by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the related knowledge relating to an asset’s operation. If we interpret the info accurately, it can give us nice insight into the asset’s situation.
Frequent monitoring of the asset can lead to much less upkeep required or extra prolonged durations without any maintenance required.
It is crucial to identify the vital thing parameters which would possibly be needed to offer us a whole image of the actual status of the transformer and the action we want to take to make sure the continued reliability of the asset to achieve the utmost lifetime.
What is the info telling us?
Has the condition of the unit changed because the final maintenance period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate score for a selected period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit before we need to think about replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally necessary to identify clear objectives as part of your technique. What do you want to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it might be a lot easier to establish the required parameters.
Health indexing of property is becoming a exceptional tool in getting a clearer picture of the condition of your transformer. Test parameters carry a numerical value-adding to the entire value of the Health Index Value of the transformer. These parameter weight values had been calculated based mostly on the international requirements for mineral oils, indicating the critical values stipulated within the varied standards.
The scope of oil analysis, interpretation of the info, and important values
At the start of this section, it’s important to state that we cope with different size transformers in the trade. Transformers are divided into courses based on the kV scores of the equipment. It is as much as the reliability or asset supervisor to make use of the rules for larger tools, thereby implementing shorter increment oil analysis and electrical exams.
The maintenance engineer or supervisor wants to discover out what sort of testing would profit him in identifying downside areas within his fleet. Some of the analysis has been recognized as to routine kind exams. Still, there’s an in depth vary of tests that can help in figuring out particular problem standards throughout the system, which might not be clear by way of the standard day-to-day evaluation normally carried out.
Please see the rating courses in TABLE 1
TABLE 2 explains which oil analyses are beneficial and how often or under which circumstances.
Oil sampling
The taking of the oil pattern is among the most crucial and significant influencers within the evaluation end result. If a pattern is not taken to the prescribed procedure, then there is a vital risk that the evaluation carried out, and the results acquired, are of no use.
Care must be taken to make sure quality control procedures are utilized in every step of the sampling process as set out by worldwide standards. A good quality pattern taken by making use of the right process is important. A pattern may be contaminated by numerous components, all of which can influence the finish result of the ends in a adverse manner.
All steps concerned in pattern taking must adhere to quality management 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 pattern, the storage and handling of the pattern, 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 information. The label must be stuck 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 ought to be waterproof, and the pen used to write down on the label should be oil- and water resistant. Otherwise, some data might be lost, making it extraordinarily troublesome for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s focus on the kinds of exams to determine the transformer’s situation, the crucial values, and the really helpful actions in each case:
a) Colour and appearance
This is a routine inspection utilized to every oil pattern.
When an oil pattern arrives at the laboratory, one of the “tests” is a visible inspection of the oil pattern in a clear vessel to determine the color, turbidity, and possible particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it would indicate a high water content in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve could be incorporated into the sample. If particles are identified as carbon, it’d indicate a possible electrical fault within the unit. The DGA evaluation of the oil will confirm if that is so.
Clear oils with out contamination will point out a good condition, and no motion is beneficial.
When oils are dark or turbid, additional analysis will verify any issues. The oil analysis results may also decide the diploma and kind 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 software
G <30
This is a routine inspection.
Breakdown voltage will point out the water content or the presence of overseas particles, or each 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 must be excessive.
If the values are Good, it is strongly recommended to continue with the current sample interval action plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil by way of oil reconditioning processes. If alternative checks point out extreme getting older, the oil can be replaced with new or reclaimed oil. Another choice could 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 parts are faraway from the oil. This process removes acid and water in addition to some other compounds. Another benefit is that the oil could be re-used, and in most situations, this can be carried out without switching off the unit, which contributes to cost-saving. If doubtful – as a substitute swap off the unit throughout this treatment course of.
If the values are Poor, it is advisable to take motion as quickly as possible and not delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content can 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 test for all classes of electrical equipment, besides class G
The outcomes of this take a look at ought to always be thought of in conjunction with the breakdown energy. If it is found that the water content material is excessive and the breakdown power is low, additional action must be taken. It is really helpful that a second pattern from the identical unit is tested to confirm the outcomes.
