by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant knowledge relating to an asset’s operation. If we interpret the information accurately, it can provide us nice perception into the asset’s situation.
Frequent monitoring of the asset can lead to much less maintenance required or extra extended durations without any maintenance required.
It is essential to identify the necessary thing parameters which may be needed to give us an entire image of the particular standing of the transformer and the action we need to take to ensure the continued reliability of the asset to realize the utmost lifetime.
What is the information telling us?
Has the situation of the unit modified because the last maintenance period?
Is it safe to function the unit?
Are there indicators of deterioration?
Is it safe 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 lengthy can we use the unit earlier than we want to consider replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring outline
It is vitally important to determine clear targets as a part of your strategy. 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 end result and what you want to accomplish, it will be much easier to identify the required parameters.
Health indexing of property is turning into a exceptional tool in getting a clearer image 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 have been calculated primarily based on the international standards for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil evaluation, interpretation of the data, and important values
At the start of this part, it’s important to state that we cope with completely different dimension transformers within the industry. Transformers are divided into lessons in accordance with the kV ratings of the tools. It is as much as the reliability or asset supervisor to use the guidelines for bigger gear, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or supervisor wants to discover out what kind of testing would benefit him in figuring out downside areas inside his fleet. Some of the evaluation has been identified as to routine sort tests. Still, there is an intensive range of exams that can assist in identifying specific drawback criteria throughout the system, which might not be clear by way of the standard day-to-day analysis often performed.
Please see the ranking courses in TABLE 1
TABLE 2 explains which oil analyses are really helpful and the way usually or underneath which circumstances.
Oil sampling
The taking of the oil sample is amongst the most crucial and significant influencers within the analysis end result. If a pattern isn’t taken to the prescribed procedure, then there’s a vital risk that the analysis carried out, and the outcomes acquired, are of no use.
Care must be taken to ensure quality management procedures are utilized in each step of the sampling process as set out by worldwide standards. A good quality pattern taken by applying the right process is essential. A pattern can be contaminated by varied components, all of which can influence the finish result of the leads to a negative manner.
All steps concerned in pattern taking must adhere to high quality control procedures, together with the container that is used, the sampling kit, the taking of the pattern, the marking of the pattern, the closing of the sample, the storage and dealing with of the sample, after which the delivery of the pattern to the laboratory.
If the pattern identification and sample data are handwritten, the human issue can lead to incorrect interpretation of the data. The label must be caught onto the container to ensure no mix-ups in pattern identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label should be waterproof, and the pen used to put in writing on the label must be oil- and water resistant. Otherwise, some information could be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s talk about the types of tests to determine the transformer’s situation, the important values, and the beneficial actions in each case:
a) Colour and appearance
This is a routine inspection applied to every oil sample.
When an oil pattern arrives at the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to determine the color, turbidity, and potential particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there might be a lot of turbidity, it would indicate a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles within the drain valve might be included into the sample. If particles are recognized as carbon, it would indicate a potential electrical fault within the unit. The DGA evaluation of the oil will confirm if that is the case.
Clear oils with out contamination will indicate a good condition, and no motion is really helpful.
When oils are darkish or turbid, additional analysis will verify any issues. The oil evaluation outcomes may even determine the degree and type 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 application
G <30
This is a routine inspection.
Breakdown voltage will indicate 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 keep away from flashover in the unit, the breakdown voltage should be high.
If the values are Good, it is suggested to proceed with the current pattern interval motion plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with different parameter results like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If various checks point out extreme aging, the oil can be replaced with new or reclaimed oil. Another choice can be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar elements are removed from the oil. This course of removes acid and water in addition to another compounds. Another advantage is that the oil can be re-used, and in most situations, this could be carried out without switching off the unit, which contributes to cost-saving. If unsure – as a substitute switch off the unit during this therapy course of.
If the values are Poor, it’s advisable to take motion as quickly as attainable and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely high water content can cause flashover within the unit, resulting in 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 lessons of electrical tools, besides class G
The results of this take a look at should at all times be considered in conjunction with the breakdown power. If it’s discovered that the water content material is excessive and the breakdown energy is low, further motion must be taken. It is beneficial that a second sample from the identical unit is examined to verify the outcomes.
In the case of switching tools, the place there is no paper current, the breakdown voltage is the determining factor.
It must be famous that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it’s discovered that the unit’s working temperature is outdoors this temperature vary, it’s best to check with Annex A of the standard.
When the value obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no additional action.
