled
moisture/impurities remain in the oil.
The oil circulation in that direction must be continued about 8 to 12 hours and
then the cycle is reversed i.e. oil is drawn from the top and fed in the bottom of the
tank. During this process the oil must not be heated beyond 85oC in the machine
and the temperature of the oil inside the transformer tank should not exceed 75oC.
If this maximum allowable temperature limit is not maintained, the oil may be
decomposed due to higher temperature operation. The process of drying out is
continued until the satisfactory insulation resistance value is achieved. The
insulation resistance or I R value of the oil mainly depends upon its moisture
content hence, by analyzing the I R value, the moisture content in the oil can be
predicted.
The I R values of the oil taken in a regular interval (at the interval of one hour) are
plotted against time. It should be observed from the graph that initially the I R
value drops down and this I R value will be low till the moisture is present inside.
Once the entire moisture is removed from the oil, as well as interior of the
transformer, the I R value again goes up and becomes steady. Initially insulation
resistance value came down because the moisture contained in the core and
winding of the transformer were removing in the hot oil thereby moisture content
of the oil was increased.

5. After, this moisture removed from the oil in the filter machine, the insulation
resistance value of the oil became again high and steady. The I R values taken at
regular intervals as should also be plotted against temperature readings. In the
beginning, I R values drop down as the temperature goes up. The I R value will be
low till the moisture is present inside.
Once the moisture is removed, the IR value will start rising and then becomes
steady. In addition to the drying out process by filter machine, the transformer can
also be heated up by short-circuiting the LV winding and supplying a reduced
voltage at the high voltage terminals. The value of the applied voltage is so
maintained that the current through the short-circuited winding should be under
70% of the rated current.
It must also be of observed during heating up the transformer winding by
applying reduced voltage, the temperature of the oil must not exceed 75oC. At the
same time temperature of the winding must not exceed 90oC.
Gaskets and Gasket Joints
Transformer gasket are neoprene bonded, cork gasket. The gaskets are supplied
loosely by the manufacturer without any bolt holes created on them. The gaskets
are usually cut to the size and shape and punched for the holes at site. Joints in the
gasket should be located away from the corner and bold holes.
Before using, the gasket must be checked for their proper flexibility and free from
local thickening. Neoprene solution is used as adhesive for joints in the gasket.
After fitting the gasket the bolts are typed in sequence, a little every time so that
uniform pressure is exerted on the gasket until the gasket is compressed to about
2/3 of its original thickness.
Fitting and Fixing Bushings of Transformer
Before fitting the bushings on the transformer, they should be checked for any
damage or crack. If they are all right, these should be cleaned by dry cotton cloth.
Bushings are normally delivered in complete assembled form. When clamping the
bushing to the tank, care should be taken to ensure that an even pressure is
obtained to the flange of the porcelain and its gasket by tightening the diagonally
opposite nuts alternatively. The power connection at the stud of the bushing
should be flexible enough to absorb any vibration. If arcing horns are supplied
with the bushing they must be fitted and the gap between the horns must be
adjusted as per the drawing supplied with the transformer.

6. Installation of Conservator Tank
Conservator tank, where fitted should be assembled with its pipe work, etc,
making sure that SKATED joints are oil tight and the pipe work is clean and free
from moisture. The mechanism of the float type oil gauge inside the conservator is
locked to prevent damage during transit. After placing the conservator in position,
it should be released by turning the locking bolt in the direction indicated on the
plate.
While feeling the conservator with transformer oil, it should be ensured that the
oil filled up to the level indicated by the oil gauge on the conservator. On the oil
gauge’s dial there is an indication of temperature, this tells the prescribed level of
oil in the conservator at that temperature.
For example, if 30oC is indicated in oil gauges dial, the oil should be filled in
conservator up to that level when oil temperature is 30oC. But during actual oil
filling in conservator tank, we should commensurate with the oil temperature at
that time.
Fittings of Silica Gel Breather
Silica gel breather always comes as loose item. The plastic plug sealing on the top
of the breather should be removed and fitted to the associated pipe work. The oil
cup at the bottom of the breather should be removed, filled with transformer oil to
the level indicated on it and then re fitted to the position. Thus the silica gel is
prevented from absorbing moisture from atmosphere.
Installation of Explosion Vent
Explosion vent is fitted with suitable gasket. Here care should be taken to ensure
that the top diaphragm with its gasket makes an air tight joint. The explosion vent
is sent blanket from works and the top blanking plate should not be removed till
the oil level inside the transformer comes up of the tank cover.
When a transformer is provided with a Buchholz device, bottom paper diaphragm
which acts as a deflector plate, is fitted at the base of the explosion vent to ensure
that any gases liberated in the transformer, will pass through the Buchholz relay
and not collect in the vent. A hole in the side permits the oil to enter the vent and
equalise the pressure on either side of the deflector diaphragm.
In transformers of large rating the space above the oil in the vent is usually
connected by a pipe to the top of the conservator to equalise the pressure and vent

