INSPECTION:
For the purpose of maintenance of transformer, it is necessary to carry-out inspection of transformer. The inspection can be divided into the following manner:-
i. External Inspection
ii. Internal Inspection
i. External Inspection:
An external inspection of a transformer is an inspection of all parts on/ or related to a transformer that can be examined with the transformer taken out of service but without opening the tank or lowering the oil. For example, the oil can be sampled and tested. Megger tests, ratio tests, can be carried out. Bushings, breather gaskets, oil level, diaphragm, tank foundation, conservator, radiators, buchholz relay and all visible/accessible auxiliaries can be thoroughly examined.
ii. Internal Inspection:
An internal inspection constitutes a careful examination of all parts inside the transformer tank, with the oil level lowered sufficiently to expose the top of the transformer. In such an inspection, radiators can be examined for sludge.
The conditions of coils and insulation, the lower ends of bushings and bushing current
transformers, and the position of thermometer can also be examined. A thorough inspection will also reveal any missing parts, such as nuts and bolts, small particles of copper or burnt and broken tape. Oil tests, megger and ratio tests should also be carried out as an external inspection.
EXTERNAL INSPECTION OF TRANSFORMER:
GENERAL MAINTENANCE
Dirt & Dust:
The external transformer surfaces shall be inspected regularly and, if required, shall be
cleaned from dust, insects, leaves and other airborne dirt.
Rust damages, Touch up painting:
A regular inspection of the surface treatment of the transformer should be carried out.
Possible rust damages are removed and the surface treatment restored to original state by
means of the primer and finish paint of the transformer to minimize the risk of corrosion and its subsequent spreading. These checks also include looking for signs of oil on gasket areas and welded areas containing oil. The touch-up paint, as and when required, as per site condition and re-painting is recommended once in a five years as per one of the
manufacturers of the transformer.
Checks to be carried out
Check of signs of corrosion.
Check for any signs of mechanical damage.
Check all joints for sign of leakage.
Check oil levels.
Check that surrounding areas are clean and tidy.
Check Procedures
i. Check for Signs of corrosion
Carry out a visual inspection on all external surfaces of the transformer and its
associated equipment. If damaged areas are seen, these must be treated immediately to
prevent the spread of corrosion. The repair procedure is detailed below:
Using a wire brush, remove all loose flakes of paint, ensuring that the surrounding
areas are given attention. Using the abrasive paper, rub the damaged areas so that all
contours to the edges are smooth and that no steps in the paint work are obvious.
Prominent dent marks and pits on external surfaces are to be filled up with knifing
putty. When putty is dry, smoothen it with emery paper and clean with clean rag.
Clean the local area with a suitable solvent like methyl alcohol that contains no salts.
Depending upon the extent of the damage, i.e. depth, apply the necessary number of
coats of paint to build up the overall thickness (min. 80 microns). The minimum
number being two: one undercoat and one top coat after two coats of primer. Please
note that paints should be applied so that the layers feather-in with each other and
there should not be any steps in layers. Spray painting (50-60 PSI) for radiators
should only be applied.
Allow each coat of paint to dry before over coating with the next coat. This
procedure must only be carried out either in warm, dry weather or while under cover
with clean plastic sheeting.Two coats of finishing paint should be applied as per specification which is generally
Light Grey.
ii. Check for any signs of mechanical damage
Checks must be carried out for mechanical damage to the fabrications and associated
equipment. Particular attention should be given to vulnerable areas such as radiators.
If damage is seen on the equipment, a decision must be taken as to its seriousness. It may be necessary to take corrective action such as the replacement of an item of
equipment.
iii. Check all joints for signs of leakage
All joints, both welded and gasketed, must be checked for signs of oil leakage. If
there is any doubt of a leak, the area must be cleaned of oil, using a suitable solvent
(methyl alcohol) and sprayed with liquid chalk. This will promote the flow of the
leak and give a good indication as to the extent location of the leak, if in fact there is
one. If a leak is suspected on a gasket, the joint must be tightened until such time that
it can be changed for a new gasket. If a leak is apparent at a welded joint, once again
clean the area, apply liquid chalk and allow to dry. This will highlight the point
exactly if, in fact, there is a leak. A temporary repair to a ‘pin hole’ leak is to peen
over the hole to stem the leak. It must be emphasized that this is a temporary repair
and must be properly repaired with welding procedures, when convenient. Prior to
leaving the leak, it must be highlighted with a marker or similar identification, so it is
not lost when permanent repair takes place.
Other areas commonly associated with oil leaks are drain plugs in radiators, valves in
the oil management, cooling system, gas relay and oil actuated relay.
iv. Check oil levels
It is good practice to check all separate oil levels associated with the equipment. This
will incorporate the expansion vessel and all oil filled bushings. This check must be
carried out with the equipment at ambient temperature. Also the oil in the oil seal
should be maintained.
v. Check that surrounding areas are clean and tidy
Once all the above checks have been carried out, a careful check should be made to
ensure that any materials or tools, used for maintenance work, have been removed.
All cloths and other debris must be disposed off. The transformer compound should
be left in a clean and tidy condition.
vi. Oil Leakage and moisture absorption:
Possible leakage: After energizing of the transformer, a certain settling may appear in
sealing joints. This applies especially to sealing joints with plain gaskets that are not
placed in grooves. These should, therefore, be re-tightened as indicated below:
Tank, Cover, Conservator: about a month after taking into service, when required.
Bushings, Turret: about a month after taking into service, when required.
The torque on the bolt should be as per manufacturer’s recommendations. If no clear
cut guidelines are recommended, the following may be used which is valid for joints
between details of steel, as per IS: 1367:-
The maximum recommended torque has been calculated on the basis of the material
properties and 70% of the proof stress has been considered as limit value for
tightening torque.
The breather contains an oil-cup that prevents spontaneous air circulation/filtration of
debris. This cup should remain filled with oil to the specified.
vii.. Marshalling Cubicle and Kiosk Check
The transformer and associated equipment need not be put out of service, isolated or
earthed while carrying out these checks. All results must be recorded in formats for
comparison during future checks.
Checks to be carried out.
Condition of paint work.
Operation of door handles.
Operation of doors and hinges.
Condition of door seal.
Door switches working.
Lights working.
Heater working.
Thermostats working.
Operation of heating and lighting switches.
Mounting of equipment secure.
Manual operation of switches satisfactory.
Checking of tightness of cable terminations.
Checking of operation of contactors (isolating the trip signals, if any).
HRC fuses and their rating.
Operation of Local alarm Annunciator by pushing push buttons provided for lamp
test, acknowledge, reset system test, mute etc. to cover all system function.
Source change over test check by putting off power sources alternatively.
Check for plugs for dummy holes and replacement, if found missing.