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Transformer Insulation Oil Testing

Laboratory Tests
The complete examination includes all the tests listed below along with the evaluation of the general condition of the oil, consideration of all results together often enables the cause of degradation or the source of a contaminant to be recognized, so that the appropriate action can be taken to ensure the reliable operation of the equipment.

  • Dielectric strength(Break Down Voltage)
  • Water Content
  • Dielectric Dissipation Factor (tan delta)
  • Specific Resistance (Resistivity)
  • Neutralisation Value (total acidity)
  • Sediment/Perceptible sludge
  • Interfacial Tension & Flash point
  • Dissolved Gas Analysis


Electric Strength (Break down Voltage)

The voltage at which the oil breaks down when subjected to an ac electric field with a continuously

increasing voltage contained in specified apparatus. The voltage is expressed in KV.

Water Content

Flash Point

The temperature at which the oil gives off so much of vapor that this vapor, when mixed with air,

forms a ignitable mixture and gives a momentary flash on application of pilot flame under the

prescribed conditions.

Precipitable Sludge

Oil deterioration products or contaminants or both, Which are insoluble after dilution of the oil

with n-heptane but are soluble in the solvent mixture of equal parts of toluene, acetone and

alcohol under prescribed conditions.


Any substance which is insoluble after dilution of the oil with n-heptane and also insoluble in the

solvent mixture of equal parts of toluene, acetone and alcohol Under prescribed conditions.

Specific Resistance (Resistivity)

It is the ratio of the dc potential gradient in volts per centimeter paralleling the current flow within the specimens to the

current density in amperes per square Centimeters at a given instant of time and under prescribed conditions. This is

numerically equal to the Resistance between opposite faces of a centimeter cube of the liquid. It is expressed in ohm-


Dielectric Dissipation Factor (Tangent Delta)

It is the tangent of the angle (delta) by which the phase difference between applied voltage and resulting current deviates

from ^/2 radian when the dielectric of the Capacitor consists exclusively of the insulating oil.

Interfacial Tension

It is a force necessary to detach a planar ring of platinum wire from the surface of the liquid of higher surface tension that

is upward from the water-oil surface. It is expressed in N/m.

Dissolved Gas Analysis

Insulating materials within transformers and electrical equipment break down to liberate gases within the unit. The

distribution of these gases can be related to the type of electrical fault, and the rate of gas generation can indicate the

severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any

preventative maintenance program.