by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the relevant knowledge regarding an asset’s operation. If we interpret the data correctly, it can give us nice perception into the asset’s condition.
Frequent monitoring of the asset can outcome in much less maintenance required or extra extended intervals without any upkeep required.
It is essential to identify the vital thing parameters which are needed to give us an entire image of the actual standing of the transformer and the motion we have to take to ensure the continued reliability of the asset to realize the maximum lifetime.
What is the info telling us?

Has the situation of the unit changed for the reason that final upkeep period?

Is it safe to operate the unit?

Are there indicators of deterioration?

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

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

How long can we use the unit earlier than we need to consider replacement?

Are the recognized issues of a recurring nature?

Effective condition monitoring define

It is vitally necessary to establish clear objectives as a part of your technique. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you wish to accomplish, it might be much easier to establish the required parameters.
Health indexing of property is becoming a exceptional device in getting a clearer image of the situation of your transformer. Test parameters carry a numerical value-adding to the entire worth of the Health Index Value of the transformer. These parameter weight values had been calculated primarily based on the worldwide standards for mineral oils, indicating the important values stipulated within the numerous requirements.
The scope of oil evaluation, interpretation of the data, and critical values

At the start of this part, it is essential to state that we take care of completely different measurement transformers within the industry. Transformers are divided into courses according to the kV rankings of the tools. It is as a lot as the reliability or asset manager to use the guidelines for bigger equipment, thereby implementing shorter increment oil analysis and electrical checks.
The maintenance engineer or supervisor wants to determine what sort of testing would profit him in figuring out downside areas within his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there could be an extensive range of checks that may help in figuring out particular problem standards throughout the system, which could not be clear via the everyday day-to-day evaluation usually performed.
Please see the score classes in TABLE 1


TABLE 2 explains which oil analyses are really helpful and the way often or beneath which circumstances.

Oil sampling

The taking of the oil pattern is among the most important and critical influencers in the analysis end result. If a pattern is not taken to the prescribed procedure, then there is a important risk that the evaluation performed, and the outcomes acquired, are of no use.
Care ought to be taken to ensure high quality control procedures are utilized in every step of the sampling course of as set out by international requirements. A good high quality sample taken by applying the proper process is essential. A sample may be contaminated by various factors, all of which might affect the outcome of the results in a negative manner.
All steps concerned 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 pattern, the storage and handling of the pattern, after which the delivery of the pattern to the laboratory.
If the pattern identification and sample information are handwritten, the human factor can lead to incorrect interpretation of the info. The label ought to be stuck onto the container to ensure no mix-ups in sample identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to write on the label should be oil- and water resistant. Otherwise, some data could be misplaced, making it extraordinarily tough for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s focus on the forms of exams to determine the transformer’s situation, the important values, and the recommended actions in each case:
a) Colour and appearance

This is a routine inspection utilized to every oil sample.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to determine the colour, turbidity, and attainable particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it would indicate a high water content within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles within the drain valve might be incorporated into the sample. If particles are recognized as carbon, it would point out a possible electrical fault in the unit. The DGA analysis of the oil will affirm if that is the case.
Clear oils with out contamination will indicate a great condition, and no action is recommended.
When oils are dark or turbid, additional analysis will affirm any issues. The oil evaluation results will also decide the degree and sort 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 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 or the presence of international particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage should be high.
If the values are Good, it is suggested to continue with the present sample interval action plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil by way of oil reconditioning processes. If alternative exams indicate extreme growing older, the oil could be replaced with new or reclaimed oil. Another choice would be to perform on-site oil reclamation using 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 some other compounds. Another advantage is that the oil could be re-used, and in most situations, this can be done without switching off the unit, which contributes to cost-saving. If unsure – as a substitute change off the unit throughout this therapy process.
If the values are Poor, it’s advisable to take action as quickly as possible and not delay the upkeep process. เกจวัดแรงดัน in the transformer system decreases the projected transformer lifetime considerably; extremely high water content can cause flashover within the unit, resulting in loss 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 test for all courses of electrical equipment, except class G

The outcomes of this test should always be thought of at the side of the breakdown energy. If it is discovered that the water content is high and the breakdown power is low, additional action needs to be taken. It is beneficial that a second pattern from the same unit is tested to verify the results.
In the case of switching equipment, where there is no paper present, the breakdown voltage is the determining factor.
It must be famous that the bounds indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is exterior this temperature vary, it’s best to check with Annex A of the usual.
When the value obtained through analyses is GOOD, the traditional sampling interval can be maintained, requiring no additional action.
When the value returns a FAIR outcome, more frequent sampling is recommended. It can be helpful to contemplate different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the action to be applied.
A POOR result would require quick motion from the asset supervisor. This would possibly include taking one other sample to verify the outcomes from the first evaluation. If it’s confirmed that the water content material is excessive, the oil could be filtered; this course of should take away a big portion of the moisture from the oil if utilized appropriately. Follow-up samples need to be taken to ensure that the moisture content continues to be within the required limits. The purpose is that the most significant slice of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil under circumstances that favour this motion. It could be found later that the oil in the water has elevated again without any obvious cause, but the supply could be the paper in the transformer.
A visible inspection can additionally be really helpful to determine if any water may move into the transformer or electrical equipment by way of leaks. This problem may be extra extreme if the transformer or electrical tools is outside and never in a coated space.
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 zero.25>0.20

