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Transformer Oil Testing

Routine testing of transformer oils and insulating oils on a regular schedule is an accepted industry practice. General and physical tests are suggested twice a year, dissolved gas analysis once a year, and Furan testing every 2 years for transformers in operation at least 5 years.

AES Laboratories provides complete transformer oil and insulating oil analysis and testing services.  The following tests are generally carried out for ensuring the good health of transformer oil or other insulating liquids.

  • Appearance
  • Dielectric Breakdown Voltage
  • Density at 15°C
  • Specific Gravity
  • Water Content
  • Sediment, Sludge
  • Interfacial Tension
  • Kinematic Viscosity
  • Neutralisation Value mg KOH/g
  • Dissipation Factor at 100°C
  • Flash Point
  • Dissolved Gas Analysis
  • Furans Analysis
  • Particle Count (NAS 1638,ISO 4406)

Some of the key analytical parameters along with their indications are given below:

Dissolved Gas Analysis

Dissolved Gas Analysis is the single most important test performed on oils from transformers. As the insulating materials in a transformer break down due to thermal and electrical stresses, gaseous byproducts are formed. The byproducts are characteristic of the type of incipient-fault condition.

Furanic Compounds In Oil

As the cellulosic insulation in a transformer ages, oil-soluble byproducts of the cellulose chain called furanic compounds are produced. High concentrations of 2-furfural, the predominant compound, are a clear indication of cellulose degradation, as this is the only type of material in transformers that yields this byproduct. When cellulosic materials are exposed to extreme temperatures, which results in charring, furanic compounds can be destroyed and the carbon oxides may be the only byproducts remaining in significant quantities. Experience is required in evaluating the furanic compound data, as there are factors such as the type of insulation preservation/oil expansion system, type of conductor wrapped insulation, the family of transformer and treatment of the oil or the transformer, which can influence the interpretation. Tests for furanic compounds should be performed initially for all power transformers (to have a baseline), for important or older transformers, when high carbon oxides are generated, for highly loaded transformers and when other tests indicate accelerated aging.

Water in Oil Analysis

The dielectric breakdown voltage of insulating material is a function of the water content. The water migrates between the solid and liquid insulation in a transformer with changes in temperature. The water content is reported in parts per million and percent of relative saturation.

Degree of Polymerization

The degree of polymerization (DP) test is another means of assessing insulation aging. This test is performed on paper samples. The DP test provides an estimate of the average polymer size of the cellulose molecules in materials such as paper and pressboard. Generally, paper in new transformers has a DP of about 1000. Aged paper with a DP of 150-200 has little remaining mechanical strength, therefore making windings more susceptible to mechanical damage during movement, particularly during extreme events such as through-faults. As insulation aging in transformers can be uneven due to thermal, moisture, oxygen and byproduct concentration, gradient samples from various locations are needed to provide the best diagnosis of the overall insulation condition.

DP testing is recommended in these cases:

  • There is other evidence of very accelerated aging of the insulation
  • The transformer is > 20 years old and an internal investigation is being performed
  • For condition assessment of older transformers for possible refurbishment
  • When a partial rewind is being considered
  • To assess cause of failure
  • For condition assessment of insulation when purchasing a service-aged transformer
  • To assess the condition of a transformer after an extreme overheating event

Oil Quality Screens

Once in service, the dielectric liquid should be tested periodically to make sure it retains its important properties such as good dielectric breakdown voltage, low acidity, and no sludge formation. The rate of deterioration of the insulating oil should be relatively slow. Accelerated aging may indicate an equipment or operating problem. AES Laboratories offers a number of Package Screen Tests to suit your testing needs for new and service-aged insulating liquids. We are equipped to perform a comprehensive Bureau of Indian Standards (BIS) specification tests, as per IS 335 and IS 1866 for new and used transformer oils respectively.


There are two types of metal-in-oil tests commonly done on insulating oil:

Wear Metals: Pumped cooling systems are susceptible to bearing wear, which when excessive can create metal particles which are harmful to the insulation system. To detect such problems, an oil sample is filtered and the particles are analyzed by atomic absorption spectroscopy for excessive amounts of copper, lead, zinc, and iron.

Dissolved Metals: The high temperatures of some incipient fault conditions will cause the number of dissolved metals associated with the problem to increase along with the dissolved gases in oil. Comparison of the metal-in-oil content with baseline values before the occurrence of the incipient fault condition can help locate the source of the gassing and the problem.

Polychlorinated Biphenyls (PCBs)

PCBs are highly regulated, therefore insulating liquids that may contain PCBs should be tested to ensure proper handling and disposal. AES Laboratories offers a testing service to quickly and accurately quantify the amount and determine the type of PCBs present in various insulating liquids, solids or other materials such as soils. 

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