Jill Duplessis - Global technical marketing manager
An important part of managing the life of a transformer involves testing the oil. Quite a bit of attention is paid to dissolved gas analysis (DGA) testing, which is a test performed on the oil to gain information about the electrical condition of the transformer, but what about the tests used to assess the condition of the oil itself? This article, a reprint of Megger’s Oil Tan Delta (OTD) Transformer Life Management (TLM) bulletin, discusses the oil Tan Delta and resistivity tests. Visit Megger’s website to access this and several more topics reviewed in TLM Bulletin series.
Transformer oil is used in transformers, circuit breakers, HV switches and cables. In transformers, oil performs several functions. It electrically insulates as well as cools, which is critical for transformer performance. Oil not only acts as a dielectric (insulator) itself but also notably enhances the dielectric properties of the solid insulation by impregnating paper and other cellulose-based insulation materials after they have been dried to fill voids in these materials. As a result, the dielectric strength of the solid insulation is notably increased beyond that of either the dry paper or oil alone. Finally, oil minimizes contact of oxygen with cellulose and other materials in the transformer that are susceptible to oxidation. Oxidation is an enemy of oil too. The transformer tank is well sealed from the atmosphere and the transformer preservation system serves to eliminate or minimize the exposure of the oil to air. If analysed regularly, oil may extend the service life of these network critical assets and may prevent catastrophic failure resulting in network down time.
As well as providing insulation, cooling and heat transfer, transformer oil has to remain stable at high temperatures and for considerably long periods, and for this reason it must have good electrical, dielectric, physical, and chemical properties.
A number of tests can be performed on the oil. Electrical/ dielectric, physical and chemical measurements on the oil provide information about the oil’s condition. Dissolved gas-in-oil analysis (DGA) testing provides information about the electrical condition of the transformer. Finally, a test for furans in the oil provides information about the condition of the cellulose. Some of the electrical, physical and chemical tests used to assess oil quality and condition include:
- Colour – visual inspection and comparison ASTM D1500 colour scale
- Moisture content – Karl Fischer (ASTM D1533, IEC 60814)
- Acidity (ASTM D1534, IEC 62021)
- Interfacial tension (ASTM D971, ISO 6295)
- Pour point
- Dielectric breakdown (BDV) (ASTM D877 & ASTM D1816, IEC 60156)
- Tan Delta (Dielectric Dissipation Factor (DDF)/ Power factor) (ASTM D924, IEC 60247, IEC 61620, BS 5737, JIS C2101, VDE 0380-2, IS 6262)
- Resistivity (ASTM D1169, IEC 60247, BS 5737, JIS C2101, VDE 0380-2, IS 6103)
Each test provides a piece of information about the oil under test, and like pieces of a puzzle they combine to form a picture of the condition and usability of the oil.
What Is Tan δ, Or Tan Delta?
Tan Delta, also called Loss Angle or Dielectric Dissipation Factor (DDF) or Power Factor (PF) testing, is an electrical dielectric test on the insulating oil used to determine its quality. The test is performed at two temperatures. Information from each test and the results considered together may form the basis for making a judgment on whether it’s suitable for a transformer to continue in service and for determining when the replacement or regeneration of the transformer oil is needed.
How Does It Work?
A Tan Delta test is performed by applying an ac voltage to a test cell of known gap, measuring the total current flow through the oil, and separating and comparing the reactive and resistance portions of the current passing through the oil.
If the insulating oil is free from contamination, the oil and the electrodes that it separates (e.g., the transformer HV windings and the grounded tank) closely exhibits the properties of a perfect, parallel plate capacitor.
In a perfect capacitor, when an ac voltage is applied across its electrodes, the resulting current flow through the capacitor is capacitive (e.g., phase shifted 90 degrees leading relative to the voltage signal); a manifestation that represents the energy being stored (and released) by the capacitor on each half cycle. However, if there are impurities in the insulation (i.e. oil) that separates the capacitor’s electrodes, the resistance of the insulation (which is typically extremely large and thereby impedes real current flow) decreases, resulting in measurable and increasing resistive current through the insulation/capacitor. The resulting total current flow through the capacitor, I TOTAL or IT, is the vector sum of the capacitive current, IC, and the resistive current, IR. The phase shift between applied ac voltage and total current through the capacitor will be less than 90 degrees when IR is present. The extent to which the phase shift is less than 90 degrees is indicative of the magnitude of IR, which in turn reflects the level of insulation contamination, hence quality and reliability of the oil.
Fig. 1- The loss angle δ
The “Loss Angle”, or δ, as shown in Fig. 1, is the angle between a purely capacitive current, IC, and the actual total current that flows through the capacitor, IT (which differs from IC when it includes the influence of real current flow, IR, arising from the losses in the dielectric that separates the capacitor’s electrodes). The loss angle, or δ, is measured and the tangent of this angle, given by IR / IC, is analysed. When the angle is zero, the Tan Delta is 0%, indicating that the oil has no losses. As losses (given in Watts) in the oil increase, the resistive current contribution to the measurement (given by IR = Watts/Volts) increases, and δ subsequently increases. The % Tan Delta is no longer zero. Increasing % Tan Delta signifies deterioration of or increasing contamination in the oil.
