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.
Some specific parameters of transformer or insulating oil should be considered to determine the serviceability of that oil.
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 the transformer or the paper insulation of the transformer, which cannot be otherwise detected without a complete overhaul of the transformer.
Colour & Appearance
Inter Facial Tension
IS 1448 [P-25]
IS 1448 [P-2]
Dielectric Dissipation Factor
Dissolved gas analysis
The most important function 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 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 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.
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 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 winding, tank walls and in cooling ducts.
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.
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.
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 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 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 which may be created with the transformer, which may or may not be associated with a change in a physical or chemical property of the insulating oil.
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 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 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.