C< hundred.15 to 0.30>0.30

F, G Not a routine check

This is a routine check for all courses besides F and G

The acids in oils are formed due to 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 have an result on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this will lead to sludge formation, usually around the lower elements of the transformer core. The sludge will eventually type a semi-solid substance that is extremely difficult to remove.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR result, the sampling interval ought to be decreased to suit the situation. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition four.0, the asset supervisor could resolve to reclaim the oil or exchange it with new or reclaimed oil, whichever choice 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< hundred.10 to zero.20>0.20

B, C<0.100.10 to 0.50>0.50

D<0.010.01 to zero.03>0.03

E< hundred.01 to 0.30>0.03

F, GNot a routine test

This is a routine test for all lessons of electrical gear, except F and G

The dielectric dissipation issue or tan delta of this test offers information concerning the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, leading to phase displacement and dielectric losses. Other impurities that may affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking extra parameters is beneficial.
When the result’s POOR, reclamation or an oil change is really helpful. The construction of the oil is broken, in impact that the chemical bonds between the molecules have damaged down, and even with filtration, the beneficial 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 one of the key parameters to assess the transformer insulation condition; that is based on the reality that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content %


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

This check is restricted to oils with this additive.
It can be advisable to contact the oil supplier to confirm the small print regarding components.
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 stop oxygen from reacting with the oil. This considerably slows the aging process in the oil and the solid insulation.
If the result is FAIR, it’s advised to top up the inhibitor stage to the prescribed degree per provider directions. It is advised to use a subject professional trained within the procedure to carry out this activity.
If the result obtained is POOR, the recommendation for this situation would suggest that the end person continues to use the oil “uninhibited,” but this will lead to extra rapid degradation of both the liquid and solid insulation.
It should be famous that some transformers already have a built-in oil preservation system; that is designed to keep dissolved oxygen at levels under 1000ppm. This can be within 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 is not required, though it would add additional 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 steady, (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 known as steel deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and scale back their price of response 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 advised 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 may deplete extra quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, regular sample intervals may be maintained.
With FAIR outcomes, keep regular monitoring.
When POOR, it is suggested to take away the oil or take away the supply of corrosivity from the oil through particular oil remedy.
I) Sediment and sludge

This just isn’t a routine take a look at.
It is advised that this test is performed when the oil results point out a excessive acid value and the dissipation issue is close to the unacceptable restrict.
The outcomes need to be lower than 0.02% by mass to be negligible. If the outcomes return a value of more than 0.02% by mass, it is advised that or not it’s reclaimed; in any other case, an oil change is recommended.
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 take a look at

F, GNot Applicable

The interfacial tension between transformer oil and water reduces through the aging course of. What this means in practical terms is there may be more polar compound present in the oil, reducing the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, the interfacial pressure turns into a excessive quality criterion: the oil must be changed under a predefined restrict.
If results are GOOD, continue the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur

This is not a routine take a look at.
Oil is both 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 injury attributable to the sulfur may be so extreme that it might trigger 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 used in transformers would possibly 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 primarily based on this institute’s risk evaluation study. [4]

l) Particle counting and sizing

Table 3: 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 test

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 suggested to perform this check when an uncommon odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This check is not to determine the condition of the transformer; this may be a well being and safety impression test. PCB is hazardous to both humans and the setting; it is vital to test for PCBs after the retro fill of a transformer. It can additionally be required whenever any maintenance has been carried out on the unit, and the risk of contamination is current. If PCB content exceeds the beneficial limits, the appropriate action must be taken.
Units with a PCB content material of greater than 50ppm require a fireplace safety plan, environmental protection plan, and further precautionary measures when maintenance is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the tools owner.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with lots of knowledge and interpretation, we are going to focus on this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this knowledge according to international standards will be discussed intimately, forming a part of the general health score willpower of the transformer.

Transformer situation monitoring is an interlaced, highly thrilling area of examine. In this article, we centered on the kinds of tests to determine the situation of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it possible to see the supposed reliability of a particular unit at a particular date and time. This makes it possible to ensure greatest follow application and optimised upkeep. It additionally make it simpler to attract up a upkeep plan and motion plan.
1. IEC 60422 Edition 4.0 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’re and the way 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 sixteen – December 2021

Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise in the trade, having previously labored as laboratory manager for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry proper from the start of her career, notably within the evaluation of test knowledge. Corné has huge sensible and theoretical knowledge of reliability upkeep programmes.

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