by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent collecting, measuring, recording, and analysis of the related knowledge concerning an asset’s operation. If we interpret the information appropriately, it can provide us great perception into the asset’s situation.
Frequent monitoring of the asset can result in less maintenance required or extra extended durations with none upkeep required.
It is essential to determine the important thing parameters that are wanted to offer us an entire image of the actual status of the transformer and the motion we have to take to make sure the continued reliability of the asset to realize the utmost lifetime.
What is the data telling us?

Has the condition of the unit modified because the final upkeep period?

Is it secure to function the unit?

Are there indicators of deterioration?

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

Are we required to implement action to make sure the continued reliability of the unit?

How lengthy can we use the unit earlier than we have to think about replacement?

Are the recognized problems of a recurring nature?

Effective situation monitoring outline

It is vitally essential to identify clear goals as a half of your strategy. What do you need to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the finish result and what you want to accomplish, it might be much easier to determine the required parameters.
Health indexing of property is turning into a exceptional tool in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the total worth of the Health Index Value of the transformer. These parameter weight values had been calculated primarily based on the international requirements for mineral oils, indicating the critical values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the info, and important values

At the start of this part, it is important to state that we cope with different measurement transformers within the industry. Transformers are divided into classes based on the kV rankings of the gear. It is up to the reliability or asset manager to make use of the rules for bigger equipment, thereby implementing shorter increment oil analysis and electrical checks.
The upkeep engineer or manager needs to determine what type of testing would benefit him in figuring out downside areas within his fleet. Some of the analysis has been recognized as to routine sort exams. Still, there’s an intensive vary of checks that may help in identifying specific drawback standards within the system, which might not be clear through the typical day-to-day evaluation normally performed.
Please see the ranking classes in TABLE 1


TABLE 2 explains which oil analyses are beneficial and the way often or under which circumstances.

Oil sampling

The taking of the oil pattern is certainly one of the most crucial and important influencers in the analysis consequence. If a sample isn’t taken to the prescribed process, then there is a important risk that the evaluation performed, and the outcomes acquired, are of no use.
Care ought to be taken to make sure quality management 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 could be contaminated by varied components, all of which may affect the finish result of the ends in a unfavorable manner.
All steps concerned in sample taking should adhere to high quality management procedures, together with the container that’s used, the sampling kit, the taking of the pattern, the marking of the sample, the closing of the sample, the storage and handling of the pattern, after which the supply of the sample to the laboratory.
If the pattern identification and pattern information are handwritten, the human factor can result in incorrect interpretation of the data. The label should be stuck onto the container to make sure no mix-ups in sample identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label should be waterproof, and the pen used to put in writing 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 particular transformer.
Now, let’s talk about the kinds of exams to determine the transformer’s situation, the crucial values, and the really helpful actions in each case:
a) Colour and look

This is a routine inspection applied to each oil sample.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a transparent vessel to discover out the color, turbidity, and attainable particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there is lots of turbidity, it would indicate a excessive water content material in the oil.
If the drain valve was not wiped clean by the sampler, the filth particles within the drain valve may be incorporated into the sample. If particles are identified as carbon, it might indicate a attainable electrical fault in the unit. The DGA analysis of the oil will verify if that is the case.
Clear oils with out contamination will point out a good condition, and no motion is beneficial.
When oils are darkish or turbid, further analysis will confirm any issues. The oil evaluation results may also decide the diploma and kind of action.
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 point application<40 kV for OLTC in delta or line-end application

G <30

This is a routine inspection.
Breakdown voltage will point out the water content or the presence of overseas particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage must be excessive.
If the values are Good, it is strongly recommended to proceed with the present sample interval motion plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with different parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil through oil reconditioning processes. If ไดอะแฟรม point out severe aging, the oil could be replaced 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 advantage that the colour of the oil is restored, and the polar parts are faraway from the oil. This process removes acid and water as well as some other compounds. Another benefit is that the oil may be re-used, and in most situations, this may be done with out switching off the unit, which contributes to cost-saving. If in doubt – as an alternative change off the unit during this treatment process.
If the values are Poor, it is advisable to take motion as soon as attainable and never delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content may cause flashover within the unit, resulting in loss of the asset.
c) Water content material (mg/kg at transformer working 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 check for all classes of electrical tools, besides class G

The results of this take a look at should all the time be considered along side the breakdown strength. If it’s discovered that the water content is excessive and the breakdown strength is low, further motion must be taken. It is recommended that a second sample from the identical unit is tested to verify the outcomes.
In the case of switching gear, where there is not a paper present, the breakdown voltage is the figuring out issue.
It should be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is outside this temperature vary, it is best to discuss with Annex A of the usual.
When the value obtained by way of analyses is GOOD, the traditional sampling interval could be maintained, requiring no additional motion.
When the worth returns a FAIR result, extra frequent sampling is really helpful. It can additionally be useful to suppose about different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the action to be carried out.
A POOR outcome would require immediate motion from the asset supervisor. This might embrace taking another sample to substantiate the results from the first analysis. If it is confirmed that the water content is high, the oil may be filtered; this course of ought to remove a big portion of the moisture from the oil if utilized accurately. Follow-up samples have to be taken to make certain that the moisture content material continues to be inside the required limits. The reason is that essentially the most significant portion of the water is caught up in the paper system within the transformer. This moisture will transfer from the paper into the oil beneath situations that favour this movement. It could be discovered later that the oil within the water has elevated again without any apparent reason, however the source can be the paper within the transformer.
A visual inspection is also beneficial to discover out if any water may transfer into the transformer or electrical tools by way of leaks. This problem could be extra extreme if the transformer or electrical gear is exterior and not in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number


