by Brenna ShumbamhiniJune 9, 2022



Corne Dames

Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the related information relating to an asset’s operation. If we interpret the data accurately, it may give us great insight into the asset’s condition.
Frequent monitoring of the asset can end result in less upkeep required or more prolonged durations without any upkeep required.
It is essential to determine the key parameters which are wanted to offer us a complete picture 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 modified for the rationale that last maintenance period?

Is it secure to operate the unit?

Are there signs of deterioration?

Is ไดอะแฟรม ซีล secure to load the unit above the nameplate rating for a selected period?

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

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

Are the identified problems of a recurring nature?

Effective condition monitoring define

It is vitally necessary to identify clear targets as part of your technique. What do you wish to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the finish result and what you wish to accomplish, it might be a lot simpler to identify the required parameters.
Health indexing of assets is becoming a outstanding tool 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 primarily based on the worldwide standards for mineral oils, indicating the important values stipulated within the numerous requirements.
The scope of oil analysis, interpretation of the info, and critical values

At the beginning of this section, it is essential to state that we take care of totally different size transformers within the trade. Transformers are divided into classes in accordance with the kV scores of the gear. It is as much as the reliability or asset supervisor to use the rules for bigger tools, thereby implementing shorter increment oil analysis and electrical checks.
The upkeep engineer or supervisor wants to determine what sort of testing would benefit 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 vary of tests that can assist in identifying specific drawback criteria inside the system, which might not be clear through the typical day-to-day analysis usually carried out.
Please see the ranking lessons 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 probably certainly one of the most important and significant influencers within the evaluation consequence. If a sample just isn’t taken to the prescribed process, then there is a significant possibility that the analysis carried out, and the results acquired, are of no use.
Care must be taken to ensure quality management procedures are utilized in every step of the sampling process as set out by international standards. A good high quality pattern taken by making use of the correct procedure is essential. A sample can be contaminated by numerous factors, all of which might influence the finish result of the results in a unfavorable method.
All steps concerned in sample taking must adhere to high quality control procedures, together with the container that’s used, the sampling kit, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the pattern, and then the delivery of the sample to the laboratory.
If the sample identification and pattern information are handwritten, the human factor can lead to incorrect interpretation of the data. The label ought to 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 must be waterproof, and the pen used to write down on the label must be oil- and waterproof. Otherwise, some knowledge could be misplaced, making it extraordinarily difficult for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s talk about the kinds of exams to determine the transformer’s situation, the critical values, and the beneficial actions in every case:
a) Colour and appearance

This is a routine inspection utilized to each oil pattern.
When an oil pattern arrives on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to determine the color, turbidity, and possible particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it might indicate a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the dust particles within the drain valve could be included into the pattern. If particles are identified as carbon, it might indicate a possible electrical fault in the unit. The DGA evaluation of the oil will confirm if that’s so.
Clear oils with out contamination will point out a good situation, and no action is really helpful.
When oils are darkish or turbid, further analysis will affirm any problems. The oil evaluation outcomes may also determine 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 material or the presence of international particles, or both in 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 current sample interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil by way of oil reconditioning processes. If different checks point out severe getting older, the oil could be changed with new or reclaimed oil. Another choice can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar elements are faraway from the oil. This process removes acid and water as nicely as another compounds. Another benefit is that the oil may be re-used, and in most situations, this might be accomplished with out switching off the unit, which contributes to cost-saving. If unsure – as a substitute switch off the unit during this treatment course of.
If the values are Poor, it’s advisable to take motion as quickly as potential and not delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content material could cause flashover within the unit, leading to loss 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 test

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

The outcomes of this test should at all times be considered at the side of the breakdown strength. If it is discovered that the water content material is high and the breakdown power is low, further action needs to be taken. It is really helpful that a second sample from the identical unit is examined to confirm the results.
In the case of switching equipment, where there is not a paper present, the breakdown voltage is the determining factor.
It should be noted that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it’s found that the unit’s operating temperature is exterior this temperature range, it’s best to check with Annex A of the usual.
When the value obtained through analyses is GOOD, the traditional sampling interval could be maintained, requiring no additional motion.
When the value returns a FAIR end result, extra frequent sampling is really helpful. It can additionally be helpful to assume about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be carried out.
A POOR end result will require instant motion from the asset supervisor. This might include taking another pattern to substantiate the outcomes from the first evaluation. If diaphragm seal ‘s confirmed that the water content material is excessive, the oil may be filtered; this course of should take away a big portion of the moisture from the oil if applied accurately. Follow-up samples have to be taken to make sure that the moisture content continues to be throughout the required limits. The reason is that the most good 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 situations that favour this motion. It may be discovered later that the oil within the water has elevated once more without any apparent reason, however the source could be the paper within the transformer.
A visible inspection can also be beneficial to discover out if any water may move into the transformer or electrical gear by way of leaks. This drawback might be more severe if the transformer or electrical equipment is exterior and never in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number


