by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the relevant data relating to an asset’s operation. If we interpret the information correctly, it can give us nice insight into the asset’s condition.
Frequent monitoring of the asset can lead to much less upkeep required or extra extended intervals with none upkeep required.
It is essential to establish the key parameters which may be wanted to offer us a whole image of the particular standing of the transformer and the motion we have to take to ensure the continued reliability of the asset to attain the maximum lifetime.
What is the info telling us?
Has the condition of the unit modified for the explanation that final upkeep 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 particular period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit earlier than we have to consider replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring define
It is vitally important to identify clear goals as part of your technique. What do you want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the finish result and what you need to accomplish, it will be much easier to identify the required parameters.
Health indexing of assets is becoming a remarkable software 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 have been calculated based on the worldwide requirements for mineral oils, indicating the critical values stipulated in the numerous requirements.
The scope of oil evaluation, interpretation of the info, and critical values
At the beginning of this part, it is essential to state that we cope with totally different size transformers in the business. Transformers are divided into courses based on the kV scores of the gear. It is as a lot as the reliability or asset manager to make use of the guidelines for larger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or manager wants to determine what type of testing would profit him in figuring out downside areas inside his fleet. Some of the analysis has been identified as to routine sort tests. Still, there’s an extensive range of checks that can assist in identifying specific drawback standards throughout the system, which might not be clear through the everyday day-to-day evaluation often performed.
Please see the ranking classes in TABLE 1
TABLE 2 explains which oil analyses are really helpful and the way typically or underneath which circumstances.
Oil sampling
The taking of the oil sample is doubtless certainly one of the most vital and significant influencers in the evaluation outcome. If a sample just isn’t taken to the prescribed process, then there is a significant chance that the evaluation performed, and the outcomes acquired, are of no use.
Care ought to be taken to make sure high quality management procedures are utilized in every step of the sampling course of as set out by international standards. A good quality sample taken by making use of the correct procedure is crucial. A sample may be contaminated by numerous elements, all of which can affect the end result of the ends in a unfavorable method.
All steps concerned in sample taking should adhere to quality control procedures, together with the container that’s used, the sampling package, the taking of the sample, the marking of the sample, the closing of the pattern, the storage and handling of the sample, after which the supply of the sample to the laboratory.
If the sample identification and sample information are handwritten, the human factor can lead to incorrect interpretation of the data. The label should 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 ought to be waterproof, and the pen used to write on the label ought to be oil- and water-resistant. Otherwise, some knowledge could be misplaced, making it extraordinarily troublesome for the laboratory personnel to complete the report or allocate the sample to a specific transformer.
Now, let’s focus on the kinds of checks to discover out the transformer’s situation, the crucial values, and the recommended actions in each case:
a) Colour and appearance
This is a routine inspection applied to every oil pattern.
When an oil pattern arrives at the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to determine the color, turbidity, and potential particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it would point out a excessive water content material within the oil.
If the drain valve was not cleaned by the sampler, the filth particles within the drain valve may be incorporated into the pattern. If particles are identified as carbon, it’d point out a potential electrical fault within the unit. เพรสเชอร์เกจวัดแรงดันน้ำ of the oil will verify if so.
Clear oils with out contamination will point out a great situation, and no motion is beneficial.
When oils are darkish or turbid, additional analysis will confirm any problems. 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 level 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 foreign particles, or both 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 current sample interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with other parameter outcomes 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 different checks point out severe growing older, the oil could be changed with new or reclaimed oil. Another choice would 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 removed from the oil. This course of removes acid and water as nicely as another compounds. Another advantage is that the oil may be re-used, and in most situations, this can be accomplished with out switching off the unit, which contributes to cost-saving. If in doubt – instead swap off the unit throughout this remedy process.
If the values are Poor, it is advisable to take motion as soon as attainable and never delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content could cause flashover in 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 check
This is a routine check for all courses of electrical equipment, except class G
The outcomes of this take a look at should all the time be thought of at the aspect of the breakdown energy. If it’s found that the water content material is high and the breakdown power is low, further action must be taken. It is beneficial that a second sample from the same unit is examined to confirm the outcomes.
In the case of switching equipment, where there isn’t any paper present, the breakdown voltage is the determining issue.
It must be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it is found that the unit’s working temperature is outdoors this temperature vary, it’s best to discuss with Annex A of the standard.
When the value obtained by way of analyses is GOOD, the normal sampling interval may be maintained, requiring no additional action.
When the worth returns a FAIR end result, more frequent sampling is beneficial. It can also be useful to assume about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be implemented.
