by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the related information concerning an asset’s operation. If we interpret the info correctly, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset can end result in less maintenance required or more extended intervals without any upkeep required.
It is essential to identify the key parameters which are wanted to offer us an entire image of the actual status 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 data telling us?
Has the condition of the unit changed since 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 rating for a particular period?
Are we required to implement motion to make sure the continued reliability of the unit?
How lengthy can we use the unit before we have to consider replacement?
Are the recognized issues of a recurring nature?
Effective condition monitoring define
It is vitally essential to identify clear targets as part of your technique. What do you need to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the outcome and what you wish to accomplish, it will be much simpler to identify the required parameters.
Health indexing of property is turning into a exceptional device in getting a clearer picture of the condition 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 based on the worldwide requirements for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the information, and important values
At the start of this section, it is important to state that we take care of completely different size transformers within the business. Transformers are divided into classes based on the kV ratings of the equipment. It is up to the reliability or asset manager to make use of the rules for larger tools, thereby implementing shorter increment oil evaluation and electrical tests.
The upkeep engineer or supervisor needs to determine what kind of testing would benefit him in figuring out problem areas within his fleet. Some of the analysis has been identified as to routine kind tests. Still, there could be an in depth vary of exams that may assist in figuring out particular downside standards within the system, which might not be clear through the standard day-to-day analysis normally carried out.
Please see the rating classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way often or beneath which circumstances.
Oil sampling
The taking of the oil pattern is one of the most vital and critical influencers within the evaluation consequence. If a sample is not taken to the prescribed process, then there’s a important risk that the analysis performed, and the outcomes acquired, are of no use.
Care should be taken to ensure quality control procedures are utilized in every step of the sampling process as set out by international standards. A good high quality sample taken by making use of the proper process is crucial. A sample may be contaminated by numerous factors, all of which may influence the result of the results in a negative manner.
All steps concerned in sample taking must adhere to high quality control procedures, together with the container that is used, the sampling equipment, the taking of the pattern, the marking of the sample, the closing of the sample, the storage and handling of the sample, after which the supply of the pattern to the laboratory.
If the sample identification and pattern information are handwritten, the human factor can result in incorrect interpretation of the information. The label ought to be caught 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 ought to be waterproof, and the pen used to put in writing on the label ought to be oil- and water-resistant. Otherwise, some data could be lost, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s discuss the kinds of checks to determine the transformer’s situation, the crucial values, and the recommended actions in every case:
a) Colour and appearance
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 pattern in a transparent vessel to determine the color, turbidity, and potential particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there may be plenty of turbidity, it’d indicate a high water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles in the drain valve could be included into the pattern. If ตัววัดแรงดันน้ำ are recognized as carbon, it’d indicate a possible electrical fault within the unit. The DGA evaluation of the oil will verify if so.
Clear oils without contamination will indicate a great situation, and no action is really helpful.
When oils are darkish or turbid, further evaluation will affirm any issues. The oil analysis results may even 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 utility
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 within the unit, the breakdown voltage must be excessive.
If the values are Good, it is recommended to continue with the current sample interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with other parameter results like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If various checks indicate severe aging, the oil may be replaced with new or reclaimed oil. Another option would 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 components are removed from the oil. This process removes acid and water in addition to some other compounds. Another advantage is that the oil can be re-used, and in most conditions, this can be done without switching off the unit, which contributes to cost-saving. If doubtful – as an alternative swap off the unit during this remedy process.
If the values are Poor, it’s advisable to take action as quickly as attainable and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely excessive water content may 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 take a look at
This is a routine check for all classes of electrical equipment, besides class G
The outcomes of this take a look at ought to always be thought of in conjunction with the breakdown energy. If it is found that the water content material is high and the breakdown power is low, additional action needs to be taken. It is really helpful that a second sample from the same unit is examined to verify the outcomes.
In the case of switching gear, where there isn’t any paper current, the breakdown voltage is the determining issue.
It must be noted that the bounds indicated by IEC 60422 Edition four apply to transformers with working temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is outdoors this temperature range, it’s best to refer to Annex A of the standard.
When the value obtained via analyses is GOOD, the conventional sampling interval may be maintained, requiring no additional action.
When the worth returns a FAIR end result, more frequent sampling is recommended. It can be helpful to contemplate different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the action to be carried out.