What is Resistivity?
Resistivity is another indicator of oil’s electrical quality, particularly apropos for used oil. Resistivity is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. For a DC resistivity measurement, a specified DC voltage is applied to the cell, and after one minute, the current flowing between the electrodes is measured. Average resistivity values are calculated from readings taken after direct and reverse polarity. It is noted that cell electrodes should be short-circuited for a prescribed amount of time (which may vary by standard) between direct polarity and reverse polarity measurements and also between Tan Delta and resistivity measurements.
The primary enemies of transformer oil are oxidation, contamination, and excessive temperature. High values of Tan Delta commensurate with reduced levels of Resistivity indicate the presence of polarised contaminants, such as moisture, particles and fibres. This testing is a sensitive way to check for contamination when purchasing unused new oil, determining the progress of the oil purification process and for quality control of oil in service.
Each test adds a piece of the puzzle combining to reveal an overall picture but the tests themselves are effective at identifying the presence of particular types of contaminants, as given in Fig. 2.
Fig.2 - Contaminants detectable by tan delta and resistivity measurement
Note: Oxidation is a chemical reaction involving the transfer of electrons. The substance that gives away electrons is oxidized. Oxidation may result in the formation of acids in the insulating oil and the formation of sludge. Transformer oil may contain an oxidation inhibitor – a chemical additive that acts as a preservative. The purpose of the inhibitor is to prevent oxygen from reacting with the oil and to thereby slow the aging rate of the oil and solid insulation.
Excessive heat is an enemy of oil. It causes decomposition of the oil and/or increases the rate of oil oxidation . Dielectric losses in the oil, given in Watts, reflect the amount of applied electrical energy that is lost to heat. Therefore, as losses in oil increase, the resultant increase in heat leads to further degradation of the oil, which, in turn, generates more heat. A loss/ heat/ loss cycle is created and gains speed as the condition of the oil worsens.
As loading on a transformer increases, increasing magnitudes of current flow through the windings, and heating rises – worsening the condition of already degraded oil which is providing heat transfer. Transformers that have oil in poor condition with inadequate cooling can become dangerously overheated and in extreme cases may cause an explosion. Awareness of the electrical or dielectric state of the oil is a key step towards averting such a scenario.
Why test at high temperature?
High temperature Tan Delta tests are more sensitive to small changes that occur in oil characteristics. Some ionic contaminants may not be detectable at 25°C but are revealed in the 100°C test . Measuring Tan Delta or Resistivity at two temperatures, e.g., ambient and 90 °C (IEC 60247), as opposed to just one temperature, may provide additional, useful information. For example, obtaining a high Tan Delta test result at ambient and an acceptable Tan Delta test result at a high temperature typically indicates the presence of moisture, since the moisture will evaporate at higher temperatures. Conversely, a high Tan Delta test result at both temperatures (or at higher temperatures only) is often attributed to the presence of contaminants.
Table 1 summarizes what is indicated by good and failing combinations of Tan Delta test results at ambient and high temperatures.
Table 1 - Diagnostic information revealed by Tan Delta test results at ambient and high temperatures
Evaluating Test Results
Test results can be greatly affected by the handling, cleaning and storage of oil and test instrument components. Erroneous results may be caused by:
- Pollution due to sampling or improper handling.
- Incomplete cleaning of the cells
- Prolonged exposure of the oil to light during storage.
Test standards provide a means to evaluate Tan Delta and resistivity test results.
IEC and IEEE provide test standards, Fig. 3, that are applicable to specific groups of transformer oil, namely New Oil, Oil in service and Reclaimed oil.
Fig. 3 - IEC and IEEE standards for acceptance and maintenance of insulating oils
Each of these standards specifies how to interpret results of Tan Delta (DDF) by comparison of measured results with published recommended levels that are transformer specific.
For new transformer oil, for example IEC 60296 specifies
And IEEE C57.106 specifies
These tables provide an easy means to compare measured values with expected values at the specified voltage class of the transformer.
For information on Megger's highly productive oil tan delta test set click here
 H. A. Pearce, “Significance of Transformer Oil Properties,” Electrical Insulating Oils, STP 998, H.G. Erdman, Ed., American Society for Testing and Materials, Philadelphia, 1988, pp. 47 – 54.
 ABB Inc. TRES – Transformer Remanufacturing and Engineering Services, North America, ABB Service Handbook for Power Transformers, January 2006.
This article, a reprint of Megger’s Oil Tan Delta (OTD) Transformer Life Management (TLM) bulletin, discusses the oil Tan Delta and resistivity tests. Visit Megger’s website to access this and several more topics reviewed in TLM Bulletin series.