7. it to the atmosphere through the breather. For transformer supplied without
equalising pipe and air cock is fitted at the top which must be opened when the
tank is being filled with the oil and closed again.
Fitting of Buchholz Relay
Buchholz relay is connected across the pipe work between conservator tank and
main tank of the transformer. There is generally a Stop Valve between Buchholz
relay and conservator tank. The Buchholz relay should be mounted on the pipe
work only after the oil level inside the transformer is above the tank cover.
The Buchholz relay is provided with an arrow marked on its housing and this
arrow should always point towards the conservator. If the gas release pipe is
provided, it must be fitted to the top pocket of the Buchholz relay. In this case the
top pockets of Buchholz relay should be opened and gas release pipes should be
full of oil during normal operation of transformer. When the gas is to be collected
through the gas release pipe, oil will flow out initially followed by the gas.
But when gas release pipe is not provided with Buchholz relay, the top pockets are
generally screwed tightly. During fittings of Buchholz relay in the pipe work, the
relay should be mechanically locked with the locking screw provided in the relay.
It prevents any mechanical damage inside the relay due to transit.
The alarm and trip contacts provided on the relay should be properly identified
from the control drawings of the relay before connecting the control wires on those
terminals of the relay. The ‘NO’ contacts for alarm and trip should be checked for
their proper functioning.
Installation of Temperature Indicator
Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) are
fitted in the Marshalling Kiosk. The thermometer bulbs are enclosed in the pockets
on the top cover of the transformer. The thermometer pockets for OTI and WTI
should be filled with transformer oil during inserting the thermometer bulbs
inside them.
Both Oil Temperature Indicator and Winding Temperature Indicator are provided
with maximum pointers and mercury switches. The steel capillary tubes are
normally quite flexible but it should not be bent sharply and repeatedly and
should be supported by clips to prevent unnecessary sagging.
Before installation, the instrument should be checked for proper calibration by hot
and cold oil bath. The mercury switches are adjusted to make contact at the

8. desired temperature depending upon the site condition i.e. ambient temperature,
loading condition, etc. The Current Transformers connection of the WTI is made to
the thermometer pocket as per instructions given on the WTI terminal board.
Installation of Cooling Equipment
The radiator units are washed by clean dry oil before fittings to the main body of
the transformer. After fittings the radiators with proper gasket work, are filled
with transformer oil. The air should be released by loosing air release pocket at the
top of the header and also at each radiator unit.
When only the drying out process of main tank is completed, the interconnecting
valves between the tank and the radiator unit are opened. This is because radiator
unit should not be subjected to the vacuum, applied in the main tank during
drying out process. The cooling fans and oil circulating pumps should also be
fitted and proper wiring should be done according to the cooler control drawings
provided with the transformer.
Fitting of On Load Tap Changer
The tap changer, if dispatched separately, it is to be fitted onto the tank, The
insulation resistance value of each tap changer lead should be measured and the
cause of low values, if any, investigated.
The lead from the tap changer should then be connected to their respective
position on the terminal board provided on the tank. The tightness of all
connections of the selector switch and terminal board should be ensured.
Sometimes it is difficult to get good insulation resistance values if there is moisture
in the tap changer board or on any of the parts of the selector switch and diverter
switch. Then it should be ensured that selector switch and diverter switch are
filled with clean dry processed oil.
To remove the moisture from the terminal board, it is preferable to circulate the
transformer oil in the selector switch tank and main transformer tank
simultaneously and at as high a temperature as permissible. If some other special
instructions are provided, the same should be followed.

9. Fitting of Off Load Tap Changer
If the transformer is supplied with off load tap changer, it is previously assembled
in the transformer itself. No extra connection to be done unlike in the case of on
load tap changer. In this case only precaution is maintained and that transformer
should be isolated from the system during changing of tap from one position to
other.
Earthing of Transformer
The main tank of the transformer should be effectively earthed at least more than
one point. This is to be noted that the earthing connection should be of sufficient
cross-section to carry the fault current for at least 30 seconds. The fans, oil pumps,
neutrals, marshalling kiosk etc. should also be properly earthed.
Completion of Erection Work for Installation of Power Transformer
Any other work such as wiring of various alarm/trip contacts from various
accessories of the transformer to marshalling kiosk, settings of relays, checking of
protection scheme, etc. should be completed before the transformer is cleared for
commissioning.