A POOR outcome would require immediate action from the asset manager. This may include taking one other sample to confirm the results from the primary analysis. If it is confirmed that the water content is high, the oil may be filtered; this process ought to take away a big portion of the moisture from the oil if applied appropriately. Follow-up samples have to be taken to ensure that the moisture content material continues to be throughout the required limits. The cause is that probably the most significant portion of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil beneath circumstances that favour this motion. It may be found later that the oil in the water has elevated again with none apparent reason, but the source can be the paper within the transformer.
A visual inspection is also recommended to determine if any water might transfer into the transformer or electrical equipment by way of leaks. This downside may be more extreme if the transformer or electrical tools is outside and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.15>0.15
B, E<0.100.10 to 0.25>0.20
C< hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine check for all courses besides F and G
The acids in oils are shaped due to 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’s going to adversely affect the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this will lead to sludge formation, normally around the lower components of the transformer core. The sludge will ultimately form a semi-solid substance that’s extraordinarily difficult to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval ought to be decreased to suit the scenario. Future evaluation ought to include a visual inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition four.0, the asset supervisor might decide to reclaim the oil or substitute it with new or reclaimed oil, whichever choice might go properly with their necessities the most effective.
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 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 check
This is a routine test for all classes of electrical tools, except 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 materials.
When oil ages, we have the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities that might influence the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is really helpful. The structure of the oil is damaged, 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 doubtless certainly one of the key parameters to assess the transformer insulation condition; this is based mostly on the truth that DC resistance is delicate to oil degradation.
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 recommended.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original worth
This check is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the details relating to 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 prevent oxygen from reacting with the oil. This significantly slows the aging course of within the oil and the strong insulation.
If the result is FAIR, it’s suggested to prime up the inhibitor stage to the prescribed stage per provider instructions. It is suggested to use a subject skilled skilled within the procedure to perform this process.
If the end result obtained is POOR, the advice for this scenario would suggest that the tip person continues to make use of the oil “uninhibited,” however this may result in more fast degradation of each the liquid and stable insulation.
It must be noted that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges beneath 1000ppm. This can 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 underneath these circumstances is not required, though it’d add additional 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 secure, (rate of decrease < 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 metal deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and scale back their rate of response with compounds within the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental types, 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 would possibly deplete more rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals may be maintained.
With FAIR outcomes, preserve common monitoring.
When POOR, it is advised to remove the oil or take away the source of corrosivity from the oil via special oil treatment.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is advised that this check is performed when the oil results indicate a high acid value and the dissipation factor is close to the unacceptable limit.
The results must be less than zero.02% by mass to be negligible. If the results return a price of more than 0.02% by mass, it is suggested that or not it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
This just isn’t a routine test
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 pressure between transformer oil and water reduces during the growing older course of. What this means in practical terms is there might be more polar compound present within the oil, lowering 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 tension becomes a prime quality criterion: the oil should be modified below a predefined limit.
If results are GOOD, proceed the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be vital. The extent of the corrosion injury brought on by the sulfur can be so severe that it’d 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 research by Doble, it has been discovered that rubber products utilized in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted 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 three: 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 check
If there is a maximum lower in flashpoints by 10%, the equipment might require further inspection. This value might differ in different countries.
It is advised to carry out 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 not to determine the condition of the transformer; it is a well being and security impact check. PCB is hazardous to both humans and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It is also required each time any upkeep has been carried out on the unit, and the potential for contamination is present. If PCB content exceeds the really helpful limits, the suitable action must be taken.
Units with a PCB content of greater than 50ppm require a hearth safety plan, environmental protection plan, and extra precautionary measures when upkeep 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 proprietor.
Local regulatory bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we are going to discuss this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this knowledge in accordance with worldwide requirements might be discussed in detail, forming part of the overall well being rating willpower of the transformer.
Transformer condition monitoring is an interlaced, highly exciting area of study. In this article, we targeted on the forms of exams to determine the condition of the transformer, the crucial values, and the beneficial 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 possible to ensure finest apply utility and optimised maintenance. It additionally make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 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 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 strength.”
6. Article initially published by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience within the business, having previously labored as laboratory supervisor for a major industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the start of her profession, notably within the analysis of check data. Corné has huge sensible and theoretical knowledge of reliability upkeep programmes.

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