Transformer Oil Testing

Transformer oils are subjected to electrical and mechanical stresses while a transformer or other electrical distribution equipment stays in operation. The analysis of insulating oil provides information about the oil, but also enables the detection of other possible problems, including contact arcing, aging insulating paper and other latent faults and is an indispensable part of the cost-efficient electrical maintenance program.

Though the risk of failure of a transformer and other oil-filled electrical equipment is small, when failures occur, they inevitably lead to high repair costs, long downtime and possible safety risks. By accurately monitoring the condition of oil, sudden faults can be discovered in time and outages can potentially be avoided.

Properties of Transformer or Insulating Oil

Following parameters of insulating oil or transformer should be considered to analyse the serviceability of  oil.

  • Physical Parameters – Colour and Appearance, Density, Inter Facial Tension, Viscosity, Flash Point and Pour Point.
  • Chemical Parameters – Water Content, Corrosive Sulphur, Acidity, Sludge Content.
  • Electrical Parameters – Dielectric Strength, Specific Resistance, Dielectric Dissipation Factor.
  • Dissolved gas analysis
  • Furan analysis
  • PCB analysis

The Furan and DGA tests are specifically not for determining the quality of transformer oil, but for determining any abnormalities in the internal winding or paper insulation of the transformer, which cannot be otherwise noticed without a complete analysis  of the transformer.

IS codes of Transformer oil tests:

1Colour & AppearanceISO 2049D-1524IS 335
2Inter Facial TensionISO 6295D-971IS 6104
3ViscosityISO 3104D-445IS 1448 [P-25]
4Flash PointIEC 2719D-92IS 1448
5Pour PointISO 3016D-97IS 1448
6Water  ContentIEC 60814D-1533 BIS 13567
7AcidityIEC 62021D-974IS 1448 [P-2]
8Dielectric strengthIEC 60156D-877IS 6792
9Specific ResistanceIEC 60247D-924IS 6103
10Dielectric Dissipation FactorIEC 60247D-924IS 6262
11Dissolved gas analysisNAD-3612-AIS 9434
12Furan analysisIEC 61198D-5837—–


Water Content or Moisture

The most important function of transformer oil is to provide electrical insulation. Any increase in moisture may result in dielectric breakdown as content can decrease the insulating properties of the oil. Many transformers contain cellulose-based paper that is used as an insulation in the winding. Excessive moisture content can result in the breakdown of this paper insulation with a resultant loss in the performance.

Corrosive sulphur

Corrosive sulphur forms acidic conditions in Transformers. This is a vital test as DGA, MAD (Moisture, Acid & Dielectric) and Furan analysis, tests may indicate normal operation even when failure is imminent. Oil gets replaced if corrosive sulphur oil is found.

Acidity or neutralisation number

Total acid number is the quantity of base (mg of KOH) that is required to neutralize acid constituents in 1g of sample. If oil becomes acidic, water content in the oil becomes more soluble to the oil. Acidity of oil deteriorates the insulation property of paper insulating the winding. Acidity triggers the oxidation process and rusting of iron in presence of moisture. An increase in acid indicates that sludge formation is beginning to occur or is occurring.


The deposition of sludge in the oil ducts blocks the free circulation of oil impairing cooling, increased temperature and more sludge. Sludge deposits itself on winding, tank walls and in cooling ducts.

Dielectric Strength or Breakdown voltage

The maximum voltage  without any electrical breakdown that can be applied across the fluid is the dielectric strength of transformer oil . These  transformer oils are designed in a manner such that under high electrical fields, any significant decrease in the dielectric strength may signal that the oil is no longer potential of performing vital functions.

Specific resistance or Resistivity

It is the DC resistance of volume of oil of unit cross sectional area and unit length. It should be as high as possible. An increase in temperature reduces the resistivity.

Dielectric Dissipation Factor or Loss Factor

When an insulating material is positioned between grounded part and live part and of an electrical equipment, current will flow. Electric current through the insulator will lead the voltage with an angle little bit shorter than 90°. Tangent of the angle by which it is short of 90° is called Dielectric Dissipation Factor or simply tan δ of transformer oil.

Dissolved Gas Analysis or DGA of Transformer Oil

It measures the concentrations of certain gases in the oil such as carbon dioxide, hydrogen, nitrogen, oxygen, carbon monoxide, methane, ethylene, acetylene and  ethane. The concentrations and relative ratios of these gases can be used to diagnose certain operational problems which may be created with the transformer, which may or may not be associated with a change in a chemical or physical property of the insulating oil.

Polychlorinated Biphenyl or PCB Analysis

PCB’s were used for the first time in the 1970’s during the oil crises to bulk up the transformer oils. Due to the high toxicity of PCB’s it is now legal to know the PCB content of your transformer oil. No PCB testing have led to ban in many countries.

Furan Analysis

They are measure of the cellulose paper’s degradation. The mechanical strength of a paper decreases when the the paper ages which reduces the  degree of polymerization. The degree of polymerization is directly proportional to the concentration of oil’s furan derivatives.

Tests to be performed on in service oils

Tests for Electrical properties and DGA of the oil of a transformer of age below 10 years should be done after every two years and of more than 10 years, it should be done every year. These tests are also required to be carried out after every dehydration.

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