Acidity mgKOH/g oilO, A, D< hundred.10 to zero.15>0.15

B, E<0.100.10 to zero.25>0.20

C< hundred.15 to 0.30>0.30

F, G Not a routine test

This is a routine test for all lessons besides F and G

The acids in oils are formed because of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will help within the formation of the acids. Because acids are polar compounds, it’ll adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this could lead to sludge formation, often around the lower components of the transformer core. The sludge will finally form a semi-solid substance that’s extremely troublesome to take away.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR end result, the sampling interval must be decreased to fit the scenario. Future evaluation ought to include a visible inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition, the asset supervisor may resolve to reclaim the oil or replace it with new or reclaimed oil, whichever option may swimsuit their necessities one of the best.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C


Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to zero.20>0.20

B, C<0.100.10 to zero.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 check

This is a routine test for all classes of electrical tools, besides F and G

The dielectric dissipation issue or tan delta of this test supplies info regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we have the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that may affect the dissipation factor embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking extra parameters is recommended.
When the result’s POOR, reclamation or an oil change is beneficial. The construction of the oil is damaged, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the recommended 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 test

DC resistivity of the oil is amongst the key parameters to assess the transformer insulation condition; that is primarily based on the reality that DC resistance is sensitive to oil degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material percent


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

This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to confirm the primary points concerning additives.
The two most typical 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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the getting older course of in the oil and the stable insulation.
If the result is FAIR, it’s advised to prime up the inhibitor degree to the prescribed degree per provider instructions. It is advised to use a subject skilled educated in the process to carry out this task.
If the end result obtained is POOR, the recommendation for this situation would recommend that the tip consumer continues to make use of the oil “uninhibited,” but this will likely lead to more fast degradation of each the liquid and strong insulation.
It must be famous that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges under 1000ppm. This would be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances isn’t required, although it would add further protection towards 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 lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and lowering at >10mg/kg/year

Passivators, also referred to as metal deactivators, react with reactive metallic surfaces and dissolved metals similar to copper and silver and cut back their price of reaction with compounds within the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary sorts, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the writer 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 rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD results, regular sample intervals can be maintained.
With FAIR results, maintain common monitoring.
When POOR, it’s suggested to take away the oil or take away the source of corrosivity from the oil via special oil therapy.
I) Sediment and sludge

This is not a routine test.
It is advised that this test is performed when the oil results indicate a high acid value and the dissipation issue is near the unacceptable limit.
The outcomes must be less than zero.02% by mass to be negligible. If the results return a price of more than zero.02% by mass, it’s suggested that it be reclaimed; otherwise, an oil change is beneficial.
j) Interfacial rigidity

This just isn’t a routine test


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

ENot a routine take a look at

F, GNot Applicable

The interfacial pressure between transformer oil and water reduces during the growing older course of. What this implies in sensible phrases is there’s more polar compound present in the oil, reducing the power of an oil to function an insulator within the transformer system. There is pressure gauge octa between interfacial tension and neutralisation number. Therefore, the interfacial rigidity turns into a quality criterion: the oil should be changed beneath a predefined restrict.
If results are GOOD, proceed the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur

This is not a routine take a look at.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be vital. The extent of the corrosion harm brought on by the sulfur may be so extreme that it’d trigger failure of the gear if not checked. The addition of a copper passivator can cut back the impression of this compound on the transformer system.
In a examine by Doble, it has been found that rubber merchandise utilized in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s threat evaluation study. [4]

l) Particle counting and sizing

Table 3: Particles

Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured using IEC 60970 [5]

m) Flashpoint ° C

Not a routine test

If there is a most decrease in flashpoints by 10%, the equipment would possibly require further inspection. This worth might differ in several international locations.
It is suggested to carry out this take a look at when an unusual odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This test is not to determine the situation of the transformer; this is a health and safety impression take a look at. PCB is hazardous to each people and the environment; it is important to check for PCBs after the retro fill of a transformer. It is also required each time any maintenance has been done on the unit, and the risk of contamination is present. If PCB content material exceeds the really helpful limits, the appropriate action must be taken.
Units with a PCB content of more than 50ppm require a hearth security plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment proprietor.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with plenty of data and interpretation, we will discuss this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data based on worldwide requirements shall be mentioned in detail, forming a half of the general well being rating willpower of the transformer.

Transformer condition monitoring is an interlaced, extremely exciting subject of research. In this article, we focused on the types of tests to determine the condition of the transformer, the important values, and the really helpful actions.
The Health Index indication makes it attainable to see the supposed reliability of a particular unit at a specific date and time. This makes it attainable to make sure best apply application and optimised maintenance. It additionally make it simpler to attract up a upkeep plan and motion plan.
1. IEC 60422 Edition four.0 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and maintenance guide)

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

3. Passivators, what they’re 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 energy.”

6. Article initially printed by Transformer Technology Magazine Issue 16 – December 2021

Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise in the trade, having previously labored as laboratory supervisor for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her career, significantly in the evaluation of check information. Corné has vast sensible and theoretical data of reliability maintenance programmes.

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