A POOR result will require instant motion from the asset supervisor. This may embody taking another sample to verify the outcomes from the primary analysis. If it’s confirmed that the water content material is excessive, the oil can be filtered; this process ought to take away a large portion of the moisture from the oil if applied appropriately. Follow-up samples must be taken to ensure that the moisture content material continues to be inside the required limits. The purpose is that the most good portion of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil beneath situations that favour this motion. It might be found later that the oil in the water has elevated again with none apparent cause, however the source would be the paper in the transformer.
A visual inspection can also be really helpful to determine if any water would possibly transfer into the transformer or electrical tools via leaks. This downside could be more severe if the transformer or electrical gear is exterior and not in a covered area.
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 0.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine take a look at
This is a routine take a look at for all classes except F and G
The acids in oils are shaped as a end result of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist in the formation of the acids. Because acids are polar compounds, it’ll adversely have an result on the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this can result in sludge formation, normally around the lower components of the transformer core. The sludge will ultimately type a semi-solid substance that is extraordinarily difficult to remove.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to fit the state of affairs. Future evaluation should embody a visible 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 could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever choice 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<0.100.10 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< hundred.01 to 0.30>0.03
F, GNot a routine test
This is a routine check for all courses of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this take a look at supplies data concerning the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities that may affect the dissipation factor include water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result’s POOR, reclamation or an oil change is really helpful. The construction of the oil is broken, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the recommended 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 among the key parameters to assess the transformer insulation situation; that is based mostly on the reality that DC resistance is delicate to grease degradation.
When the result is FAIR, extra frequent sampling and checking extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of authentic value
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil provider to verify the small print regarding additives.
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 significantly slows the aging process in the oil and the stable insulation.
If the result’s FAIR, it’s advised to top up the inhibitor degree to the prescribed level per supplier instructions. It is suggested to use a area skilled trained within the process to carry out this task.
If the end result obtained is POOR, the recommendation for this scenario would suggest that the end user continues to use the oil “uninhibited,” but this will result in more speedy 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 maintain dissolved oxygen at ranges beneath 1000ppm. This could 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 under these circumstances isn’t required, although it’d add additional safety in opposition to 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 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 known as steel deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and cut back their price of response with compounds in the oil. This consists of oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary varieties, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the writer 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 more rapidly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD results, common sample intervals could be maintained.
With FAIR results, maintain regular monitoring.
When POOR, it is suggested to take away the oil or take away the supply of corrosivity from the oil by way of special oil therapy.
I) Sediment and sludge
This is not a routine take a look at.
It is suggested that this take a look at is carried out when the oil outcomes point out a excessive acid worth and the dissipation factor is near the unacceptable restrict.
The results have to be lower than 0.02% by mass to be negligible. If the outcomes return a price of more than 0.02% by mass, it is instructed that it be reclaimed; in any other case, an oil change is really helpful.
j) Interfacial pressure
This is not a routine test
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 in the course of the aging course of. What this means in sensible terms is there may be extra polar compound current in the oil, lowering the ability of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation number. Therefore, the interfacial pressure turns into a quality criterion: the oil must be modified under a predefined restrict.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, lower 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 either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system can be vital. The extent of the corrosion injury brought on by the sulfur may be so severe that it would cause failure of the tools if not checked. The addition of a copper passivator can reduce the impact 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 permitted for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s threat evaluation research. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on power transformer insulating oil as measured utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there is a most decrease in flashpoints by 10%, the equipment would possibly require further inspection. This value may differ in several countries.
It is advised to perform this take a look at when an unusual odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not determine the situation of the transformer; this is a well being and safety impression check. PCB is hazardous to both humans and the environment; it’s critical to test for PCBs after the retro fill of a transformer. It is also required every time any upkeep has been accomplished on the unit, and the possibility of contamination is current. If PCB content material exceeds the beneficial limits, the appropriate motion must be taken.
Units with a PCB content material of greater than 50ppm require a fire security plan, environmental protection plan, and additional precautionary measures when maintenance is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the tools proprietor.
Local regulatory our bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we’ll discuss this phenomenon partially II of the article. The limits for the different gases and the interpretation of this knowledge in accordance with international requirements might be mentioned in detail, 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 focused on the types of tests to determine the condition of the transformer, the critical values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a specific unit at a selected date and time. This makes it attainable to ensure greatest follow utility and optimised maintenance. It additionally make it easier 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 maintenance 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 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise in the business, having beforehand labored as laboratory supervisor for a significant industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the beginning of her profession, particularly within the evaluation of check information. Corné has vast practical and theoretical knowledge of reliability maintenance programmes.

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