by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent accumulating, measuring, recording, and analysis of the related data concerning an asset’s operation. If we interpret the info accurately, it may give us great insight into the asset’s situation.
Frequent monitoring of the asset may end up in less maintenance required or extra prolonged durations without any upkeep required.
It is essential to identify the important thing parameters that are needed to offer us a whole picture of the actual standing of the transformer and the motion we want to take to make sure the continued reliability of the asset to achieve the utmost lifetime.
What is the information telling us?

Has the situation of the unit changed because the last maintenance period?

Is it safe to operate the unit?

Are there signs of deterioration?

Is it safe to load the unit above the nameplate ranking for a selected 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 contemplate replacement?

Are the recognized problems of a recurring nature?

Effective situation monitoring define

It is vitally important to identify clear objectives as a half of your technique. What do you want to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating เพรสเชอร์เกจ and what you want to accomplish, it might be a lot simpler to determine the required parameters.
Health indexing of belongings is becoming a exceptional device in getting a clearer picture of the condition 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 have been calculated based on the worldwide standards for mineral oils, indicating the crucial values stipulated within the numerous standards.
The scope of oil evaluation, interpretation of the data, and critical values

At the beginning of this section, it’s important to state that we take care of different measurement transformers within the industry. Transformers are divided into lessons according to the kV ratings of the gear. It is up to the reliability or asset manager to make use of the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical tests.
The maintenance engineer or supervisor wants to determine what type of testing would profit him in identifying downside areas inside his fleet. Some of the evaluation has been identified as to routine kind tests. Still, there’s an intensive range of checks that may assist in identifying particular drawback criteria inside the system, which might not be clear through the typical day-to-day analysis usually performed.
Please see the rating classes in TABLE 1


TABLE 2 explains which oil analyses are really helpful and how usually or underneath which circumstances.

Oil sampling

The taking of the oil pattern is certainly one of the most crucial and significant influencers within the evaluation consequence. If a pattern just isn’t taken to the prescribed procedure, then there is a important chance that the analysis carried out, and the results acquired, are of no use.
Care must be taken to make sure quality control procedures are utilized in every step of the sampling course of as set out by worldwide requirements. A good high quality sample taken by applying the proper procedure is crucial. A pattern can be contaminated by various elements, all of which might influence the finish result of the leads to a unfavorable manner.
All steps involved in pattern taking should adhere to high quality management procedures, including the container that’s used, the sampling package, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and dealing with of the sample, and then the supply of the sample to the laboratory.
If the pattern identification and sample data are handwritten, the human issue can lead to incorrect interpretation of the data. The label ought to be stuck onto the container to make sure no mix-ups in pattern identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label should be waterproof, and the pen used to write on the label must be oil- and waterproof. Otherwise, some data could be lost, making it extraordinarily troublesome for the laboratory personnel to finish the report or allocate the sample to a particular transformer.
Now, let’s talk about the kinds of exams to find out the transformer’s condition, the crucial values, and the recommended actions in each case:
a) Colour and appearance

This is a routine inspection applied to each oil sample.
When an oil sample arrives on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a transparent vessel to discover out the colour, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it might point out a high water content in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve may be integrated into the pattern. If particles are recognized as carbon, it would point out a possible electrical fault within the unit. The DGA analysis of the oil will confirm if 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 problems. The oil analysis outcomes may also decide the diploma and sort 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 software

G <30

This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of overseas 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 must be excessive.
If the values are Good, it is suggested to proceed with the current pattern interval action plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil by way of oil reconditioning processes. If different checks point out extreme getting older, the oil can be changed with new or reclaimed oil. Another option can 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 course of removes acid and water as well as another compounds. Another advantage is that the oil may be re-used, and in most conditions, this can be carried out without switching off the unit, which contributes to cost-saving. If unsure – as an alternative switch off the unit throughout this therapy process.
If the values are Poor, it is advisable to take action as quickly 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 could cause flashover within the unit, resulting in lack of the asset.
c) Water content (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 check for all classes of electrical gear, except class G

The outcomes of this check ought to at all times be considered at the facet of the breakdown power. If it is discovered that the water content material is excessive and the breakdown strength is low, additional motion needs to be taken. It is beneficial that a second sample from the identical unit is tested to verify the outcomes.
In the case of switching equipment, where there is not any paper current, the breakdown voltage is the determining factor.
It should be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is outdoors this temperature vary, it’s best to discuss with Annex A of the standard.
When the worth obtained by way of analyses is GOOD, the conventional sampling interval can be maintained, requiring no further action.
When the value returns a FAIR end result, more frequent sampling is really helpful. It is also useful to contemplate other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the action to be implemented.
A POOR end result will require instant motion from the asset supervisor. This might include taking another pattern to substantiate the outcomes from the primary analysis. If it’s confirmed that the water content is excessive, the oil may be filtered; this process ought to take away a big portion of the moisture from the oil if utilized accurately. Follow-up samples need to be taken to ensure that the moisture content continues to be inside the required limits. The reason is that essentially the most significant portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil underneath conditions that favour this movement. It might be found later that the oil within the water has increased again without any apparent reason, however the source would be the paper in the transformer.
A visual inspection is also recommended to discover out if any water may transfer into the transformer or electrical equipment through leaks. This downside may be extra severe if the transformer or electrical equipment is outside and never in a lined area.
d) Acidity (mgKOH/g oil) Neutralization Number


Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15

B, E< hundred.10 to 0.25>0.20

C< hundred.15 to zero.30>0.30

F, G Not a routine take a look at

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

The acids in oils are fashioned as a result of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help in the formation of the acids. Because acids are polar compounds, it will adversely affect the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, usually across the lower elements of the transformer core. The sludge will ultimately kind a semi-solid substance that is extraordinarily difficult to take away.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR result, the sampling interval must be decreased to fit the state of affairs. Future evaluation should include a visual inspection of the oil for sediment and sludge.
If the result is POOR in accordance with 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 choice would possibly suit 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.a hundred.10 to 0.20>0.20

B, C<0.100.10 to 0.50>0.50

D<0.010.01 to zero.03>0.03

E<0.100.01 to 0.30>0.03

F, GNot a routine take a look at

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

The dielectric dissipation factor or tan delta of this check offers info relating to the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we now have the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities that may influence the dissipation issue include water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is beneficial. The structure of the oil is broken, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the really helpful 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 amongst the key parameters to assess the transformer insulation condition; this is primarily based on the truth that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content material percent


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

This check is restricted to oils with this additive.
It can be advisable to contact the oil provider to verify the details regarding additives.
The two most common 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 stop oxygen from reacting with the oil. This significantly slows the growing older process in the oil and the strong insulation.
If the result’s FAIR, it is advised to top up the inhibitor degree to the prescribed degree per supplier directions. It is suggested to make use of a subject skilled educated within the process to carry out this activity.
If the end result obtained is POOR, the advice for this scenario would counsel that the top person continues to use the oil “uninhibited,” however this may result in extra rapid degradation of both 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 keep dissolved oxygen at levels under 1000ppm. เกจวัดแรงดัน could be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances isn’t required, although it might add additional safety towards oil oxidation if the preservation system ever fails. [2]

h) Passivator content material


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 decreasing at >10mg/kg/year

Passivators, also identified as steel deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and reduce their price of response with compounds in the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental sorts, sulfur-based and nitrogen-based. The first advised 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 extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals may be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it’s suggested to remove the oil or take away the source of corrosivity from the oil by way of particular oil therapy.
I) Sediment and sludge

This isn’t a routine test.
It is advised that this test is performed when the oil outcomes point out a excessive acid value and the dissipation factor is close to the unacceptable restrict.
The results have to be lower than 0.02% by mass to be negligible. If the results return a value of more than zero.02% by mass, it is advised that or not it’s reclaimed; in any other case, an oil change is beneficial.
j) Interfacial rigidity

This 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 check

F, GNot Applicable

The interfacial rigidity between transformer oil and water reduces through the aging course of. What this means in sensible phrases is there’s extra polar compound current within the oil, lowering the power 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 rigidity becomes a quality criterion: the oil should be modified under a predefined limit.
If results are GOOD, continue the common sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, check 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 can be important. The extent of the corrosion injury caused by the sulfur may be so extreme that it might cause failure of the gear if not checked. The addition of a copper passivator can scale back the influence of this compound on the transformer system.
In a study by Doble, it has been discovered that rubber products utilized in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s threat assessment examine. [4]

l) Particle counting and sizing

Table 3: Particles

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

m) Flashpoint ° C

Not a routine check

If there is a maximum lower in flashpoints by 10%, the gear might require additional inspection. This worth might differ in numerous countries.
It is suggested to perform this test when an unusual odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This take a look at is to not determine the situation of the transformer; this can be a health and security impact 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 whenever any upkeep has been accomplished on the unit, and the risk of contamination is present. If PCB content exceeds the beneficial limits, the suitable action needs to be taken.
Units with a PCB content material of more than 50ppm require a hearth security plan, environmental protection plan, and additional precautionary measures when maintenance is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the tools owner.
Local regulatory bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with a lot of information and interpretation, we are going to talk about this phenomenon in part II of the article. The limits for the totally different gases and the interpretation of this data in accordance with international requirements shall be mentioned in detail, forming a half of the general health ranking determination of the transformer.

Transformer condition monitoring is an interlaced, highly exciting subject of examine. In this article, we centered on the forms of tests to determine the condition of the transformer, the critical values, and the beneficial actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a particular date and time. This makes it potential to make sure greatest practice utility and optimised maintenance. It also make it simpler to draw up a maintenance plan and action 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

three. Passivators, what they’re and the way 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 strength.”

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

Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience within the business, having previously labored as laboratory supervisor for a major industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry right from the beginning of her profession, significantly in the evaluation of take a look at data. Corné has vast practical and theoretical data of reliability upkeep programmes.

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