by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the relevant information concerning an asset’s operation. If we interpret the information correctly, it can provide us nice insight into the asset’s situation.
Frequent monitoring of the asset can outcome in much less upkeep required or extra prolonged intervals without any maintenance required.
It is essential to establish the necessary thing parameters that are wanted to give us a whole picture of the actual status of the transformer and the action we want to take to ensure the continued reliability of the asset to attain the utmost lifetime.
What is the data telling us?
Has the condition of the unit modified because the last maintenance period?
Is it secure to function the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate score for a particular period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit earlier than we need to consider replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring define
It is vitally necessary to establish clear goals as a part of your technique. What do you need to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it will be much simpler to identify the required parameters.
Health indexing of assets is changing into a exceptional device in getting a clearer image of the situation 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 had been calculated based mostly on the worldwide standards for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil evaluation, interpretation of the data, and significant values
At the beginning of this part, it is essential to state that we cope with different measurement transformers within the trade. Transformers are divided into courses based on the kV rankings of the gear. It is up to the reliability or asset manager to make use of the guidelines for bigger tools, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or supervisor wants to find out what kind of testing would profit him in figuring out problem areas within his fleet. Some of the analysis has been recognized as to routine kind exams. Still, there might be an in depth vary of checks that may assist in identifying specific downside criteria within the system, which might not be clear through the everyday day-to-day evaluation normally performed.
Please see the ranking courses in TABLE 1
TABLE 2 explains which oil analyses are recommended and how typically or under which circumstances.
Oil sampling
The taking of the oil pattern is probably considered one of the most vital and significant influencers within the evaluation consequence. If a sample is not taken to the prescribed process, then there is a significant risk that the evaluation carried out, and the results acquired, are of no use.
Care should be taken to ensure quality control procedures are utilized in every step of the sampling course of as set out by worldwide requirements. A good high quality sample taken by applying the proper process is crucial. A pattern may be contaminated by various elements, all of which might affect the end result of the results in a unfavorable method.
All steps involved in pattern taking should adhere to quality control procedures, together with the container that is used, the sampling kit, the taking of the sample, the marking of the pattern, the closing of the pattern, the storage and handling of the sample, after which the supply 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 info. 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 write on the label should be oil- and waterproof. Otherwise, some data may be misplaced, making it extremely tough for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s discuss the types of checks to discover out the transformer’s condition, the important values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection utilized to every oil pattern.
When an oil sample arrives at the laboratory, one of many “tests” is a visual inspection of the oil sample in a transparent vessel to find out the colour, turbidity, and potential particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it might point out a excessive water content within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles within the drain valve might be incorporated into the pattern. If particles are recognized as carbon, it would indicate a potential electrical fault within the unit. The DGA evaluation of the oil will verify if this is the case.
Clear oils without contamination will point out a good condition, and no action is beneficial.
When oils are darkish or turbid, further evaluation will affirm any problems. The oil evaluation outcomes may even decide 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 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 material or the presence of foreign particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage have to be excessive.
If the values are Good, it is strongly recommended to continue with the current pattern interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with different parameter results just 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 alternative checks indicate extreme growing older, the oil can be replaced with new or reclaimed oil. Another option would 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 components are removed from the oil. This course of removes acid and water as nicely as another compounds. Another advantage is that the oil could be re-used, and in most conditions, this may be carried out without switching off the unit, which contributes to cost-saving. If unsure – 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 course of. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content could cause flashover within the unit, resulting in 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 take a look at
This is a routine take a look at for all classes of electrical gear, except class G
The outcomes of this test ought to always be thought-about along side the breakdown strength. If it’s found that the water content material is high and the breakdown power is low, further motion must be taken. It is recommended that a second sample from the identical unit is tested to confirm the outcomes.
In the case of switching tools, the place there is no paper present, the breakdown voltage is the figuring out issue.
It ought to be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s operating temperature is exterior this temperature vary, it’s best to refer to Annex A of the usual.
When the value obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no additional action.
When the value returns a FAIR result, more frequent sampling is really helpful. It can be useful to consider other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be applied.
A POOR outcome will require quick action from the asset supervisor. This may embody taking another sample to verify the outcomes from the primary analysis. If it is confirmed that the water content material is high, the oil can be filtered; this process should take away a large portion of the moisture from the oil if utilized accurately. Follow-up samples have to be taken to ensure that the moisture content is still throughout the required limits. The reason is that the most good portion of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under conditions that favour this motion. It could be discovered later that the oil in the water has increased once more without any apparent cause, however the source could be the paper within the transformer.
