by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and analysis of the related data regarding an asset’s operation. If we interpret the information appropriately, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset can end result in less upkeep required or extra extended durations with none maintenance required.
It is crucial to identify the key parameters that are needed to give us a complete image of the actual standing of the transformer and the action we have to take to ensure the continued reliability of the asset to achieve the maximum lifetime.
What is the data telling us?
Has the situation of the unit modified because the last upkeep period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate rating for a selected period?
Are we required to implement motion to ensure the continued reliability of the unit?
How lengthy can we use the unit earlier than we need to consider replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring define
It is vitally necessary to identify clear goals as a half of your technique. What do you need to obtain 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 would be a lot simpler to determine the required parameters.
Health indexing of belongings is changing into a exceptional device in getting a clearer picture of the situation 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 were calculated based on the international standards for mineral oils, indicating the crucial values stipulated in the various requirements.
The scope of oil evaluation, interpretation of the info, and important values
At the beginning of this section, it’s important to state that we take care of different dimension transformers in the trade. Transformers are divided into lessons according to the kV ratings of the tools. It is as much as the reliability or asset manager to use the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical checks.
The maintenance engineer or supervisor needs to find out what kind of testing would profit him in identifying drawback areas within his fleet. Some of the evaluation has been identified as to routine type tests. Still, there is an in depth vary of exams that can assist in figuring out specific drawback standards inside the system, which could not be clear by way of the everyday day-to-day analysis normally performed.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way typically or under which circumstances.
Oil sampling
The taking of the oil pattern is among the most crucial and significant influencers within the analysis end result. If a sample isn’t taken to the prescribed procedure, then there is a vital chance that the analysis performed, and the results acquired, are of no use.
Care ought to be taken to make sure quality management procedures are utilized in every step of the sampling course of as set out by international standards. A good high quality sample taken by applying the correct process is important. A pattern may be contaminated by numerous factors, all of which may affect the finish result of the ends in a negative method.
All steps concerned in sample taking must adhere to quality management procedures, including the container that’s used, the sampling equipment, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and handling of the pattern, after which the supply of the sample to the laboratory.
If the sample identification and pattern data are handwritten, the human factor can result in incorrect interpretation of the data. The label should be stuck onto the container to ensure 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 down on the label ought to be oil- and waterproof. Otherwise, some data may be lost, making it extraordinarily troublesome for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s focus on the types 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 on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a transparent vessel to determine the color, turbidity, and potential particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there is lots of turbidity, it’d point out a high water content within the oil.
If the drain valve was not wiped clean by the sampler, the filth particles in the drain valve could be incorporated into the pattern. If particles are recognized as carbon, it would indicate a potential electrical fault in the unit. The DGA evaluation of the oil will verify if that is the case.
Clear oils with out contamination will point out a good situation, and no motion is beneficial.
When oils are dark or turbid, further analysis will verify any problems. The oil evaluation results will also determine the degree and type 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 application
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of foreign particles, or both in 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 strongly recommended to proceed with the current sample interval motion plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil through oil reconditioning processes. If alternative checks point out extreme aging, the oil could be changed with new or reclaimed oil. Another choice can be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar components are faraway from the oil. This process removes acid and water in addition to another compounds. Another benefit is that the oil could be re-used, and in most conditions, this can be accomplished with out switching off the unit, which contributes to cost-saving. If unsure – as an alternative swap off the unit during this therapy course of.
If the values are Poor, it is advisable to take motion as soon as potential and not delay the maintenance course of. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content material can cause flashover in the unit, leading to lack 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 test
This is a routine take a look at for all lessons of electrical tools, except class G
The outcomes of this check ought to at all times be thought of in conjunction with the breakdown strength. If it’s found that the water content is high and the breakdown strength is low, additional action must be taken. It is beneficial that a second pattern from the same unit is tested to substantiate the outcomes.
In the case of switching gear, where there isn’t any paper present, the breakdown voltage is the determining issue.
It should be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it’s found that the unit’s operating temperature is exterior this temperature vary, it is best to refer to Annex A of the standard.
When the worth obtained via analyses is GOOD, the conventional sampling interval could be maintained, requiring no additional action.
When the value returns a FAIR outcome, more frequent sampling is beneficial. It can also be helpful to suppose about different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be applied.
