by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the related data 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 more prolonged intervals without any maintenance required.
It is essential to identify the important thing parameters which are needed to offer us a whole picture of the particular status of the transformer and the motion we need to take to ensure the continued reliability of the asset to realize the utmost lifetime.
What is the data telling us?
Has the situation of the unit changed because the last maintenance period?
Is it safe to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate rating for a specific period?
Are we required to implement action to make sure the continued reliability of the unit?
How lengthy can we use the unit before we have to think about replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring outline
It is vitally important to determine clear targets as part of your strategy. What do you want to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the end result and what you need to accomplish, it might be a lot easier to identify the required parameters.
Health indexing of assets is becoming a exceptional device in getting a clearer image of the condition of your transformer. Test parameters carry a numerical value-adding to the whole value of the Health Index Value of the transformer. These parameter weight values have been calculated based mostly on the international standards for mineral oils, indicating the important values stipulated within the varied standards.
The scope of oil evaluation, interpretation of the information, and critical values
At the start of this part, it’s important to state that we take care of completely different size transformers within the trade. Transformers are divided into lessons according to the kV ratings of the gear. It is as much as the reliability or asset supervisor to use the rules for larger equipment, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or manager needs to discover out what type of testing would benefit him in figuring out drawback areas inside his fleet. Some of the evaluation has been identified as to routine kind exams. Still, there is an intensive vary of exams that may help in figuring out particular drawback standards throughout the system, which could not be clear by way of the everyday day-to-day evaluation usually performed.
Please see the score courses in TABLE 1
TABLE 2 explains which oil analyses are beneficial and the way typically or under which circumstances.
Oil sampling
The taking of the oil sample is considered one of the most vital and critical influencers within the analysis outcome. If a pattern is not taken to the prescribed procedure, then there’s a significant possibility that the evaluation carried out, and the results acquired, are of no use.
Care should be taken to make sure high quality control procedures are utilized in each step of the sampling course of as set out by worldwide standards. A good high quality pattern taken by making use of the proper procedure is important. A pattern could be contaminated by varied factors, all of which might affect the outcome of the leads to a negative method.
All steps involved in pattern taking should adhere to high quality control procedures, together with the container that’s used, the sampling package, the taking of the pattern, the marking of the pattern, the closing of the sample, the storage and dealing with of the pattern, and then the supply of the pattern to the laboratory.
If the pattern identification and pattern knowledge 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 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 put in writing on the label must be oil- and water resistant. Otherwise, some information could be lost, making it extremely tough for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s talk about the kinds of exams to determine the transformer’s situation, the critical values, and the beneficial actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil sample.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to determine the color, turbidity, and potential particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there might be 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 may be included into the sample. If particles are recognized as carbon, it would point out a potential electrical fault in the unit. The DGA evaluation of the oil will confirm if that is the case.
Clear oils with out contamination will point out a good condition, and no motion is really helpful.
When oils are dark or turbid, further analysis will verify any problems. The oil analysis outcomes may even determine the diploma 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 point application<40 kV for OLTC in delta or line-end application
G <30
This is a routine inspection.
Breakdown voltage will point out the water content material 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 in the unit, the breakdown voltage should be excessive.
If the values are Good, it is strongly recommended to continue with the present sample interval motion 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 strongly recommended to recondition the oil via oil reconditioning processes. If different checks indicate extreme aging, the oil can be replaced with new or reclaimed oil. Another option can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar parts are removed from the oil. This course of removes acid and water as properly as some other compounds. Another advantage is that the oil could be re-used, and in most conditions, this may be accomplished without switching off the unit, which contributes to cost-saving. If in doubt – as an alternative swap off the unit throughout this remedy process.
If Ridiculous are Poor, it is advisable to take action as soon as attainable and not delay the upkeep process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content could cause flashover in the unit, resulting in loss of the asset.
c) Water content material (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 take a look at
This is a routine test for all courses of electrical tools, besides class G
The results of this test ought to always be thought of in conjunction with the breakdown strength. If it is found that the water content material is excessive and the breakdown strength is low, further motion must be taken. It is recommended that a second pattern from the same unit is examined to confirm the outcomes.
In the case of switching gear, the place there is no paper present, the breakdown voltage is the figuring out factor.
It should be famous that the boundaries indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it is discovered that the unit’s working temperature is outside this temperature range, it is best to discuss with Annex A of the standard.
When the worth obtained by way of analyses is GOOD, the normal sampling interval could be maintained, requiring no additional motion.
When the worth returns a FAIR result, extra frequent sampling is recommended. It is also helpful to suppose about different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be applied.
