by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the relevant data regarding an asset’s operation. If we interpret the data appropriately, it can give us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to less maintenance required or more prolonged durations without any maintenance required.
It is crucial to determine the important thing parameters which are wanted to give us a complete picture of the actual status of the transformer and the motion we need to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the info telling us?

Has the situation of the unit changed for the reason that last maintenance period?

Is it protected to operate the unit?

Are there indicators of deterioration?

Is it protected to load the unit above the nameplate score for a selected period?

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

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

Are the recognized problems of a recurring nature?

Effective condition monitoring define

It is vitally necessary to establish clear targets as part of your strategy. What do you need to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the outcome and what you wish to accomplish, it will be much easier to determine the required parameters.
Health indexing of property is changing into a remarkable device in getting a clearer image of the situation of your transformer. Test parameters carry a numerical value-adding to the total value of the Health Index Value of the transformer. These parameter weight values had been calculated primarily based on the worldwide requirements for mineral oils, indicating the crucial values stipulated in the numerous requirements.
The scope of oil evaluation, interpretation of the information, and critical values

At the beginning of this part, it is essential to state that we cope with totally different size transformers in the trade. Transformers are divided into lessons in accordance with the kV ratings of the tools. It is up to the reliability or asset supervisor to make use of the guidelines for larger equipment, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or manager needs to determine what type of testing would benefit him in figuring out problem areas inside his fleet. Some of the analysis has been identified as to routine sort tests. Still, there’s an extensive vary of checks that can help in identifying specific problem criteria throughout the system, which might not be clear through the everyday day-to-day analysis normally performed.
Please see the ranking courses in TABLE 1


TABLE 2 explains which oil analyses are recommended and the way typically or underneath which circumstances.

Oil sampling

The taking of the oil sample is one of the most crucial and significant influencers in the evaluation consequence. If a pattern is not taken to the prescribed process, then there is a important risk that the evaluation carried out, and the outcomes acquired, are of no use.
Care must be taken to make sure quality control procedures are applied in every step of the sampling process as set out by worldwide requirements. A good high quality sample taken by making use of the proper process is important. A sample can be contaminated by numerous factors, all of which can affect the outcome of the leads to a negative manner.
All steps involved in sample taking should adhere to high quality management procedures, together with the container that’s used, the sampling package, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and dealing with of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and sample information are handwritten, the human factor can result in incorrect interpretation of the info. The label should be caught onto the container to make sure no mix-ups in sample identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to put in writing on the label ought to be oil- and waterproof. Otherwise, some data could be lost, making it extraordinarily troublesome for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s focus on the types of checks to determine the transformer’s condition, the important values, and the really helpful actions in every case:
a) Colour and look

This is a routine inspection utilized to each oil sample.
When an oil pattern arrives on the laboratory, one of many “tests” is a visual inspection of the oil sample in a clear vessel to determine the color, turbidity, and possible particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there is lots of turbidity, it’d point out a excessive water content material within the oil.
If the drain valve was not cleaned by the sampler, the dust particles in the drain valve might be included into the pattern. If particles are recognized as carbon, it would point out a attainable electrical fault within the unit. The DGA evaluation of the oil will verify if that’s so.
Clear oils without contamination will point out a great condition, and no action is beneficial.
When oils are darkish or turbid, additional evaluation will confirm any issues. The oil analysis results may also decide the degree 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.
diaphragm seal will point out the water content or the presence of foreign particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to keep away from flashover within the unit, the breakdown voltage should be excessive.
If the values are Good, it is recommended to proceed with the current sample interval motion plan.
If the values are Fair, more frequent sampling is recommended in collaboration with different parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil by way of oil reconditioning processes. If alternative tests indicate severe aging, the oil can be changed with new or reclaimed oil. Another possibility can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar components are removed from the oil. This course of removes acid and water in addition to another compounds. Another advantage is that the oil could be re-used, and in most conditions, this may be done with out switching off the unit, which contributes to cost-saving. If doubtful – as an alternative swap off the unit throughout this remedy course of.
If the values are Poor, it is advisable to take motion as quickly as potential and never delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content may cause flashover within the unit, leading to lack of the asset.
c) Water content (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 take a look at

This is a routine test for all courses of electrical tools, besides class G

The outcomes of this take a look at should all the time be thought-about at the aspect of the breakdown power. If it is discovered that the water content material is high and the breakdown strength is low, further action must be taken. It is beneficial that a second sample from the same unit is tested to confirm the outcomes.
In the case of switching equipment, where there isn’t a paper present, the breakdown voltage is the figuring out issue.
It must be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between 40 and 70 °C. If it is found that the unit’s working temperature is outside this temperature range, it is best to check with Annex A of the usual.
When the worth obtained via analyses is GOOD, the traditional sampling interval could be maintained, requiring no further action.
When the worth returns a FAIR outcome, extra frequent sampling is really helpful. It can be helpful to consider different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR outcome would require instant action from the asset manager. This might embody taking one other sample to substantiate the outcomes from the primary analysis. If it is confirmed that the water content is high, the oil can be filtered; this course of ought to take away a large portion of the moisture from the oil if applied appropriately. Follow-up samples must be taken to guarantee that the moisture content material remains to be inside the required limits. The reason is that probably the most significant 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 conditions that favour this movement. It may be found later that the oil in the water has elevated once more with none apparent cause, however the source can be the paper within the transformer.
A visual inspection can additionally be really helpful to discover out if any water might move into the transformer or electrical gear through leaks. This problem may be more extreme if the transformer or electrical tools is outdoors and never in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number


