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Transformer Oil Testing

Transformer Oil Testing maybe a proven loss prevention technique that should be a neighbourhood of any condition-based predictive maintenance program. Transformer Oil (also known as insulating oil) is a particular type of oil with exceptional electrical shielding properties and is stable at high temperatures. Transformer oil is used in oil-field electrical power transformers to cloister, prevent arcing and partial discharge, and scatter the transformer (i.e., act as a coolant).
The transformer’s liquid not only serves as a heat transfer medium; it also is part of the transformer’s insulation system. Therefore, it is sensible to occasionally perform tests on the oil to determine whether it is proficient in pleasing its role as an insulant. Some of the most common transformer oil tests are: Screen Tests, Water Content, Metals-in-Oil, and Polychlorinated biphenyl, and Dissolved Gas in Oil Analysis.

Transformer Oil Testing Methods are a vital part of any maintenance program. An annual DGA is the most crucial test for the liquid lining. Analysis of gases in the oil can specify insulation overheating/ overloading, corona discharge, liquid overheating, or arcing in the transformer. The screen test comprises additional physical, chemical, and electrical tests; this includes power factor, interfacial tension, acidity, colour, and dielectric breakdown. No single test alone can indicate the authentic stature of the liquid. Thus, it is necessary to perform all the tests. Furthermore, a useful test is performed by transformer oil labs that include those for water content, PCBs furanic compounds, and metals-in-oil.

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:

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 insulation in the winding. Excessive moisture content can result in the breakdown of this paper insulation with a resultant loss in 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.

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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 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 a 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 the 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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• What is the transformer oil?

 Transformer oil is also known as insulating oil is a type of oil having outstanding electrical insulating properties which remains stable at high temperatures used in oil-filled electrical transformers to insulate, prevent arcing, corona discharge and act as a coolant.

• What is transformer oil testing?

Transformer oil testing is a method of measuring breakdown voltage and various chemical and physical properties of the transformer oil, either in the laboratory or by using any sort of portable test equipment on site.

• What are oil transformer testing methods?

Breakdown Voltage Test (BDV)

Colour and Appearance Test (IS335; ASTM D-1524)

Water content or Moisture Test (IS1356; ASTM-D-1533B)

Specific Resistance Test (IS6103; ASTM-D-924)

Dissolved gas analysis (IS9434;  ASTM-D-3612-A)

Furan analysis (ASTM-D-5837)

• Why Transformer oil testing is required?

To maintain and expand the life of power transformers avoiding severe breakdowns by determining the problem conditions with oil or transformer as the quality of transformer oil predicts the quality of transformers.

• How often transformer oil should be tested?

Routine testing of transformer oil should be done every 2 years on a regular schedule.

• Is transformer oil tested?

Transformer Oil is an insulting and cooling oil used in transformers and other electrical equipment, required to be tested periodically to make sure that it is still apt for the purpose.

• How often should transformer oil be tested?

Standard testing of transformer oils and insulating oils on a regular schedule is a recognized industry practice. General and physical tests are recommended twice a year, dissolved gas consideration once a year, and Furan Testing should be done every two years for transformers in operation at least five years.

• What is the transformer BDV test of oil?

BDV test means the Breakdown Voltage Test. This test is performed for verifying the dielectric strength of the oil of the transformer. Dielectric Strength is the maximum capacity to resist the voltage of insulating oil. This test demonstrates the dielectric Strength of Transformer Oil.

• What is ppm in transformer oil?

PPM stands for Parts Per Million. It is used to measure the weight of moisture divided by the value of oil. The moisture content in oil lowers the insulating system dielectric strength that permits flashover to harm a transformer. For example, for mineral oil, a generally accepted maximum moisture content is 35 ppm.

• Is 335 a transformer oil?

IS:335 is a trace subdued oil because, as per IS, oxidation inhibitor up to 0.05% is to be measured as the absence of DBPC while as per IEC 60296 clause 3.6, such oil is treated as trace reserved as the latest IEC standards.

• What is the Colour of transformer oil?

Typically, oil is yellow, orange, or somewhat red when fresh and will work as proposed. As the oil ages, it transforms into brown or black colour, and its efficiency weakens.