Transformer Oil Testing
Transformer oils are subject to electrical and mechanical stresses while a transformer or other electrical distribution equipment is in operation. The analysis of insulating oils 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 a cost-efficient electrical maintenance program.
Though the failure risk 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, suddenly occurring faults can be discovered in time and outages can potentially be avoided.
Properties of Transformer or Insulating Oil
Some specific parameters of transformer or insulating oil should be considered to determine the serviceability of that 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 windings of the transformer or the paper insulation of the transformer, which cannot be otherwise detected without a complete overhaul of the transformer.
IS codes of Transformer oil tests:
Colour & Appearance
Inter Facial Tension
IS 1448 [P-25]
IS 1448 [P-2]
Dielectric Dissipation Factor
Dissolved gas analysis
Water Content or Moisture
The most important functions of transformer oil is to provide electrical insulation. Any increase in moisture content can reduce the insulating properties of the oil, which may result in dielectric breakdown. Many transformers contain cellulose-based paper used as insulation in the windings. Excessive moisture content can result in the breakdown of this paper insulation with a resultant loss in performance.
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. If corrosive sulphur oil is found, oil will need to be replaced.
Acidity or neutralisation number
Total acid number is the quantity of base (mg of KOH) that is required to neutralise 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 insulation of winding. Acidity accelerates the oxidation process in the oil and also includes 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 windings, tank walls and in cooling ducts.
Dielectric Strength or Breakdown voltage
The dielectric strength of transformer oil is defined as the maximum voltage that can be applied across the fluid without electrical breakdown. Because transformer oils are designed to provide electrical insulation under high electrical fields, any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function.
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 placed between live part and grounded part of an electrical equipment, leakage current will flow. Electric current through the insulator will lead the voltage with an angle little bit shorter than 90o. Tangent of the angle by which it is short of 90o is called Dielectric Dissipation Factor or simply tan delta of transformer oil.
Dissolved Gas Analysis or DGA of Transformer Oil
Dissolved gas analysis determines the concentrations of certain gases in the oil such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene The concentrations and relative ratios of these gases can be used to diagnose certain operational problems with the transformer, which may or may not be associated with a change in a physical or chemical property of the insulating oil.
Polychlorinated Biphenyl or PCB Analysis
PCB’s were realised 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 legislation to know the PCB content of your transformer oil and have to led banning in many countries.
Furan derivatives are a measure of the degradation of cellulose paper. When the paper ages, its degree of polymerization reduces, so its mechanical strength decreases. The degree of polymerization of the paper can be directly related to the concentration of furan derivatives in the oil.
Tests to be performed on in service oils
Tests for Electrical properties and DGA of the oil of the transformer of the 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.