Techincal Information Regarding Transformer Oil

In this section we would like to educate our viewers regarding the technicalities of electrical transformers, substations, their maintenance, etc. We are very sure that after going through this informative resource, viewers would be happy to have stumbled upon this vast amounts of information which can be useful professionally and academically.

Transformer Testing

If transformers are used to their full potential, or used in critical operations, it is recommended that oil testing be performed annually. There are several screenings and tests performed to determine the quality of oil such as:

Dielectric Strength

It is the voltage at which the insulating value of the oil breaks down and a spark jumps the gap between two electrodes. A low dielectric indicates contamination, usually from moisture in the oil. However this test does not give the true indication of deteriorated condition of oil.

Karl Fischer( Total Water Content In Oil )

This test measures moisture in the oil in ppm. It will also give values for the percentage saturation of the oil and the percent of moisture by dry weight of the paper insulation.


It is a measure of the total acid concentration in the oil. It is measured by titrating the oil against an alcoholic solution of potassium hydroxide (KOH) of known concentration. Acids not only attack the insulation and core of the transformer but also play a part in the formation of sludge. New oil typically has an acid concentration of less than 0.03 mg-KOH/g. When concentrations in excess of 0.5 mg-KOH/g are reached, sludge is imminent and damage will occur if allowed to remain.

Interfacial Tension( IFT )

It is a measure of the surface tension of oil in contact with water. Many of the contaminants that form in oil are colloidal particles with different types of ends which tend to attach themselves to ionic particles on one hand and non-ionic on the other. Thus they tend to align themselves on the water-oil interface and lower the surface tension. When the concentration of these polar particles is high enough, they line up like iron filings in a magnetic field, particularly in areas of high stress, forming conducting bridge's and sludge which impairs the insulating and cooling functions of the oil.
New oils typically have an IFT value in excess of 40 dynes/cm. When the value drops to near 20 dynes/cm, sludge formation is imminent. Below 15 dynes/cm sludge formation is a certainty.

Specific Resistance( Resistivity )

It is the DC resistance of volume of oil of unit Cross Sectional Area & unit length prevalent unit is OHM-cm. It is desirable to have specific resistance of oil as high as possible. This test gives a measure of the oil soluble contaminants in the oil, resulting from oxidation of the oil itself, or from the solution of the external containers or materials used in the construction of the equipment. This test may be carried out at room temperature as well as at 90°C for getting better information.

Dielectric Dissipation Factor

It is also known as tan-delta or loss factor. The power factor is the ratio of the power dissipated in the oil in Watts to the product of the effective voltage & current in volt amperes, when tested with a sinusoidal field under prescribed conditions i.e. at 90°C. A high power factor value is an indication of the presence of contaminants or deterioration products. This test is a sensitive method of measuring the presence of soluble contaminants & aging products in the oil. Changes in the characteristics may be monitored even when the oil is heavily contaminated.
Lower the Liquid Power Factor, the better. Transformer oils that look good at 25°C can fail the 90°C test. Because transformers have been designed to operate at a Hottest Spot temperature of 110°C, the PF Test 90°C is tremendously important. If the oil fails to protect the transformer under design stress a failure can occur.

Dissolved Gas-In-Oil Analysis

This is a valuable tool in the preventative maintenance of transformers. Internal transformer faults(three types are corona, hot spots, and arcing) generate gases at an early stage and a portion of these gases dissolve and remain in the transformer oil. Each type of fault will generate gases in quantity and proportion that may be recognizably different than those generated by the other fault types. This "uniqueness" in gas generation for each fault type is the basis for transformer fault diagnosis. By sampling the transformer oil and measuring the dissolved gases, an indication is obtained as to the possible source of a gas generation. This in turn aids in performing preventative maintenance on the transformer. Test analyzes the oil for combustible gases in ppm, which are generated by faults in the transformer. Cellulose Paper and transformer oil under stress will generate gases, some of which are combustible. These gases can be found in transformer oil and give vital information on the type of problem being experienced by the unit. Obvious advantages that fault gas analyses can provide are:

  •     • Advance warning of developing faults.
  •     • Determining the improper use of units.
  •     • Status checks on new and repaired units.
  •     • Convenient scheduling of repairs.
  •     • Monitoring of units under overload.

Why Test?

One of the most obvious questions people might be having is Why to test transformers? Well the answer(s) are obvious and self explanatory.Testing yields information about the condition of the material and the equipment from which the sample was taken.

Testing Protects Your Bottom Line:

Transformers are critical wherever electrical power is being generated. A breakdown or failure can cause total or partial plant shutdown, which can threaten your competitive edge as your uptime percentage plummets and your revenues are compromised. Maintenance of transformer oil and transformers can help prevent costly transformer breakdowns and failures.

