TESLA TRANSFORMERS LTD

MAINTENANCE OF TRANSFORMER

NOTE : This activity should be performed by qualified person having required knowledge
about maintenance of transformer/electrical equipments generating high voltages.

1. General :

If a transformer is to give long and trouble free service, it should receive a reasonable
amount of attention and maintenance. Maintenance consists of regular inspection,
testing and reconditioning where necessary. Records should be kept giving details of any usual
occurrence and also of any previous results taken.

The principal object of maintenance is to maintain the insulation in good condition. Moisture
dirt and excessive heat in contact with oxygen are the main causes of insulation deterioration
and avoidance of these will in general, keep the insulation in good condition. Factors lead to
aging of insulation during aging process, are due to chemical and physical effects. The
decay of the insulation follows the chemical reaction rate. In case of oil immersed transformer,
if the sustained operating temperature of 750C is higher by 60C to 100C it will reduce the life
of the
transformer. Following are the factors affecting the life of
transformer.

(a) Moisture :
Due to higher affinity of water, the transformer oil and the insulation paper absorb
moisture from the air, which result in decrease of dielectric strength. Hence
preventive steps should be taken to guard against moisture penetration to the transformer.
This will include blocking of all openings for free access of air n storage and frequent
reactivation of breathers in service.

(b) Oxygen :
Oxygen may be present inside the transformer oil due to air pockets trapped in the
windings, etc. The oxygen reacts on the cellulose of insulation and decomposes it,
which will result in sludge formation, blocking free circulation of oil. Sometimes, the
oxygen may act as catalytic agent and increase the operating between hot oil and bare
copper.

(c) Solid impurities :

Dielectric strength of oil diminishes appreciable by minute quantities of solid impurities.
It is therefore a good practice to filter the oil after it has been in service for reasonable
time.

(d) Varnishes :
Some varnishes particularly of the oxidizing type enter readily in reaction with
transformer oil and precipitate sludge on the windings. Synthetic varnishes having acid
inhibiting properties generally delay the natural formation of acid and sludge in the oil.
This should be done in mind by the maintenance engineer when rewinding and
replacing the coils during repairs of transformers.

(e) Effects of slackness of

TESLA TRANSFORMERS LTD
windings:
Coils are fully clamped before the transformer leaves the factory. Natural setting of
coils may take place during first few months of operation. Slackness of winding may
cause a
TESLA TRANSFORMERS LTD

failure due to repeated movement of coils which may wear the conductor insulation
at some places and lead to an interterm failure. The coils may also get displaced under the
load conditions or momentary short circuit which may cause electric and magnetic
unbalance. It is therefore a good practice to lift the core and winding of a transformer and
remove any slackness by tightening the rods of pressure screws.

2. Maintenance Procedure :

(a) No work should be done on the transformer unless it is disconnected from all
external electrical circuits and all windings have been solidity earthed.

General notes on information given under storage handling and inspection are applicable
for routine maintenance. Oil level shall be maintained to keep the insulation under oil and
lowered to reasonable before slackness and removing bolts, nuts and conservators,
radiators, etc.

No fire shall be near the transformer while work is going on. Precautions are to be
taken to secure tools with tape outside the tank, to prevent them from dropping inside the
tank.

(b) Core and Winding :

It is recommended that the core and windings be removed from the tank for visual
inspection as per the maintenance schedule at the end. The windings should be
examined to ensure the no sludge has deposited blocking the oil ducts. Any loose nut and
bolt should be tightened.

Adjusting rods / coil clamping screws provided should be tightened to bear evenly
and firmly on the coil clamping block/rings, if there is any slacking of windings.
Before lifting the core and windings from the tanks it is necessary to disconnect
the
windings from the bushings inside the tank, to disconnect the off circuit tap switch
handle, to remove earthing strip between the core and the tank, and remove fixing bars
between core clamps and tanks guide bars.

The core and windings must be removed with great care under cover and in dry lace.
If this is not possible a visual inspection of as much of the transformer as can be
seen within its tanks should be carried out. Refer untaking instruction also.

(c) Off circuit tap switch :

i) This is an integral part of the transformer. Care should be taken to switch off
the transformer from line before operating the switch handle. To operate the
switch, unlock the operating handle by removing the locking strip/pin. Move the
handle to required position and relock. Tapping position number can be seen
through the windows of handle alternatively on the handle when window not
provided.

ii) Off Circuit tap changing links :

Tap changing links are provided inside the tank. They are located under oil, and
are accessible from the inspection opening to operate the links. Links should be
unbolted by loosening nuts on studs and fixed in desired tap position.
 iii) On Load tap changer :
On load tap changer is normally mounted on the tank is a separate housing and
connected to winding leads through copper studs fixed on a insulated terminal
board Terminal board is on leak proof. Oil in the tank need not be lowered
down for a attending to OLTC gear. Please see OLTC leaflet for the operation
and maintenance instructions.