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

B, E< hundred.10 to zero.25>0.20

C<0.100.15 to 0.30>0.30

F, G Not a routine take a look at

This is a routine take a look at for all lessons except F and G

The acids in oils are shaped due to chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist within the formation of the acids. Because acids are polar compounds, it’ll adversely affect the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this will result in sludge formation, normally around the decrease parts of the transformer core. The sludge will ultimately kind a semi-solid substance that is extremely difficult to remove.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to fit the situation. Future analysis ought to embrace a visual 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 manager could decide to reclaim the oil or replace it with new or reclaimed oil, whichever possibility might swimsuit their necessities 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 zero.20>0.20

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

D<0.010.01 to 0.03>0.03

E< hundred.01 to 0.30>0.03

F, GNot a routine test

This is a routine take a look at for all courses of electrical tools, except F and G

The dielectric dissipation issue or tan delta of this check provides info relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we now have the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities that may affect the dissipation issue embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking extra parameters is recommended.
When the result is 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 damaged down, and even with filtration, the really helpful dielectric values can’t be achieved.
f) Resistivity (GΩm) at 20 °C or ninety °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 check

DC resistivity of the oil is amongst the key parameters to evaluate the transformer insulation situation; that is primarily based on the truth that DC resistance is delicate to oil degradation.
When the result is FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content %


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

This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the major points concerning components.
The two commonest oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a couple of,6-di-tertiary butyl-phenol (DBP). The function of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the getting older course of in the oil and the strong insulation.
If the result is FAIR, it is advised to top up the inhibitor stage to the prescribed degree per provider instructions. It is suggested to make use of a field professional trained within the procedure to perform this task.
If the outcome obtained is POOR, the advice for this scenario would suggest that the top consumer continues to use the oil “uninhibited,” however this may result in more speedy degradation of each the liquid and stable insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels beneath 1000ppm. This could be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, though it’d add further protection against 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 steady, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year

Passivators, also referred to as steel deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and scale back their fee of response with compounds in the oil. This contains oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary sorts, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the author is conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.[3]

As the oil ages, the passivator might deplete more quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, common pattern intervals can be maintained.
With FAIR results, preserve common monitoring.
When POOR, it’s advised to take away the oil or take away the source of corrosivity from the oil through particular oil treatment.
I) Sediment and sludge

This is not a routine take a look at.
It is advised that this test is carried out when the oil outcomes point out a excessive acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes have to be less than zero.02% by mass to be negligible. If the outcomes return a worth of more than 0.02% by mass, it’s instructed that it be reclaimed; in any other case, an oil change is really helpful.
j) Interfacial pressure

This isn’t a routine take a look at


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

ENot a routine check

F, GNot Applicable

The interfacial pressure between transformer oil and water reduces through the growing older course of. What this means in practical terms is there might be more polar compound present in the oil, lowering the flexibility of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial tension turns into a high quality criterion: the oil have to be changed under a predefined limit.
If outcomes are GOOD, continue the common sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur

This isn’t a routine test.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be significant. The extent of the corrosion injury caused by the sulfur can be so severe that it’d cause failure of the tools if not checked. The addition of a copper passivator can cut back the impact of this compound on the transformer system.
In a study 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 may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s danger evaluation study. [4]

l) Particle counting and sizing

Table three: Particles

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

m) Flashpoint ° C

Not a routine take a look at

If there’s a maximum lower in flashpoints by 10%, the equipment would possibly require additional inspection. This worth may differ in different nations.
It is advised to perform this test when an unusual odour is observed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This test is to not determine the situation of the transformer; this is a health and safety influence test. PCB is hazardous to both people and the setting; it is vital to test for PCBs after the retro fill of a transformer. It can additionally be required every time any upkeep has been accomplished on the unit, and the potential of contamination is current. If PCB content exceeds the beneficial limits, the appropriate motion must be taken.
Units with a PCB content material of more than 50ppm require a hearth security plan, environmental safety plan, and additional precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of protected disposal issued to the tools proprietor.
Local regulatory our bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with lots of data and interpretation, we’ll discuss this phenomenon partially II of the article. The limits for the completely different gases and the interpretation of this data according to international requirements shall be mentioned in detail, forming part of the general health ranking dedication of the transformer.

Transformer condition monitoring is an interlaced, highly exciting area of research. In this article, we focused on the kinds of checks to determine the situation 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 particular unit at a particular date and time. This makes it potential to make sure greatest apply software and optimised maintenance. It also make it easier to draw up a maintenance plan and action 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

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

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

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

Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise in the business, having beforehand worked as laboratory supervisor for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, particularly in the evaluation of test knowledge. Corné has vast sensible and theoretical knowledge of reliability upkeep programmes.

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