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Transformer Oil Analysis Methods
Moisture Content – IEC 60814
Moisture can reduce transformer oil’s effectiveness as an insulator. Elevated moisture content also accelerates the degradation of the paper.A progressive reduction of the paper-oil insulating properties can result in electrical defects such as partial discharge, which can evolve into electrical discharges and power arcs with electrical faults in the transformer.A transformer with less than one percent moisture content in the paper could typically last 40 years, whereas a moisture content of four percent or higher could reduce the anticipated life to 10 – 15 years.
Dissipation & Resistivity Factor – IEC 60247
The dissipation test measures the leakage current through oil, which is the measure of the contamination or deterioration. The test is not specific in what it detects i.e. more a screening test. Resistivity is a reliable indicator of the condition of transformer oil, low resistivity values being a sign of oil, which contains particulate contaminants and oxidative products.
Flash Point (Pensky-Martens Closed Cup Tester) (ASTM D93-IP34; ASTM D3941)
Flash point is a physical property of the oil. Transformer oil has to operate safely within the transformer environment. When standard design practices began to incorporate higher hot spot temperatures, the specification limit for flash point of the oil had to be increased to maintain a margin of safety.
Total Acid Number (TAN) – IEC 62021-1
The TAN value indicates the potential of corrosion problems within a system. As the acid number increase (usually due to oxidation of the oil) the insulating quality of the oil (and paper) decreases.
Furan Analysis Of Transformer Oil -ASTM D5837 - IEC 61198
Furanic compounds are typically present from 50 ppb to 9000 ppb according to the age of the transformer. Therefore, direct analysis and quantification of these compounds serves as an indicator of the age and health of the transformer.
Dissolved Gas Analysis – ASTM D3612
The amounts and types of gases found in the oil are indicative of the severity and type of fault occurring in the transformer. The types of gases we are concerned with are hydrogen, methane, ethane, ethene, acetylene, carbon dioxide and carbon monoxide.There are various international guidelines on interpreting DGA. The most frequently used is the Rogers Ratio Method
The Rogers Ratio Method diagnoses faults using 3 gas ratios:
C2H2/C2H4, CH4/H2, C2H4/C2H6
This indicates the following types of faults:
Normal ageing, partial discharge, low and high energy density
High energy of thermal faults and electrical faults
Breakdown Voltage or Dielectric Strength – IEC 60156/1995-05
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Interfacial Tension – ISO 6295
The interfacial tension (IFT) test provides an indicator of the presence of oil decay products (acids) and soluble polar contaminants from solid insulating materials. The greater the concentration of contaminants, the lower the IFT.
Relative Density (ASTM D1298)
This is a test of a physical property that relates to the oil’s composition and function. Specific gravity directly affects heat transfer. Specific gravity of oil should not change because of aging. Significant changes, while in-service, are an indication that the oil has been contaminated.
Degree of Polymerization (DP) (Calculation based on Furans content – IEC 61198 )
The mechanical properties of insulating paper can be established by measurement of the degree of polymerisation (DP). This is determined by analysing the oil for the presence of 2-furaldehyde.
The paper in a new transformer is mechanically strong (has high tensile strength) and has a DP value of between 800 and 1200. As the paper ages, it breaks down due to the effects of heat, moisture, oxygen, and acids. As this breakdown occurs, the chains become successively shorter, and thus the DP declines, resulting in weakened paper. When the DP reaches a level of 200, it is brittle and is considered to be at the end of its useful life. At this level, the electrical and mechanical strength of the transformer is severely compromised.
Corrosive Sulphur – (DIN51353 and IEC62535)
Most recent failures due to corrosive sulphur are related to the formation of copper sulphide deposits in and on the surface of winding cellulosic paper.
Free, elemental sulphur (S8) and some sulphur containing compounds (DBDS) in oil will react with metals in a transformer, particularly copper and silver, which leads to corrosion of conductor, connections, and soldered or braised joints. The growth of copper sulphide on bare copper may cause the presence of conductive particulates in the oil, which can act as nuclei for electrical discharge and may lead to a fault.
The IEC62535 method uses a copper conductor, wrapped with one layer of paper, immersed in the oil and heated to evaluate the capability of the oil to yield copper sulphide and transfer it to paper layers.
The DIN51353 method uses a silver strip immersed in the oil and heated. After this process the silver strip is inspected for any discolouration.
Oxidation Inhibitor Content (DBPC) (IEC60666)
Most mineral oils contain an added oxidation inhibitor which is a chemical additive that acts as a preservative. Inhibitor is used to prevent oxygen reacting with the oil and as the result slowing the ageing rate of the oil as well as solid insulation. An inhibitor concentration of less than 0.1% is deemed unacceptable as the concentration of inhibitor is too low to adequately protect the oil from oxidation. So, when the concentration drops to below 0.1%, the oil should be re-inhibited to bring the concentration up to an acceptable 0.3%.
Polychlorinated Biphenyls (PCB) Analysis – IEC 61619
PCB analysis detects the presence of polychlorinated biphenyls in transformer oil. These toxic organic pollutants were used extensively in transformers for decades because of their ideal properties for use in transformers. The analysis is performed under ISO 17025 accreditation using the IEC61619 standard method.
Sulphides Species (DBDS) Analysis – IEC 62697
Oil containing reactive sulphur species identified in most cases as dibenzyldisulfide (DBDS) can react with copper to form copper sulphide on the surface of the conductors and on the paper insulation surfaces even under normal operating conditions of transformers. Oil with a high concentration of DBDS is susceptible to the formation of corrosive sulphur.
Other tests
- Particle Count Distribution (ISO4406)
- Viscosity at 40oC (ASTM D445)
- Rotating Pressure Vessel Oxidation Test (RPVOT)