(d) Conservator :
Conservator is so designed that the lower part acts as sump in which any
impurities entering the conservator will collect. A drain plug is fitted at the lowest point of
the conservator for draining and sampling oil. While sampling, care must be taken to
run off any contaminated oil before taking a sample for testing. The inside of the
conservator should be cleaned every 2 to 3 years by flushing clean transformer oil to
remove sludge and other impurities. Oil level should be maintained at “Filling level” mark
(+300C).

(e) Oil gauges :

Plain oil gauge : Oil gauge glass should be kept clean so that the gauge can fulfill their
purpose. Broken glasses should be replaced immediately.

Magnetic oil gauge : When the conservator is stripped for cleaning, the mechanism of the
float type oil gauge should be inspected and cleaned. The function of alarm and
trip contacts should be checked.

(f) Silicagel dehydrating breather :

Breather should be examined frequently to ascertain if the silica gel requires changing.
The frequency of inspection depends on local climate and operating conditions. More
frequent inspection are needed when the climate is humid and when transformer is
subject to fluctuating load. The crystals of silica gel in the rather act as an indicator and
changed from blue to pink, on becoming saturated with moisture. When majority of
crystals have turned pink, the silica gel should b heated in or over a shallow pan at a
temperature of 1500C to 2000C until the original blue colour is regained.

This usually takes 2 to 3 hours. Oil should be cleaned to remove dust and dirty oil.
The level in oil seal should be maintained with fresh oil. Check that inside of connecting
pipe between breather and conservator is clean and not rusted. If necessary clean and
paint inside with oil resistant paint.

(g) Buchhloz relay :

Routine operation and mechanism inspection tests should be carried our at one or two
yearly intervals respectively. Te operation is tested by injecting air through lower
level petcock of double float buchhloz relay. (refer installation manual of Buchhloz
relay for testing). After inspection, any air, which has accumulated in the gas chamber,
must be released at the upper level petcock leaving the chamber full of oil.
 To carry out mechanical inspection, the oil level should first be brought below the level
of
relay or the shut off valve between conservator and tanks closed and the mechanism
removed. The float should be tightly clamped and be in good order. If a mercury switch is
defective owing to the glass being cracked. There will b clouding of the glass. In
such cases the switch must be replaced. During operation if there is an alarm and gas
is found to be collected, the transformer should be isolated from lines and the gas should
be tested and analysed to find out the nature of fault. Sometimes if may be noticed that
the gas collected is only air. The reason for this may be that the oil is releasing any
absorbed air due to change in temperature. Internal faults can be identified to a great
extent by a chemical analysis of gas.

By examining the gas it is possible to detect the nature of

fault.
i) Colorless and odorless gas with faint odor of oil is air trapped in the oil or insulation.
ii) Grayish white, non inflammable gas with sharp penetrating odor may by due to
over heating or faulty insulation.
iii) Yellowish and inflammable gas may be due to surface leakage on material like wood.
iv) Dark gray and inflammable gas may be due to flashover in oil or due to
excessive overheating of oil caused by a fault in the winding or core.

(h) Explosion vent :

The diaphragm, which is fitted at the exposed end of the vent should be inspected at
frequently intervals and replaced if damaged. Failure to replace the diaphragm
quickly may allow the ingress of moisture in the transformer. If the bottom diaphragm has
broken because of a faulty in the transformer, inspection should be carried our to determine
the natural and cause of the fault.

(i) Gasket :
Gasket sometimes shrink during service. It is therefore, necessary to check the
tightness of all bolts / fasteners of gasketed joints. The bolts should be tightened
evenly around the joints to avoid uneven pressure. Leaky gaskets should be replaced as
soon as the circumstances permit.

(j) Pipe Work

The pipe work should be inspected at least once a year. Leak may be due to badly sealed
joints caused by misalignment, in which case the pipes should be aligned and
joints remade.

(k) Temperature Indicators

At each yearly maintenance inspecting, the level of oil in the pockets holding
thermometer bulbs should be checked and the oil replenished if required. The capillary
tubing should be fastened down again if it has become lose. Dial glasses should be kept
clean. Temperature indicators, if found to be reading incorrectly, should be calibrated with
standard thermometer immersed in hot oil
bath.
(L) Paint work :
During storage and service, the paint work should be inspected once a year
and necessary painting or retouching carried out.
If the metal surface is exposed and becomes dirty rusty or greasy because of delay
in
repairing the paint work, the surface must be thoroughly cleaned before repainting to
ensure a good bond between metal and paint. If recommended paints are not
available, any good quality synthetic enamel paint may be used.

