by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the related data relating to an asset’s operation. If we interpret the data correctly, it can give us great perception into the asset’s condition.
Frequent monitoring of the asset may find yourself in less upkeep required or more prolonged intervals without any upkeep required.
It is essential to determine the key parameters that are needed to give us a whole picture of the particular standing of the transformer and the action we have to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the data telling us?

Has the condition of the unit changed for the reason that last upkeep period?

Is it protected to operate the unit?

Are there signs of deterioration?

Is it secure to load the unit above the nameplate rating for a specific period?

Are we required to implement motion to ensure the continued reliability of the unit?

How long can we use the unit before we need to think about replacement?

Are the recognized issues of a recurring nature?

Effective situation monitoring outline

It is vitally important to identify clear objectives as part of your strategy. What do you wish to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you want to accomplish, it would be a lot simpler to identify the required parameters.
Health indexing of property is changing into a exceptional device in getting a clearer picture of the condition of your transformer. Test parameters carry a numerical value-adding to the whole worth of the Health Index Value of the transformer. These parameter weight values were calculated based on the international standards for mineral oils, indicating the important values stipulated in the various standards.
The scope of oil evaluation, interpretation of the information, and important values

At the beginning of this part, it’s essential to state that we cope with completely different dimension transformers within the business. Transformers are divided into classes according to the kV ratings of the equipment. It is up to the reliability or asset manager to make use of the rules for larger tools, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager wants to discover out what type of testing would profit him in identifying drawback areas inside his fleet. Some of the evaluation has been identified as to routine kind tests. Still, there could be an in depth range of exams that may assist in identifying particular downside criteria throughout the system, which could not be clear by way of the everyday day-to-day evaluation normally performed.
Please see the score classes in TABLE 1


TABLE 2 explains which oil analyses are really helpful and the way usually or under which circumstances.

Oil sampling

The taking of the oil pattern is considered one of the most crucial and significant influencers in the evaluation outcome. If a sample just isn’t taken to the prescribed process, then there’s a significant risk that the analysis carried out, and the outcomes acquired, are of no use.
หลักการทํางานของpressuregauge must be taken to make sure high quality control procedures are utilized in each step of the sampling process as set out by international standards. A good high quality sample taken by applying the right procedure is crucial. A sample can be contaminated by numerous components, all of which may influence the result of the leads to a unfavorable manner.
All steps involved in pattern taking should adhere to high quality control procedures, including the container that is used, the sampling package, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the sample, after which the delivery of the sample to the laboratory.
If the sample identification and sample data are handwritten, the human factor can lead to incorrect interpretation of the data. The label should be stuck onto the container to ensure no mix-ups in pattern identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write on the label ought to be oil- and water-resistant. Otherwise, some information could be lost, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s focus on the kinds of tests to determine the transformer’s situation, the crucial values, and the beneficial actions in every case:
a) Colour and appearance

This is a routine inspection utilized to every oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to determine the color, turbidity, and potential particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there is a lot of turbidity, it’d indicate a high water content material in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve could be included into the sample. If particles are recognized as carbon, it might point out a potential electrical fault within the unit. The DGA analysis of the oil will affirm if that is so.
Clear oils with out contamination will indicate an excellent situation, and no action is beneficial.
When oils are darkish or turbid, additional analysis will affirm any problems. The oil evaluation results will also determine the diploma and type of motion.
b) Breakdown Voltage


Breakdown Voltage (kV)O, A, D>6050 to 60<50

B, E>5040 to 50<40

C>4030 to 40<30

F<30 kV for OLTC in star level application<40 kV for OLTC in delta or line-end software