A visible inspection is also recommended to discover out if any water may transfer into the transformer or electrical tools through leaks. This problem might be extra severe if the transformer or electrical equipment is outside and not in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.15>0.15
B, E< hundred.10 to zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine check for all classes besides F and G
The acids in oils are fashioned as a result of chemical reactions between the oil, water, and paper. Higher temperatures or load increases 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 enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, usually around the lower parts of the transformer core. The sludge will ultimately form a semi-solid substance that’s extraordinarily tough to take away.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval ought to be decreased to suit the scenario. Future evaluation should include a visible inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition, the asset manager might resolve to reclaim the oil or exchange it with new or reclaimed oil, whichever possibility may suit 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 zero.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 test for all classes of electrical gear, besides F and G
The dielectric dissipation factor or tan delta of this test supplies information regarding the extent of the dielectric losses in transformer oil. This test 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 that might influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking extra parameters is recommended.
When the result is 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 damaged down, and even with filtration, the beneficial 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 doubtless considered one of the key parameters to assess the transformer insulation condition; this is based on the reality that DC resistance is sensitive to grease degradation.
When the result’s FAIR, more frequent sampling and checking additional parameters is beneficial.
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 authentic value<40% of authentic worth
This check is restricted to oils with this additive.
It could be advisable to contact the oil provider to verify the small print concerning components.
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 purpose of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the getting older process in the oil and the stable insulation.
If the result’s FAIR, it’s suggested to top up the inhibitor degree to the prescribed stage per supplier instructions. It is suggested to make use of a field professional skilled within the process to carry out this activity.
If the outcome obtained is POOR, the recommendation for this situation would counsel that the tip consumer continues to make use of the oil “uninhibited,” however this will likely result in more fast degradation of each the liquid and solid insulation.
It must be noted that some transformers have already got a built-in oil preservation system; that is designed to keep dissolved oxygen at levels beneath 1000ppm. This can be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, although it would add additional protection 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 stable, (rate of lower < 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 metallic deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and scale back their price of response with compounds within the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic types, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the author is conscious, was in 1967 by J.J. pressure gauge น้ำมัน and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator would possibly deplete more rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals could 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 via special oil therapy.
I) Sediment and sludge
This just isn’t a routine check.
It is advised that this take a look at is performed when the oil outcomes point out a excessive acid worth and the dissipation factor is close to the unacceptable limit.
The results have to be lower than zero.02% by mass to be negligible. If the results return a value of more than 0.02% by mass, it’s suggested that it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial pressure
This is not 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 test
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces during the aging course of. What this means in practical terms is there may be more polar compound current within the oil, reducing the power of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial rigidity turns into a prime quality criterion: the oil must be modified below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, check 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 impact on the transformer system could be significant. The extent of the corrosion injury attributable to the sulfur may be so extreme that it would trigger failure of the equipment if not checked. The addition of a copper passivator can scale back the impression of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber merchandise utilized 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 need of corrective actions based on this institute’s risk assessment study. [4]
l) Particle counting and sizing
Table three: 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 test
If there’s a maximum lower in flashpoints by 10%, the equipment would possibly require further inspection. This worth would possibly differ in several countries.
It is suggested to perform this test when an unusual odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not decide the condition of the transformer; it is a well being and safety influence check. PCB is hazardous to each humans and the environment; it’s critical to test for PCBs after the retro fill of a transformer. It can be required every time any maintenance has been carried out on the unit, and the potential of contamination is current. If PCB content material exceeds the recommended limits, the appropriate action must be taken.
Units with a PCB content material of more than 50ppm require a fire security plan, environmental protection plan, and further precautionary measures when upkeep is done. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the tools owner.
Local regulatory our bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of information and interpretation, we will talk about this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data based on international requirements shall be mentioned in detail, forming part of the overall health score determination of the transformer.
Transformer situation monitoring is an interlaced, extremely thrilling subject of research. In this article, we focused on the forms of tests to discover out the condition of the transformer, the important values, and the really helpful 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 potential to make sure best apply utility and optimised upkeep. It additionally 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
3. Passivators, what they’re 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 revealed by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise in the business, having beforehand labored as laboratory manager for a major industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the beginning of her profession, particularly within the evaluation of take a look at information. Corné has vast sensible and theoretical knowledge of reliability upkeep programmes.