A POOR result will require instant action from the asset supervisor. This may embody taking another pattern to verify the outcomes from the primary analysis. If it’s confirmed that the water content material is excessive, the oil could be filtered; this course of should remove a large portion of the moisture from the oil if applied appropriately. Follow-up samples need to be taken to make certain that the moisture content material is still inside the required limits. The reason is that probably the most good portion of the water is caught up in the paper system in the transformer. This moisture will move from the paper into the oil under conditions that favour this movement. It may be discovered later that the oil within the water has increased again without any apparent purpose, however the supply could be the paper in the transformer.
A visible inspection can also be beneficial to find out if any water might move into the transformer or electrical gear via leaks. ตัววัดแรงดันน้ำ might be more extreme 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 0.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C< hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine take a look at for all courses except F and G
The acids in oils are formed as a end result of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help within the formation of the acids. Because acids are polar compounds, it’s going to adversely have an effect on the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this could lead to sludge formation, usually around the decrease parts of the transformer core. The sludge will eventually form a semi-solid substance that is extraordinarily troublesome 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 according to the prescribed values in IEC 60422 Edition four.0, the asset manager might determine to reclaim the oil or substitute it with new or reclaimed oil, whichever option would possibly suit 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<0.100.10 to zero.20>0.20
B, C<0.a hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E<0.a hundred.01 to zero.30>0.03
F, GNot a routine take a look at
This is a routine take a look at for all courses of electrical tools, besides F and G
The dielectric dissipation issue or tan delta of this take a look at supplies data relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we now have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities which may affect the dissipation issue embody 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 structure of the oil is broken, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the beneficial 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 likely one of the key parameters to evaluate the transformer insulation situation; this is based on the truth that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
When the result is 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 authentic value
This test is restricted to oils with this additive.
It can be advisable to contact the oil provider to confirm the details concerning additives.
The two most common 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 stop oxygen from reacting with the oil. This considerably slows the aging process in the oil and the stable insulation.
If the result’s FAIR, it is advised to high up the inhibitor degree to the prescribed level per supplier instructions. It is advised to use a subject professional skilled in the procedure to perform this task.
If the result obtained is POOR, the recommendation for this situation would counsel that the tip user continues to use the oil “uninhibited,” but this will likely lead to more fast degradation of each the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges below 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 underneath these circumstances is not required, though it might add further protection against 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 decrease of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called steel deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and cut back their rate of reaction with compounds within the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary types, sulfur-based and nitrogen-based. The first suggested 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 more quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals could be maintained.
With FAIR results, preserve common monitoring.
When POOR, it’s suggested to remove the oil or take away the supply of corrosivity from the oil via special oil therapy.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this test is carried out when the oil results point out a excessive acid value and the dissipation issue is close to the unacceptable restrict.
The results have to be less than zero.02% by mass to be negligible. If the outcomes return a value of greater than zero.02% by mass, it’s instructed that it’s reclaimed; in any other case, an oil change is really helpful.
j) Interfacial pressure
This just isn’t a routine check
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 during the growing older course of. What this means in sensible phrases is there’s extra polar compound present within the oil, decreasing the power of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure becomes a quality criterion: the oil have to be modified beneath a predefined restrict.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system can be significant. The extent of the corrosion injury attributable to the sulfur may be so severe that it might trigger failure of the equipment if not checked. The addition of a copper passivator can reduce the impression of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber merchandise utilized in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved 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’s a maximum lower in flashpoints by 10%, the equipment may require additional inspection. This worth may differ in numerous international locations.
It is advised to carry out this test when an uncommon odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not decide the situation of the transformer; this could be a well being and safety impression check. PCB is hazardous to each people and the surroundings; it is important to test for PCBs after the retro fill of a transformer. It can be required whenever any upkeep has been accomplished on the unit, and the risk of contamination is current. If PCB content material exceeds the beneficial limits, the suitable motion must be taken.
Units with a PCB content material of greater than 50ppm require a fireplace safety plan, environmental protection plan, and extra precautionary measures when upkeep is finished. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear owner.
Local regulatory our bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of information and interpretation, we will talk about this phenomenon partially II of the article. The limits for the different gases and the interpretation of this information according to worldwide standards might be mentioned in detail, forming part of the overall well being ranking dedication of the transformer.
Transformer situation monitoring is an interlaced, highly exciting field of research. In this text, we centered on the forms of checks to discover out the situation of the transformer, the important values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a particular unit at a selected date and time. This makes it attainable to ensure finest follow application and optimised upkeep. It also make it easier to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition 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 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 energy.”
6. Article initially published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise in the trade, having beforehand labored as laboratory manager for a serious industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her profession, particularly in the analysis of take a look at information. Corné has huge sensible and theoretical data of reliability maintenance programmes.

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