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Oil Testing Definitions

esi oil testing


Throughout this section ESI’s recommended limits have been included as a guide.

AC              Acceptable
QU             Questionnable
UN             Inacceptable
BD              Bad
VB              Very Bad
EB              Extremely Bad


Moisture Content



Measures the water content of the oil at a particular temperature. This is an extremely important test as it can be related to the actual moisture content, by dry weight, of the transformer winding. Moisture is one of the critical factors associated with degradation of cellulosic insulation through loss of tensile strength.
Moisture content can be directly related to % moisture by dry weight of the transformer winding.

Winding moisture contents are considered as:

0 - 2% Dry
2 - 4% Entering risk zone
4 - 6% Considerable risk
7% Failure imminent


Moisture in oil


AC              < 10 ppm
QU            10 – 20 ppm
UN              > 20 ppm


Dielectric Dissipation Factor

Measures the leakage current through oil. This is a measure of the contamination or deterioration. (Polar contaminants, ageing products or colloids.)
When the DDF exceeds 0. 5% an investigation is indicated. If a 100C reading is more than 7 to 10 times the value at 25C this usually indicates a soluble component other than water.




Neutralisation Number (or Acidity)


This is a measure of the amount of acidity (from oxidation) in the oil. Acid destroys the cellulosic insulation by reducing the tensile strength of the paper.



Interfacial Tension Test

IFT measures the tension at the interface between oil and water. This test is extremely sensitive to the presence of oil decay products and soluble polar contaminants from solid insulating materials. This test is thus directly related to the degree of the oils oxidation. (sludge formation)



Relationship between IFT and NN

An increase in NN is normally followed by a characteristic drop in IFT. IFT is a powerful tool for determining how insulating oil has performed and how much life is left in the oil before some correction is needed to prevent sludge. ( See curve Transformer Life Extension) A low IFT not accompanied by a corresponding increase in acidity and colour, indicates polar contaminants which have not come from normal oxidation of the oil.




Resistivity is a measure of the amount of conductive contaminants in the oil.


Specific Gravity


Specific gravity is the ratio of the mass of a given volume of oil to the mass of an equal volume of water at a specific temperature. It can be an indicator of PCB or water contamination but will vary with base stock.



Dielectric Strength

A measure of the oils ability to withstand electric stress without failure. Free water and solid particles will tend to migrate to areas of high electric strength and reduce the breakdown voltage dramatically.


Oil Colour Comparison Test

As an individual test the colour has little value. It is important however, when noting marked colour changes. Very dark colour can be an indicator of gassing and oil
oxidation, Colour level is determined by comparison with a series of colour standards.


Visual Examination


Good oil should be clear and sparkling. Cloudiness indicates moisture, carbon and/or sludge.


Metals in Oil

Measures the dissolved metals in the oil. This can aid in identifying a fault location


Inhibitor Content

Oxidation in a transformer is an ongoing process. By monitoring and replenishing the level of inhibitor (an anti-oxidant) present, the oil can be prevented from oxidising. Inhibitor type DEPC is added at the rate of 0.3% by weight of oil.



Furans are formed through the degradation of cellulosic paper insulating materials. Overheating of insulation, oxidation and degradation by moisture in insulation contribute to formation of a variety of compounds that include several derivatives of furan. These are known as "furans". They can be accurately measured and are directly related to cellulose breakdown.


There are five commonly formed furan compounds measured.

5H2F 5 - hydroxymethyl - 2 - furaldehyde Indicative of an active fault.

2ACF 2 -acetyl furan, indicates presence of a bad furan but it does not allow itself to be measured as any

particular item.

2FOL 2- furfurol Indicative of wet insulation.

5M2F 5 - methyl -2 - furaldehyde. System has had a shock such as a lightning strike.

2FAL 2 - furaldehyde. This furan is always present. High concentrations represent a critical situation with

very rapid cellulose deterioration.


Furan measurement is of most value when assessed in company with, or as support to other oil tests (NN, IFT, Dielectric, Colour, Dissipation Factor, DGA and Moisture Content.)


Furan Levels

As a guide S D Myers would recommend the following: Expressed in parts per billion (10 minus 9. [ppb]) i.e. 5000ppb=5ppm


Total Furans

Healthy Transformer < 25ppb
Questionable Transformer 100-250ppb
Unacceptable Transformer 250ppb



Dissolved Gas Analysis (DG )

Certain combustible gases are generated when a transformer experiences abnormal thermal and electrical stresses. The rate at which these gases evolve is also important as the normal ageing of a transformer will also produce these gases at an extremely slow rate.
The interpretation of DGA results requires considerable skill and a number of methods are used to assist.


As a general guide however the following can be considered.

Nine gases are analysed:




The percentages in which different gases are represented is an indicator of the type of fault from which the gas has evolved.

The faults that generate gases are:









One method of analysis is by identifying the "key" gas. Accurate interpretation of results requires special skill. As a general guide the following conditions may be occurring when the percentage of combustibles are as indicated














Traces of acetylene may be formed if the fault is severe or involves electrical contacts.



Total Combustible Gases.


Combustible gases in the range of 0 - 500 ppm are normally an indication of satisfactory operation of a transformer.
A range of 500 – I000 ppm of combustible gas indicates that decomposition may be in excess of normal ageing. ( suggesting more frequent analysis)
More than 1000 ppm of combustibles usually means that decomposition is significant. (A trend should now be established to monitor the position.)


In Summary

A great deal of information is available through the oil of a transformer. Understanding the information that is available and being able to interpret test results provides the asset owner with a powerful tool.

ESI can provide oil sampling, testing and reporting on the current condition of your assets, We will explain the analysis and recommend action necessary to control the three critical criteria, thereby extending the operating life of the transformer.

It is also important to establish a regular sampling program to monitor and detect any trends that may be occurring. This may also lead to more frequent monitoring of particular prevailing or developing conditions in a transformer that require special attention.

ESI can provide a complete service of regular sampling, testing, interpretation, recommendation and as well, the provision of a data base to record and “trend” ongoing information.

Regular monitoring of transformer insulation condition through the oil and the consequent early advice and corrective action, as required, is an extremely cost effective means of extending the operating life of the most expensive item of plant in the switchyard.





Transformer Oil Reclamation The Economic & Environmental Solution
Energy Services International
Transformer Maintenance Institute (A Division of S D Myers Inc.)
An Introduction to the Half Century Transformer
Fifty + Years A Guide to Transformer Maintenance JJ Kelly, SD Myers RH Parris

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