Testing and Erection of Transformers


Part to be tested Test to be performed
Winding Resistance Across Windings
Turns Ratio/Polarity/Phase
Excitation Current at all tap positions
Short Circuit impedance
Insulation resistance to ground (megohmmeter)
Capacitance (doble)
Power factor/dissipation Factor (doble)
Induced Voltage /Partial Discharge/Riv
Bushings Capacitance (doble)
Dielectric Loss (doble)
Power factor/dissipation factor(doble)
Partial discharge (doble)
Temperature (infrared)
Oil Level (Sight Glass)
Visual Inspection (Cracks and Cleanliness )
DGA Insulating Oil Dissolved gas analysis
Dielectric Strength
Interfacial Tension
Acid Number
Visual Inspection
Water Content
Oxygen Inhibitor
Power Factor/Dissipation factor
Tap Changers -Load Contact Pressure and Continuity
Temperature (Infrared )
Turns Ratio at all Position
Motor Load Current
Limit Switch Operation and Continuity
Tap Changers -Load Contact Pressure and Continuity
Temperature (Infrared )
Turns Ratio at all Position
Motor Load Current
Limit Switch Operation and Continuity
Tap Changers – No Load Contact Pressure and Continuity
Turns Ratio at all Position
Visual Inspection
Core Core Insulation Resistance to tank
Ground Test (Megohmmeter)
Tanks and Associated Devices Pressure /Vaccum/Temperature Gages – Calibration
Temperature (Infrared)
Visual Inspection (Leaks and Corrosion)
Conservator Visual Inspection (Leaks and Corrosion)
Air Direr Desiccant Proper Color
Valves in Proper Position
Sudden Pressure Relay Calibration and Continuity
Buchholz Relay Proper Operation and Continuity
Cooling System Temperature (Infrared)
Heat Exchanger Radiators Clear Air Flow
Visual (Leaks, Cleaning, and Corrosion)
Fans Controls
Visual Inspection and Unusual Noise
Pumps Rotation and Flow Indicator
Motor Load Current


Indian code of Practice for Institution and Maintenance of Transformer is: 10028, that generally cover all the relevant aspects of the special attention required at the time of the erection of transformer, should be referred before commissioning the installation works.


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Instruction To Erection Personnel: All the people having access to the interior of the transformer while inspection, fitting bushing or any other fittings, should empty their pockets and all loose articles. Spanners and other tools in use should be securely tied with a cotton tape to a suitable point so that they can be recovered if accidentally dropped inside.

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Cleaning: Fibrous materials should not be used for cleaning. The presence of loose fibers in suspension with transformer oil can reduce its insulation property. If cleaning or wiping is necessary, this should be done with clean dry oil using soft and non-fluffy cloth.

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Fire risk: Transformer oil is inflammable and under certain circumstances in a confined place, it becomes explosive. Naked light and flame should never be used near the transformer.

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Condensation: Transformer oil and other solid insulation that is normally used in transformer construction absorb moisture easily, particularly when cold. Condensation can be caused if the interior of a tank is exposed to the atmosphere. But, this can be minimized by taking adequate care. The transformer can be protected from “damp” hazard by circulating warm and dry oil through it until a temperature is 50 to 100 C above ambient. This should be done before allowing access to the interior of the tank. Warm oil should be circulated all the time while the transformer is open to atmosphere.

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Painting: Corroded components should be cleaned thoroughly to remove the rust, dirt, and grease and repaint with one coat of red oxide primer and two coats of enamel paint should be done. Retouching of paint may be required at the places where the paint is damaged during transit.

Location and Foundation:

A leveled and firm foundation that is strong enough to support the complete weight of the transformer and prevent accumulation of water should be provided. Continuous external vibrations may set up physical movements on the core and windings and in turn, may lead to a breakdown. While locating the transformer, care should be taken to provide adequate clearances from the live parts of the transformer to earth parts. Easy access should be provided all around like, to diagram plate, thermometer, valves, oil gauges etc. so that they can be easily reached or read.

Suitable rails or tracks should be used for rollers and their wheels should be locked to prevent any accidental movement…
At the place where the transformer is enclosed in the chamber, it should be ensured that it gets goods ventilation.

Wherever the transformers are mounted on poles, the structure should be strong enough to withstand the weight of the transformer, wind pressure and other atmospheric conditions.

