Transformer Oil

The functions of the transformer oil are

i. to provide dielectric strength.

ii. to protect the insulation system, and

iii. cooling.

Transformer Oil

Transformer oil is a mineral insulating oil derived from crude petroleum. It is a mixture of various hydrocarbons.

Aliphatic compounds

(open chain compounds) with the general formula Cn H2n+2 and Cn H2n.

Many oils also contain certain aromatic compounds (closed chain or ring compounds) related to benzene, naphthalene and derivatives of these with aliphatic chains.

Good transformer oil must

• insulate
• prevent flash over of the exposed parts within the equipment and
• it must effectively transform the heat from the core to the radiating surface.

Transformer Oil – Characteristics

The characteristics of new transformer oil is given in I S 335 / 1983.

Characteristics	Requirement
Appearance	Clear and Transparent, Free from suspended particles
Density at 27 0 C (Max.)	0.89 g / cc
Dielectric Strength (BDV) - New - After Filtration	30 kV (rms) 50 kV (rms)
Dielectric Dissipation Factor (Tan d) At 90 0 C – ( Max.)	0.005
Neutralization Value Total Acidity (Max.) b. Inorganic acidity / alkalinity	0.03 mg. KOH / gm. Nil

Characteristics	Requirement
Kinematic Viscosity at 27 0 C (Max.)	27 cSt.
Interfacial Tension at 27 0 C (Max.)	0.04 N / m
Flash Point	140 0 C
Pour Point	-9 0 C
Corrosive Sulphur	Nil
Oxidation Stability Neutralization Value after oxidation (Max.) Total Sludge after oxidation (Max.)	0.40 mg. KOH / gm. 0.10 percent by weight
Specific resistance (Resistivity) At 90 0 C ( Min.) At 27 0 C (Min.)	30 x 10 12 ohm-cm. 500 x 10 12 ohm-cm.

TRANSFORMER OIL - QUALITY CHECKING

There is no single test to judge the quality of transformer oil. Each test is significant within its limits and is able to give only general information about the overall condition of the oil.

The most popular test is the dielectric strength test. If the dielectric strength is low, the oil is unfit for use regardless of any other condition. Good transformer oil must have 60 kV withstand voltage between two brass electrodes - 12.7 to 13 mm dia, arranged horizontally with their common axis not less than 40 mm above the bottom of the cell, with a gap of 2.5 mm.