A POOR outcome will require quick motion from the asset supervisor. This might embrace taking another pattern to confirm the outcomes from the primary evaluation. If it’s confirmed that the water content material is excessive, the oil could be filtered; this process should take away a large portion of the moisture from the oil if applied appropriately. Follow-up samples need to be taken to ensure that the moisture content material remains to be inside the required limits. The cause is that the most good 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 conditions that favour this movement. It might be found later that the oil in the water has elevated again with none obvious purpose, however the supply would be the paper in the transformer.
A visual inspection can additionally be really helpful to determine if any water may transfer into the transformer or electrical tools via leaks. This problem may be more extreme if the transformer or electrical gear is outdoors and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E< hundred.10 to 0.25>0.20
C<0.100.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine take a look at for all lessons except F and G
The acids in oils are formed as a result of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will help within the formation of the acids. Because acids are polar compounds, it’ll adversely have an effect on the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this can result in sludge formation, often across the lower parts of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily troublesome 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 suit the scenario. Future evaluation ought to embody a visual inspection of the oil for sediment and sludge.
If the result’s POOR according to the prescribed values in IEC 60422 Edition, the asset supervisor might resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever possibility may swimsuit 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< hundred.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<0.a hundred.01 to zero.30>0.03
F, GNot a routine check
This is a routine test for all courses of electrical tools, except F and G
The dielectric dissipation factor or tan delta of this check offers data concerning the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that may affect the dissipation factor embrace water, dissolved insulating resin, and paper.
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 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 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 one of the key parameters to evaluate the transformer insulation condition; this is based on the reality that DC resistance is delicate to oil degradation.
When the result’s FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of original value
This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to verify the details regarding components.
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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the growing older process in the oil and the solid insulation.
If the result is FAIR, it’s advised to high up the inhibitor stage to the prescribed level per supplier directions. It is suggested to use a field professional educated within the procedure to carry out this activity.
If the outcome obtained is POOR, the recommendation for this scenario would counsel that the tip user continues to use the oil “uninhibited,” but this may lead to more speedy degradation of each the liquid and strong insulation.
It should be noted that some transformers already have a built-in oil preservation system; that is designed to keep dissolved oxygen at ranges beneath 1000ppm. This would be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances is not required, although it would add additional safety towards 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 decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also known as steel deactivators, react with reactive steel surfaces and dissolved metals similar to copper and silver and reduce their fee of response with compounds in the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary varieties, sulfur-based and nitrogen-based. The first advised 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 might deplete more quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD results, regular sample intervals could be maintained.
With FAIR results, maintain common monitoring.
When POOR, it is suggested to take away the oil or remove the source of corrosivity from the oil by way of particular oil treatment.
I) Sediment and sludge
This isn’t a routine check.
It is advised that this check is carried out when the oil results indicate a high acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes have to be lower than zero.02% by mass to be negligible. If the results return a price of greater than 0.02% by mass, it’s instructed that or not it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial rigidity
This just isn’t 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 through the getting older course of. What this implies in sensible terms is there is more polar compound present within the oil, reducing the ability 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 tension becomes a prime quality criterion: the oil have to be modified below a predefined limit.
If results are GOOD, proceed the common sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine test.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be significant. The extent of the corrosion harm brought on by the sulfur can be so severe that it would trigger failure of the gear 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 found that rubber products utilized in transformers would possibly 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 need of corrective actions based on this institute’s risk evaluation examine. [4]
l) Particle counting and sizing
Table 3: 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 test
If there is a maximum decrease in flashpoints by 10%, the gear would possibly require further inspection. This worth may differ in different international locations.
It is advised to carry out this take a look at when an uncommon odour is observed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not determine the condition of the transformer; this is a health and security impact take a look at. PCB is hazardous to both people and the surroundings; it is important to check for PCBs after the retro fill of a transformer. It can additionally be required every time any upkeep has been carried out on the unit, and the potential of contamination is current. If PCB content exceeds the recommended limits, the appropriate motion needs to be taken.
Units with a PCB content material of greater than 50ppm require a hearth safety plan, environmental protection plan, and extra precautionary measures when upkeep is done. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear owner.
Local regulatory bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of information and interpretation, we’ll discuss this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this knowledge in accordance with international standards might be discussed in detail, forming part of the general health rating determination of the transformer.
Transformer situation monitoring is an interlaced, highly exciting subject of examine. In this text, we centered on the kinds of exams to determine the situation of the transformer, the important values, and the really helpful actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a specific date and time. This makes it possible to ensure best follow application and optimised upkeep. 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 tools – 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 power.”
6. Article initially revealed by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the industry, 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 career, notably within the analysis of take a look at knowledge. Corné has vast sensible and theoretical information of reliability upkeep programmes.

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