(M) Unmaking and tanking

details : General :

For transformers assembled with bushings, conservator, pipe work etc.

Before removing the top cover from the tanks following care should be
taken:

(i) All the bushing/cable boxes/pockets mounted on cover (connected to core

and winding ) should be mechanically disconnected off the transformer and
removed carefully.
(ii) External cable connections laid over the cover connected to terminal box,
buchholz relay, OTI etc. Should be disconnected and removed.
(iii) Thermometer bulbs along with capillary tubes to be removed.
(iv) Pipe work from the cover to the conservator to be removed, if
conservator is separately supported on tank brackets.
(v) Oil should be drained out below the tank cover
level. (vi) All isolating valves should be closed.
(vii) Disconnect CT secondary leads connection when CTs are mounted on the live part.
(viii) Disconnect core and winding (live part) from all external connections, to
avoid damages while removing cover from the tank.
(ix) Detach the fixing bolted to the square angular guide bars on the tank.
(x) Before lifting core and windings cheek that the lifters are rigidly fixed to core clamps.

(A) Untanking the core and windings of transformer with off circuit switch:
(i) Drain out oil below switch handle level from transformer tank.
(ii) It is important that the switch handle assembly should be dismantled before
untanking the core and windings.

(B) Untanking of core and windings of transformer with OLTC

(I) Drain out oil below the cover level of transformer and OLTC.
(II) Detach the winding leads from the terminal board between transformer and OLTC.
(III) Ensure that the lead number tags are in position on the leads. If not, tag the
leads correctly to reconnect the leads in correct position on terminal board.
(IV) Give temporary support under the OLTC housing before lifting the core and windings
from the tank. It is likely that the tank may become unstable due to weight of OLTC
 without core and windings in main tank.
(V) Refer OLTC leaflet for further information.

DRYING OF TRANSFORMER :

If as a result of tests carried out in section 6.4, presence of moisture is indicated or the oil
does not withstand dielectric strength or the insulation resistance readings are not
satisfactory is shall be necessary to dry out the transformer.

Method of drying out

Normally hot oil circulation method should be used for drying out the transformer. In special
circumstances where the above method does not give satisfactory results, short circuit
method with hot oil circulation as described in 8.1.2 should be used. In this method both oil and
the core and winding inside the tank are simultaneously dried out with stream line or
other filter with heater and vacuum pump. The moisture is driven out from the windings into
the oil and removed from the oil by evaporation and filtering. Constant circulation of hot oil
through a filter will lead to gradual deterioration of the oil and probably a partial breakdown of
the constituents of the oil due to the continuous mechanical action and filtering under high
temperature. Further this tends to increase the acidity of oil. Great care should. Therefore,
be exercised in circulating the oil through the filter during the drying out period.

The tank sides and top cover should be covered with some covering like tarpaulin. The
LV winding should be short circuited and a three phase supply of 415 volts be applied to HV
side of the transformer. It should be ensured that the short circuit current does not
exceed the rated current of transformer. The temperature of the top oil should be
measured by a thermometer. The oil temperature should not exceed the limit of 80 degree C.

During the drying out process temperature of the top oil and the insulation resistance
of the winding should be taken every two hours.

As the temperature of the oil rises, the insulation resistance may fall and will eventually reach a
steady value. The temperature should be kept constant until the insulation resistance
shows steady increasing upward trend. When this poking is reached the drying out process is
complete and the application of heat may be discontinued. As during the cooling down
process, the transformer reaches 60dgree C the insulation resistance should again be
measured. The insulation resistance in mega ohms varies inversely with the temperature and
for a 10dgree C change of temperature, mega ohms change by ration generally in the range
2.1 to 1.4 :1.
Precautions during drying out :
Drying out is a delicate operation and great care and judgment are necessary in carrying it
out correctly to avoid damage to the transformer winding or oil.
Care should be taken to see that maximum limits of temperature specified are never
exceed i.e.80 degree c.

It is essential that the insulation tester and the thermometer used for the purpose are
reliable.
Only spirit type thermometer are to be used for temperature measurement.

Mercury thermometer shall not be used everywhere except in the pocket provided for this
purpose.

In no case transformer should be left unattended during any part of the drying out period. The
transformer should be observed continuously throughout the drying out process and all
observations shall be recorded.
Insulation resistance should be measured at least with a 100 volts
meggar.
The life of the insulation decreased with every drying out process. Therefore, it would
be desirable to dry out the transformer at 50dgree c less that maximum temperature
stipulated i.e. 80 degree c.