G <30

This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of international particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage should be high.
If the values are Good, it is strongly recommended to proceed with the current sample interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with different parameter results like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If different tests indicate severe growing older, the oil can be changed with new or reclaimed oil. Another option could be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are removed from the oil. This process removes acid and water as properly as another compounds. Another advantage is that the oil could be re-used, and in most conditions, this can be accomplished without switching off the unit, which contributes to cost-saving. If unsure – as an alternative swap off the unit throughout this treatment course of.
If the values are Poor, it’s advisable to take motion as soon as possible and not delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely excessive water content material could cause flashover within the unit, leading to lack of the asset.
c) Water content material (mg/kg at transformer operating temperature)


Water Content(mg/kg at Transformer Operating Temperature)O, A<1515 to 20>20

B, D<2020 to 30>30

C, E<3030 to 40>40

FAction necessity >40

GNot a routine check

This is a routine take a look at for all lessons of electrical gear, besides class G

The results of this test should at all times be considered at the side of the breakdown energy. If it’s found that the water content material is excessive and the breakdown strength is low, further action must be taken. It is recommended that a second sample from the identical unit is examined to confirm the results.
In the case of switching tools, where there is no paper current, the breakdown voltage is the determining factor.
It should be noted that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is found that the unit’s operating temperature is outside this temperature range, it’s best to refer to Annex A of the standard.
When the worth obtained through analyses is GOOD, the conventional sampling interval could be maintained, requiring no further action.
When the value returns a FAIR outcome, extra frequent sampling is beneficial. It can be useful to consider other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be carried out.
A POOR result would require quick action from the asset manager. This may embrace taking another sample to confirm the outcomes from the first evaluation. If it is confirmed that the water content material is high, the oil can be filtered; this process should remove a big portion of the moisture from the oil if applied appropriately. Follow-up samples need to be taken to make sure that the moisture content material is still within the required limits. The reason is that the most good portion of the water is caught up within the paper system within the transformer. This moisture will transfer from the paper into the oil underneath circumstances that favour this movement. It may be found later that the oil within the water has increased once more with none apparent reason, but the supply can be the paper in the transformer.
A visual inspection can also be really helpful to discover out if any water would possibly move into the transformer or electrical equipment through leaks. This downside may be extra extreme if the transformer or electrical tools is outside and never in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number


Acidity mgKOH/g oilO, A, D<0.100.10 to zero.15>0.15

B, E<0.a hundred.10 to 0.25>0.20

C< hundred.15 to 0.30>0.30

F, G Not a routine check

This is a routine check for all lessons except F and G

The acids in oils are fashioned due to chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help within the formation of the acids. Because acids are polar compounds, it’ll adversely have an effect on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this could lead to sludge formation, normally across the decrease parts of the transformer core. The sludge will finally type a semi-solid substance that’s extraordinarily troublesome to remove.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval ought to be decreased to fit the state of affairs. Future evaluation should embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition, the asset supervisor might resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever option may suit their necessities one of the best.
e) Dielectric Dissipation issue at 40Hz to 60Hz at 90° C


Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to 0.20>0.20

B, C<0.a hundred.10 to 0.50>0.50

D<0.010.01 to 0.03>0.03

E<0.a hundred.01 to 0.30>0.03

F, GNot a routine take a look at

This is a routine test for all courses of electrical equipment, except F and G

The dielectric dissipation factor or tan delta of this test provides info regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities which may affect the dissipation factor embody water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is really helpful. The structure of the oil is broken, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the beneficial dielectric values can’t be achieved.
f) Resistivity (GΩm) at 20 °C or 90 °C


Resistivity(GΩm)At 20° CO, A>20020 to 200<20

B, C>604 to 60<4

D>800250 to 800<250

E>607 to 60<7

This is NOT a routine take a look at

DC resistivity of the oil is among the key parameters to evaluate the transformer insulation condition; that is based mostly on the fact that DC resistance is delicate to grease degradation.
When the result is FAIR, extra frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content percent


Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of original value

This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to verify the major points relating to additives.
The two commonest oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a pair of,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the getting older process in the oil and the strong insulation.
If the result’s FAIR, it’s suggested to prime up the inhibitor degree to the prescribed level per supplier directions. It is advised to make use of a field skilled skilled in the procedure to carry out this process.
If the end result obtained is POOR, the recommendation for this scenario would recommend that the top consumer continues to use the oil “uninhibited,” but this will likely result in extra rapid degradation of each the liquid and solid insulation.
It must be famous that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels below 1000ppm. This can be in the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, though it would add additional safety in opposition to oil oxidation if the preservation system ever fails. [2]

h) Passivator content


Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and stable, (rate of decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and reducing at >10mg/kg/year

Passivators, also called metal deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and scale back their price of reaction with compounds in the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental varieties, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the creator is aware, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.[3]

As the oil ages, the passivator may deplete more quickly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals could be maintained.
With FAIR results, maintain common monitoring.
When POOR, it is advised to take away the oil or remove the source of corrosivity from the oil through special oil remedy.
I) Sediment and sludge

This isn’t a routine take a look at.
It is advised that this test is carried out when the oil outcomes indicate a excessive acid worth and the dissipation factor is near the unacceptable limit.
The results must be lower than zero.02% by mass to be negligible. If the outcomes return a worth of more than 0.02% by mass, it is suggested that it be reclaimed; in any other case, an oil change is really helpful.
j) Interfacial rigidity

This just isn’t a routine check


Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to twenty-eight 20 to 25<22 <20

ENot a routine test

F, GNot Applicable

The interfacial pressure between transformer oil and water reduces during the growing older process. What this means in sensible terms is there is extra polar compound current in the oil, decreasing the flexibility of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure becomes a excessive quality criterion: the oil should be modified beneath a predefined restrict.
If results are GOOD, proceed the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur

This just isn’t a routine check.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system can be vital. The extent of the corrosion harm attributable to the sulfur can be so extreme that it’d trigger failure of the equipment if not checked. The addition of a copper passivator can scale back the impression of this compound on the transformer system.
In a research by Doble, it has been found that rubber products used in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s risk evaluation examine. [4]

l) Particle counting and sizing

Table three: Particles

Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured utilizing IEC 60970 [5]

m) Flashpoint ° C

Not a routine check

If there is a maximum decrease in flashpoints by 10%, the equipment might require further inspection. This value would possibly differ in different nations.
It is suggested to perform this check when an uncommon odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This take a look at is not to decide the condition of the transformer; it is a health and security influence check. PCB is hazardous to both humans and the environment; it’s critical to check for PCBs after the retro fill of a transformer. It is also required whenever any maintenance has been accomplished on the unit, and the potential of contamination is current. If PCB content exceeds the really helpful limits, the suitable motion needs to be taken.
Units with a PCB content of greater than 50ppm require a fireplace safety plan, environmental safety plan, and further precautionary measures when maintenance is done. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the equipment owner.
Local regulatory bodies outline the limits.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with lots of information and interpretation, we’ll discuss this phenomenon in part II of the article. The limits for the different gases and the interpretation of this data in accordance with worldwide requirements will be mentioned intimately, forming part of the general health score dedication of the transformer.

Transformer condition monitoring is an interlaced, highly exciting field of research. In this text, we focused on the kinds of exams to determine the condition of the transformer, the critical values, and the really helpful actions.
The Health Index indication makes it potential to see the supposed reliability of a specific unit at a particular date and time. This makes it possible to ensure best apply application and optimised maintenance. It also make it simpler to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and upkeep guide)

2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik

3. Passivators, what they are and how they work, by Lance Lewand, Doble Engineering Company.
4. CIGRE technical brochure 378, 2009 “Copper sulfide in Transformer Insulation.”

5. CIGRE Technical Brochure 157, 2000 “Effect of particles on transformer dielectric strength.”

6. Article initially revealed by Transformer Technology Magazine Issue sixteen – December 2021

Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience in the business, having previously labored as laboratory supervisor for a major industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her profession, particularly in the analysis of test information. Corné has huge sensible and theoretical data of reliability maintenance programmes.

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