A Proven Early Warning System:

Even though transformers are considered to be reliable equipment, breakdowns and failures do occur. Most breakdowns are the result of insulation deterioration or deteriorated oil. Internal electrical faults such as overheating, arcing or partial discharge(corona) will cause the transformer insulation to break down over time.
When the transformer insulation is subjected to an electrical fault, combustible gases become dissolved in the oil. Chemical analysis of transformer oil for these gases can reveal indications of current and future problems. This information is valuable in developing maintenance programs that help prevent costly failures and power interruptions.

More Than Testing - You Gain Knowledge:

Knowing the condition of your equipment not only helps avoid losses, it also helps you work smarter. The interpretive engineering report gives your maintenance staff the knowledge needed to establish priorities, plan work assignment schedules, arrange outside service, and order parts and materials.

IS 1866-2000 Tables

Classification of categories of equipment are as follows:

Category O. Power Transformer > 420 KV
Category A Power Transformer > 170 KV <= 420 KV
Category B Power Transformer > 72.5 KV <= 170 KV
Category C Power Transformer <= 72.5 KV
Rating Instrument Transformer > 170 KV
Category E Instrument Transformer <= 170 KV
Category F Diverter Tanks Of On Load Tap Changer(OLTC)
Category G Oil Filled Circuit Breakers


  • 1.Separate Selector Tanks Of On Load Tap Changers belong to the same category as the associated transformer
  • 2.For transformer up to 1 MVA & 36 KV, the guidelines given in Category C are adequate
  • 3.Oil Impregnated paper bushing & other hermetically sealed equipment may be placed in Category D

Application & interpretation of tests


Equipment Category Permissible Values Recommended Action
Power Trans > 420 KV
Power Trans > 170 KV <= 420 KV
Power Trans > 72.5 KV <= 170 KV
Power Trans <= 72.5 KV
Instrument Trans > 170 KV
Instrument Trans <= 170 KV
Clear, without visible contamination As dictated by other tests

Breakdown Voltage

Equipment Category Permissible Values Recommended Action
Power Trans > 420 KV
Power Trans > 170 KV <= 420 KV
Power Trans > 72.5 KV <= 170 KV
Power Trans <= 72.5 KV
Instrument Trans > 170 KV
Instrument Trans <= 170 KV
> 50 KV
> 50 KV
> 40 KV
> 30 KV
> 50 KV
> 50 KV
Recondition oil or alternatively if more economical or other tests dictates replace oil

Water Content

Equipment Category Permissible Values Recommended Action
Power Trans > 420 KV
Power Trans > 170 KV <= 420 KV
Power Trans > 72.5 KV <= 170 KV
Power Trans <= 72.5 KV
Instrument Trans > 170 KV
Instrument Trans <= 170 KV
<= 20 ppm
<= 20 ppm
<= 40 ppm
No free moisture at room temp.
<= 40 ppm
<= 30 ppm
Check source of water and consider reconditioning
Note: The given values are applicable only when acidity < 0.1

Neutralization Value

Equipment Category Permissible Values Recommended Action
All equipment categories <= 0.3 mg KOH/gm Replace or Reclaim oil
Note: Perform tests more frequently when acidity > 0.2mg KOH/gm

Sediment and precipitable sludge

Equipment Category Permissible Values Recommended Action
All equipment categories No sediment or precipitable sludge should be present When sediment is detected recondition When precipitable sludge is detected Replace/reclaim


Equipment Category Permissible Values
(1012 ohm-cm)
Recommended Action
Power Trans > 420 KV
Power Trans > 170 KV <= 420 KV
Power Trans > 72.5 KV <= 170 KV
Power Trans <= 72.5 KV
Instrument Trans > 170 KV
Instrument Trans <= 170 KV
20°C <= 90°C Investigate

Dielectric Dissipation Factor at 90°C AND 40-60 Hz

Equipment Category Permissible Values Recommended Action
Power Trans > 420 KV
Power Trans > 170 KV <= 420 KV
Power Trans > 72.5 KV <= 170 KV
Power Trans <= 72.5 KV
Instrument Trans > 170 KV
Instrument Trans <= 170 KV
0.2 max
0.2 max
1.0 max
1.0 max
0.2 max
0.3 max
Note: Comply with manufacturer instructions if other frequency and limits are recommended

Interfacial Tension

Equipment Category Permissible Values Recommended Action
All equipment categories 0.015 N/m Investigate

Flash Point

Equipment Category Permisible Values Recommended Action
All equipment categories Max decrease by 15°C Replace oil, Equipment require inspection

Gas Content

Equipment Category Permissible Values Recommended Action
All equipment categories IS: 10593 Investigate
Note: Comply with manufacturer instructions

Residual life assesment of transformers

How to be sure your transformer has not weakened by heat

When Oil Soaked paper is damaged by heat, some unique oil soluble compounds are released into the oil along with carbon mono oxide and carbon dioxide, which are derivatives of the aromatic compound called Furan.