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

B, E<0.100.10 to zero.25>0.20

C<0.100.15 to 0.30>0.30

F, G Not a routine test

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

The acids in oils are formed as a result of 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 will adversely affect the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this could lead to sludge formation, usually around the lower elements of the transformer core. The sludge will finally form a semi-solid substance that’s extraordinarily difficult to take away.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to suit the state of affairs. Future evaluation ought to embrace a visual inspection of the oil for sediment and sludge.
If the result’s POOR according to the prescribed values in IEC 60422 Edition, the asset manager may decide to reclaim the oil or replace it with new or reclaimed oil, whichever option may suit their necessities the most effective.
e) Dielectric Dissipation issue 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.a hundred.10 to zero.50>0.50

D<0.010.01 to zero.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 courses of electrical gear, except F and G

The dielectric dissipation issue or tan delta of this test provides info regarding the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that might influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking further parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended. The construction of the oil is damaged, in effect 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 take a look at

DC resistivity of the oil is amongst the key parameters to evaluate the transformer insulation condition; this is based on the fact that DC resistance is delicate to grease degradation.
When the result is FAIR, extra frequent sampling and checking additional parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material %


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

This check is restricted to oils with this additive.
It could be advisable to contact the oil provider to confirm the details concerning additives.
The two most typical 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 forestall oxygen from reacting with the oil. This considerably slows the aging course of in the oil and the stable insulation.
If the result’s FAIR, it is advised to high up the inhibitor level to the prescribed level per provider instructions. It is advised to make use of a area skilled trained within the procedure to perform this activity.
If the result obtained is POOR, the advice for this scenario would recommend that the tip person continues to make use of the oil “uninhibited,” but this will lead to more speedy degradation of each the liquid and strong insulation.
It ought to be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges under 1000ppm. This can be in 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 beneath these circumstances isn’t required, although it would add further safety in opposition to 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 secure, (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 referred to as steel deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and scale 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 types, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the author 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 would possibly deplete extra rapidly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, common sample intervals could be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it’s suggested to remove the oil or take away the source of corrosivity from the oil via special oil remedy.
I) Sediment and sludge

This just isn’t a routine take a look at.
It is suggested that this check is carried out when the oil results indicate a high acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes have to be less than 0.02% by mass to be negligible. If the results return a worth of greater than 0.02% by mass, it is advised that it be reclaimed; otherwise, an oil change is beneficial.
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 rigidity between transformer oil and water reduces through the getting older course of. What this implies in sensible phrases is there is extra polar compound present in the oil, reducing the power of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial rigidity becomes a high quality criterion: the oil have to be changed beneath a predefined restrict.
If outcomes are GOOD, proceed the common sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur

This isn’t 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 may be important. The extent of the corrosion harm brought on by the sulfur could be so severe that it’d trigger failure of the gear if not checked. The addition of a copper passivator can reduce the impact of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber merchandise 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 threat assessment examine. [4]

l) Particle counting and sizing

Table 3: Particles

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

m) Flashpoint ° C

Not a routine check

If there is a most decrease in flashpoints by 10%, the equipment might require further inspection. This value may differ in several nations.
It is suggested to carry out this test when an unusual odour is noticed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)

This test is to not decide the situation of the transformer; this is a health and security impact check. PCB is hazardous to each humans and the environment; it is important to check for PCBs after the retro fill of a transformer. It can be required whenever any maintenance has been done on the unit, and the risk of contamination is present. If PCB content exceeds the beneficial limits, the appropriate action needs to be taken.
Units with a PCB content material of more than 50ppm require a fireplace security plan, environmental protection plan, and extra precautionary measures when upkeep is completed. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the gear owner.
Local regulatory bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)

As DGA is an intricate science with lots of information and interpretation, we will focus on this phenomenon partially II of the article. The limits for the different gases and the interpretation of this knowledge according to international requirements might be discussed in detail, forming a half of the general well being score willpower of the transformer.

Transformer condition monitoring is an interlaced, extremely thrilling subject of research. In this article, we centered on the kinds of checks to discover out the situation of the transformer, the critical values, and the recommended actions.
The Health Index indication makes it possible to see the supposed reliability of a specific unit at a particular date and time. This makes it possible to make sure greatest practice utility and optimised upkeep. It also make it simpler to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition four.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

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 printed by Transformer Technology Magazine Issue sixteen – December 2021

Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ experience within the business, having beforehand worked as laboratory manager for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, notably within the analysis of check data. Corné has vast sensible and theoretical data of reliability upkeep programmes.

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