Page 192 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

• little deterioration with rise in temperature (The insulating properties should not change much with the rise in
temperature i.e. when electrical machines are loaded.)
• non-absorption of moisture, when exposed to damp atmospheric condition. (The insulating properties,
specially specific resistance and dielectric strength decrease considerably with the absorption of even a slight
amount of moisture.)
Classification of insulators (Fig 1)

Air is an example of a gaseous insulator. Other examples are hydrogen, nitrogen and inert gases.
Liquid insulators
Mineral oils, synthetic liquids, resins and varnishes are the liquid insulators.

Transformer oil
In transformers the oil is used as an insulator and also forcooling of the transformer windings by convection.
Therefore, the transformer oil should be dry and purified, since the presence of moisture will reduce the dielectric
strength of the oil.
Purpose of transformer oil
• Transfer of heat by convection, from winding and core to the cooling surfaces.

• It maintains the insulation of winding and also extinguishes fire that occurs due to faults occurring in the
windings.
Precaution
The insulating value of a transformer oil is reduced due to the formation of sludge as a result of oxidation due to
air and temperature. To minimise oxidation, the oil should not be exposed to air.
Sludge is also formed due to the presence of acids and alkalis.
Sludge formation

• Reduces the rate of heat transfer.
• Blocks the ducts.
• Increases the operating temperature.
To prevent moisture from entering the oil, the whole apparatus is made airtight, and calcium chloride, silicagel
fillets are used.
Testing of transformer oil as per ISI Standard (Fig 2)
Dielectrical strength test (Refer to Fig 2) : The oil should be 40 mm above and 40 mm below the electrodes. The
gap between the two electrodes should be kept at 4 mm ± 0.02 mm).
A high voltage is applied across the electrodes through a step-up transformer, and increased till there is a spark
in between the electrodes. The voltage noted on the voltmeters, when the spark occurs, is the breakdown voltage
or dielectric strength of the oil. This is the maximum voltage the oil can withstand.
According to ISI specifications, the oil should be able to withstand 40 kV for one minute with a gap (4 mm ± 0.02
mm) between the electrodes and with the diameter of the electrodes as 13 mm.

Moisture test : In this test, an oil sample is cooled in a closed vessel down to 15-25°. A dry test tube, 12.5 mm in
diameter and 125 mm long, is taken and an adequate quantity of oil is poured into it.

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CITS : WCS - Electrical - Exercise 18