Paper is made of cellulose which is a polymer consisting of long chains of glucose rings joined by glycosidic bonds. During the many degradation processes the glycosidic bonds are broken and the glucose rings are opened. Glucose level in the transformer may indicate the extent of paper degradation, but glucose is unstable and has a very low solubility in oil. Glucose further degrades to produce furans, which are more stable and oil soluble. Furan is a heterocyclic aromatic system consisting of four carbons and one oxygen in a five membered ring with each of the carbons having hydrogen attached; hence the molecular formula is C4H4O.

The five most prevalent derivatives of furan that arise from the degradation of the cellulose and that are soluble in the oil to an appreciable degree are
the following:

  •    •  2-Furaldehyde
  •    •  Furfuryl alcohol
  •    •  2-Acetylfuran
  •    •  5-Methyl-2-furaldehyde
  •    •  5-Hydroxymethyl-2-furaldehyde

Because of the furfuryl family of compounds(known as furans or PIBP or Paper Insulation Breakdown Products) can only be derived from the destructive heating of cellulose(i.e. paper & wood products).


The entire details of the procedure for determining the quantity of furanic compounds in an insulating fluid are given in the ASTM D 5837 method and are only briefly mentioned here.

A sample of the oil is extracted with either another liquid such as acetonitrile or with a solid phase extraction(SPE) device. The extract is then analyzed using high performance liquid chromatography(HPLC). The five compounds mentioned above are separated on an appropriate column and each is detected by use of a Photodiode Array Detector that is adjusted automatically to the appropriate wavelength for each of the five components. Calibration solutions are made up for each of the components to be analyzed and these are used to standardize the instrument responses. From the data on the standard solutions, the extraction efficiencies for each component can be calculated and corrections can be made accordingly. The results are usually reported in terms of parts per billion(ppb).


The five furanic materials normally analyzed in this procedure are aromatic compounds that arise from the degradation of the cellulosic materials within a transformer either by normal aging or from being involved with an incipient fault. Thus the amount of these products present in the oil might be a good indication of the condition of the cellulosic insulation. Research is being carried out to find if there are any useful correlations that can be used but at the present it might only be useful to follow trends rather than absolute amounts in the oil. Another test that can be used to assess the condition of the cellulose within a transformer is to determine the average degree of polymerization. However, this is an intrusive test that requires a sample of the cellulose. One has to take the unit out of service to obtain the sample and a portion of the unit is destroyed in the process.

Advantages of PIBP Analysis over Degree of Polymerisation of Paper:

  •    •  It is a non-intrusive procedure
  •    •  It does not require interruption of service to obtain a sample
  •    •  The laboratory analysis requires less time and it is a more sensitive determination

History of Analysis

Previous furan study have focused either on the details of the analytical techniques or on experiment with laboratory analysis of transformer. A typical study involves some copper & steel present to make the system more similar to transformer.

The co-relation shows that when you cook paper in oil well enough to weaken it it also releases furaldehyde into the oil.

Such studies are excellent for testing of materials but are not very similar to real transformer because

  •    • Transformer are dynamic system with constantly fluctuating electrical loads & temperature
  •    • It does not have uniform internal temperature. As the test bottle in the oven
  •    • Transformers in the field are subject to various uses, which may affect the oxygen and/or water content of oil.

This study is an effort to move beyond interesting pilot studies and deal with transformer under actual operating condition.

Individual Furan Measurements

The most abundant furan by far was 2-furaldehyde. However the other four furans plus 2-furaldehyde constitutes Total Furans.

In our testing of different furans we have arrived at an estimated detection limit of 1 ppb with an accurate quantification being near to 5 ppb level.

Furans & gases

The strongest co-relation of total furans is with carbon oxide gases. Such relation is expected since the breakdown of cellulose insulation can be described by the following equation:

Cellulose = Glucose + H2O + CO + CO2 + Organic Acids

The combination of gas & furan testing is especially powerful as it allows for a determination of the extent of paper damage. In the case of severe localized paper damage, high furans with high gas content can be seen. In case of general overheating of the paper insulation or general aging one can see a slow building of the furans concentrations without necessarily seeing an increase of gas content. With both DGA & Furan Test one can usually diagnose the problem and undertake remedial action. The nature of action will be depending on the severity of problem.