Erection of transformer:

Generally, small transformers, up to 1000 KVA, are dispatched oil fitted with all fittings. The fittings that are dismantled to facilitate transport may be reassembled in the following sequence, depending on the site conditions:

A sequence of erection:

Indian code of Practice for Institution and a Maintenance of Transformer IS:10028 generally covers all the relevant aspects of the special attention required at the time of the erection of transformer, should be referred before commissioning installation works.

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Fitting of rollers

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Oil check

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Buchholz relay and pipework

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Silica gel dehydrating

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Explosion Vent

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Terminals HV-LV bushings or cable boxes, CT terminals etc

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Topping up of oil

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Temperature indicators

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Marshaling boxes

Details of erection:

All fittings and accessories should be assembled as shown in outline drawing.


Before mounting of rollers, they should be checked for soundness of casting and to ensure that bearing surface is not damaged. The bearing surface should be greased or oiled before mounting.

Assembly of rollers becomes simple if the transformer is lifted by overhead crane or pulley block. A transformer may be jacked up with mechanical or hydraulic jacks (Jacking pads are provided for transformers 1600 KVA and above). The height of jacking pads from the ground, without wheels in positions, should not be less than 300 mm. Suitable wooden or steel packing may be given under jacks to raise the jack height.

b. Oil check:

A sample of oil should be taken out from the bottom of tank and tested for the Electric strength, (break – down voltage) which should not be below 30 KV (RMS) when tested on a standard test as specified in IEC – 600296 Table 1-9 (Electrode dia. 120.5 mm polished, electrode gap 2.5 mm # 0.1). The electric strength should be the arithmetic mean of the six results that are taken on the same sample of oil.

c. Conservator :

When Dismantled for transport (Ref .3.1)Before assembly of a conservator, make sure that inside of vessel and pipework is free from rust, dust, grease, and moisture. If found corroded, clean the rust and repaint with oil resistant paint from inside.
Enamel paint given is on the outside on a coat of red oxide-zinc chromate primer. Mount conservator on the corresponding supports in the tank cover. Pipework should be assembled in the following sequence. Conservator through Buchholz relay to tank, conservator to silica gel breather.Magnetic oil gauge should be assembled at the correct position.Plain oil gauge window glass should be checked for the cracks which may lead to oil leaks and entry of moisture in the conservator.The gasket should be tightened uniformly to make the joints leak proof. Deteriorated and leak gaskets should be replaced.

d. Buchholz relay:

Assembly precautions (if dismantled )
Relay pipework should be 3 deg. to 7 deg. upwards towards conservator.
Arrowheads on the body should be pointing towards the conservator.
Floats, which operate elements, should be free.
Check that mercury switches are intact.
Test alarm and trip mechanism. In order to test the alarm and trip mechanism, the relay is provided with two separate petcocks which enable air to be injected into the body of the relay. For testing the injected air, use of a cycle pump or any other suitable device is made, until switch operates (check continuity of contacts)To test trip elements, the valve controlling the bottle is opened quickly so that air or oil rushes in, impinges on the flap and depresses it for operating the switch.On each relay, petcock is provided for taking out samples and releasing gas from the top of relay housing.Testing of the relay may be carried out after fitting in the conservator pipe and filling of oil in the conservator. It is recommended that the relay should not be taken apart and under no circumstances should any alteration be made to operating floats or flaps.

e. Silica gel Dehydrating breather :

Breather is connected to the conservator through pipework. Breather assembly should be suitably clamped so as to prevent vibration. Fill up transformer oil through filling a hole, in the bottom compartment of the breather, up to the filling level mark. Air entry hole is sealed with the adhesive tape, which should be removed for free passage of air.

Check that silica gel is blue in color, if not, reactivate or replace it with fresh gel.

Remove cork/ Blanking tape from a top screwed hole and fix breathe on the pipework.

f. Explosion Vent

Explosion vent is provided with a Bakelite diaphragm. Normally, the transformer is dispatched fitted with explosion vent. If dismantled for the purpose of transport, remove blanking plate from the bottom of the vent before assembling it on the tank. Check whether the vent is corroded from inside. Rust should be completely removed and the inside section should be painted with oil resistant paint.

Before assembly, make sure that the top and bottom diaphragms, if provided, are intact.

The diaphragm should be held between two gaskets and care should be taken to clamp the flanges uniformly to prevent cracking of diaphragm.

Pressure equalizer pipe may be provided against specific requirement which connects to the top of explosion vent and then vents it to the atmosphere through the breather.