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.
Following parameters of insulating oil or transformer should be considered to analyse the serviceability of 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 paper insulation of the transformer, which cannot be otherwise noticed without a complete analysis of the transformer.
|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||—–|
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 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.
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.
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 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.
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 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.
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.
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.
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 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.