Furans with water

The correlation is weak. But it has long been known that the water speeds the breakdown of cellulose chains by breaking the bonds between the cellulose rings. However the transformer is a dynamic system, water migrates from paper to fluid depending on the temperature. Furans possibly behaves in the same way. It may be ultimately necessary to take into account fluid temperature in order to understand furan behaviour completely.

Furans & IFT

Also high furans tend to be more common in bad oil. Still furans correlate very weakly with IFT. The relationship arises because furans are trace constituents of the oil. IFT measures the total hydrophilic molecules that have appeared in the oil through oxidation, etc. Furans fall in that category, but other types of molecules make the dominant contribution to low IFT.

According to Myers, sludge begins to precipitate out of the oil and into the paper of the transformer in the range of 0.10 to 0.60 mg KOH/gm roughly corresponding to 0.032 to 0.015 N/m of IFT Measurement. What may be happening in this area is that sludge begins to form so does the paper insulation begins to age due to acids and sludge's in the oil attacking the paper. In the range the acids are formed the furans are produced from the paper breakdown. As this happens they migrate to the oil portion of the insulation system and are attracted to the sludge in suspension. When the sludge precipitates out of the fluid to the paper insulation, the furans are pulled out of the fluid along with the sludge. It also helps us to support some long held beliefs that the acids are an integral part of the paper breakdown process.

Effect of maintenance of furans in oil

   Treatment of Fuller's Earth

Dripping the oil through Fuller Earth through gravity does reclamation of the oil. Fullers Earth is known to strongly retain polar molecules. In the field multiple passes through the earth at an elevated temperature. The earth more effectively purifies the oil.

Oil Treatment Total Furans before treatment Total Furans after treatment
Reclamation ratio of oil: earth (2: 1) 4388 269
Reclamation ratio of oil: earth (1: 1) 4381 24
Reclamation ratio of oil: earth (2: 1) 11269 9
Heat 65 And Vacuum < 10 Microns 5754 2930

The fuller's earth is extremely effective at the removal of furans. The effectiveness depends on the volume ratio of oil to earth and also initial abundance of furans. One can confidently conclude that reclaiming with fuller's earth will remove essentially all furans.

Treatment by Vacuum Processing

Condition 4 of the above table the oil was heated to 65°C and vacuum of less than 10 microns. The oil was cycled for 10 times. The heat and vacuum did remove some furans, but not very effectively. Thus Heat and vacuum degassing do remove the furans. But does not completely reset the clock as does the fuller's earth.

Acceptable Limits

  •    • 0 - 100 ppb of total furans should be acceptable
  •    • 101 - 250 ppb of total furans should be Questionable
  •    • 251 - 1000 ppb of total furans should be Detiorated
  •    • 1001 - 2500 ppb of total furans should be Low Reliability
  •    • More than 2500 ppb of total furans should be Rewind/Replace

It is also very important to look at the rate of formation prior to forming an opinion on the meaning of the data. A transformer that produces furans at the rate of 25ppb per month would definitely be of more interest than one that has been holding steady at 250 ppb for the last few years. One must be careful in attaching a life expectancy value to transformer based upon the furan analysis. The benefit that is to be gained with this test is that we can see the paper being degraded in time to perform some maintenance on the unit to stall the rate of degradation, or in extreme cases, pull it off line for immediate action prior to a failure.

To conclude the breakdown of paper insulation results in formation of many different products. No individual measurement gives the total picture of paper degradation. By looking at all of the pertinent test(i.e. Furans, Gas-in-Oil Analysis and oil screening tests) we can get a more complete picture of the extent of the damage and the reasons for it. But caution is necessary if one is to attempt understanding this complex process by merely viewing one facet of this whole picture.


  •    • Furfuraldehyde is the easiest to measure. It offers the best available indication of the heat damage to the paper
  •    • One can be certain that, if a transformer has no furfuraldehyde in its oil, none of its paper has been seriously weakened by heat
  •    • PIBP analysis is useful for heat runs, If the transformer has nothing wrong, you will see no rise in PIBP levels
  •    • Rate of generation of furans are of more interest than the Total value of furans in one sample
  •    • If the Total Furans does not change for a very long time, it can be removed by Fuller's Earth treatment. Although a very high level of Total Furans takes more                 than one pass through Fuller's Earth
  •    • If the furans have been extracted from oil, they should be analyzed within 12 hours, as some furans tends to disappear at up to 5 % per day