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, ageing 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 the oil, sudden faults can be discovered in time and outages can potentially be avoided.

Properties of Transformer Oil Or Insulating Oil

Following parameters of insulating oil or transformer oil 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 of paper insulation of the transformer, which cannot be otherwise noticed without a complete analysis of the transformer.

IS Codes Of Transformer Oil Tests:

1 Colour & Appearance ISO 2049 D-1524 IS 335
2 Inter Facial Tension ISO 6295 D-971 IS 6104
3 Viscosity ISO 3104 D-445 IS 1448 [P-25]
4 Flash Point IEC 2719 D-92 IS 1448
5 Pour Point ISO 3016 D-97 IS 1448
6 Water  Content IEC 60814 D-1533 B IS 13567
7 Acidity IEC 62021 D-974 IS 1448 [P-2]
8 Dielectric strength IEC 60156 D-877 IS 6792
9 Specific Resistance IEC 60247 D-924 IS 6103
10 Dielectric Dissipation Factor IEC 60247 D-924 IS 6262
11 Dissolved gas analysis NA D-3612-A IS 9434
12 Furan analysis IEC 61198 D-5837 —–


Water Content or Moisture Test

The most important function of transformer oil is to provide electrical insulation. Any increase in moisture may result in the 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 SulphurTest

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. However, in this test, oil gets replaced if corrosive sulphur oil is found.

Acidity Or Neutralisation Number Test

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

Sludge Test

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 Test

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 no longer has the potential to perform vital functions.

Specific Resistance or Resistivity Test

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 Test

When an insulating material is positioned between the grounded part and live part and of electrical equipment, a 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 1970s 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 has led to a ban in many countries.

Furan Analysis

They are a measure of the cellulose paper’s degradation. The mechanical strength of a paper decreases when 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 for those of age 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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