When the worth returns a FAIR result, more frequent sampling is really helpful. It can additionally be useful to contemplate other parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR result would require quick action from the asset supervisor. This might embrace taking another pattern to substantiate the results from the first evaluation. If it is confirmed that the water content material is high, the oil could be filtered; this course of ought to take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples need to be taken to ensure that the moisture content material continues to be within the required limits. The cause is that essentially the most significant portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil under circumstances that favour this movement. It could be discovered later that the oil in the water has elevated again with none obvious purpose, however the supply would be the paper within the transformer.
A visual inspection is also really helpful to determine if any water would possibly move into the transformer or electrical equipment by way of leaks. This drawback may be extra severe if the transformer or electrical gear is outside and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D< hundred.10 to 0.15>0.15
B, E< hundred.10 to 0.25>0.20
C< hundred.15 to 0.30>0.30
F, G Not a routine check
This is a routine test for all lessons besides F and G
The acids in oils are fashioned as a end result of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist in the formation of the acids. Because acids are polar compounds, it will adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this will lead to sludge formation, usually across the lower components of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily difficult to remove.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR result, the sampling interval should be decreased to fit the state of affairs. Future evaluation ought to include a visual 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 manager could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever choice may suit their requirements 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 0.20>0.20
B, C< hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E< 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 classes of electrical equipment, besides F and G
The dielectric dissipation factor or tan delta of this check offers data regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities which may influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is recommended. The construction of the oil is broken, in effect that the chemical bonds between the molecules have broken down, and even with filtration, the recommended 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 test
DC resistivity of the oil is likely one of the key parameters to evaluate the transformer insulation situation; that is based on the truth that DC resistance is sensitive to grease degradation.
When the result’s FAIR, more frequent sampling and checking additional parameters is recommended.
When the result’s POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of unique worth
This test is restricted to oils with this additive.
It could be advisable to contact the oil supplier to verify the details concerning additives.
The two most common oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The objective of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the aging course of within the oil and the solid insulation.
If the result’s FAIR, it’s advised to high up the inhibitor stage to the prescribed degree per supplier instructions. It is suggested to use a subject professional skilled within the process to carry out this process.
If the result obtained is POOR, the recommendation for this state of affairs would counsel that the tip person continues to make use of the oil “uninhibited,” but this will likely lead to extra fast degradation of both the liquid and stable 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 levels below 1000ppm. This could be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances just isn’t required, though it would add additional 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 lower of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also known as metal deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and cut back their rate of response with compounds in the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary varieties, 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 might deplete extra quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals can be maintained.
With FAIR outcomes, keep regular monitoring.
When POOR, it’s suggested to take away the oil or remove the source of corrosivity from the oil through particular oil treatment.
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 high acid value and the dissipation issue is near the unacceptable restrict.
The outcomes have to be less than zero.02% by mass to be negligible. If the results return a worth of more than 0.02% by mass, it is suggested that or not it’s reclaimed; otherwise, 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 twenty-eight 20 to 25<22 <20
ENot a routine test
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces in the course of the getting older course of. What this means in practical terms is there might be extra polar compound current in the oil, reducing the flexibility of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial tension becomes a top quality criterion: the oil should be changed below a predefined limit.
If outcomes are GOOD, continue the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
pressure gauge วัด แรง ดัน ) Corrosive sulfur
This 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 damage brought on by the sulfur can be so extreme that it’d cause failure of the gear if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a research by Doble, it has been found that rubber products used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions primarily based on this institute’s risk assessment study. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination levels (particles) encountered on energy transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there is a most lower in flashpoints by 10%, the gear might require further inspection. This value may differ in different countries.
It is suggested to perform this take a look at when an uncommon odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the condition of the transformer; this may be a health and safety impression test. PCB is hazardous to both people and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It is also required whenever any maintenance has been done on the unit, and the potential of contamination is present. If PCB content exceeds the really helpful limits, the suitable motion needs to be taken.
Units with a PCB content material of greater than 50ppm require a fireplace safety plan, environmental safety plan, and further precautionary measures when upkeep is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the equipment proprietor.
Local regulatory bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of data and interpretation, we will talk about this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this knowledge based on international standards shall be mentioned in detail, forming part of the general health ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, extremely thrilling field of research. In this text, we centered on the types of exams to determine the situation of the transformer, the important values, and the beneficial actions.
The Health Index indication makes it potential to see the supposed reliability of a particular unit at a specific date and time. This makes it potential to make sure best practice software and optimised maintenance. It additionally make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 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 are 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 power.”
6. Article initially published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ experience in the business, having beforehand labored as laboratory manager for a significant industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the start of her profession, particularly within the evaluation of check knowledge. Corné has vast practical and theoretical information of reliability maintenance programmes.

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