In the case of switching equipment, the place there isn’t a paper present, the breakdown voltage is the figuring out issue.
It ought to be famous that the limits indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it is found that the unit’s working temperature is outdoors this temperature range, it is best to discuss with Annex A of the standard.
When the value obtained through analyses is GOOD, the traditional sampling interval could be maintained, requiring no further motion.
When the value returns a FAIR end result, more frequent sampling is really helpful. It is also helpful to assume about different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR result would require immediate motion from the asset supervisor. This would possibly include taking one other sample to substantiate the results from the first evaluation. If it’s confirmed that the water content is excessive, the oil can be filtered; this course of should remove a large portion of the moisture from the oil if applied appropriately. Follow-up samples have to be taken to ensure that the moisture content material continues to be throughout the required limits. The purpose is that probably the most significant 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 circumstances that favour this motion. It might be found later that the oil within the water has elevated once more with none obvious purpose, however the source could be the paper within the transformer.
A visual inspection can additionally be really helpful to determine if any water might transfer into the transformer or electrical equipment through leaks. This problem might be extra extreme if the transformer or electrical gear is exterior and not in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D< hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C<0.100.15 to 0.30>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 outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist 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 will result in sludge formation, normally around the lower parts of the transformer core. The sludge will ultimately form a semi-solid substance that’s extraordinarily troublesome to remove.
If the result is GOOD, the common sampling interval can continue.
In case of a FAIR end result, the sampling interval should be decreased to suit the state of affairs. Future evaluation should include a visual inspection of the oil for sediment and sludge.
If เกจ์แรงดัน is POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset manager could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever possibility would possibly swimsuit 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< hundred.10 to zero.20>0.20
B, C< hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to zero.30>0.03
F, GNot a routine test
This is a routine check for all courses of electrical tools, besides F and G
The dielectric dissipation factor or tan delta of this check supplies information concerning 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 factor include water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking additional parameters is beneficial.
When the result’s 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 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 amongst the key parameters to assess the transformer insulation condition; this is based on the fact that DC resistance is delicate to oil degradation.
When the result’s FAIR, extra frequent sampling and checking further parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of unique worth
This take a look at is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the details relating to 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 function of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the growing older process within the oil and the solid insulation.
If the result is FAIR, it’s advised to prime up the inhibitor level to the prescribed stage per supplier instructions. It is suggested to use a subject skilled educated within the process to carry out this activity.
If the result obtained is POOR, the advice for this situation would counsel that the top user continues to use the oil “uninhibited,” however this will likely result in extra speedy degradation of each the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges 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 underneath these circumstances is not required, though it would add further protection in opposition to 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 lower < 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 called metallic deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and scale back their price 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 types, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the creator 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 would possibly deplete extra quickly; this depletion may accelerate when the oil is un-inhibited.
With GOOD results, regular pattern intervals could be maintained.
With FAIR results, keep common monitoring.
When POOR, it is advised to remove the oil or remove the supply of corrosivity from the oil via special oil remedy.
I) Sediment and sludge
This is not a routine test.
It is suggested that this test is carried out when the oil results indicate a excessive acid value and the dissipation factor is close to the unacceptable limit.
The results must be less than zero.02% by mass to be negligible. If the results return a worth of more than zero.02% by mass, it is suggested that it be reclaimed; otherwise, an oil change is really helpful.
j) Interfacial pressure
This is not 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 take a look at
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces during the aging process. What this implies in practical phrases is there is extra polar compound current in the oil, decreasing the ability of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure becomes a excessive quality criterion: the oil should be changed beneath a predefined limit.
If results are GOOD, continue the regular sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine take a look at.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system may be vital. The extent of the corrosion injury brought on by the sulfur could be so extreme that it’d trigger failure of the equipment if not checked. The addition of a copper passivator can scale back the influence of this compound on the transformer system.
In a research by Doble, it has been found that rubber merchandise used 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 necessity of corrective actions based mostly on this institute’s threat assessment study. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured utilizing 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 would possibly require additional inspection. This value would possibly differ in different nations.
It is suggested to carry out this check when an uncommon odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to determine the condition of the transformer; it is a health and security impact take a look at. PCB is hazardous to each humans and the surroundings; it is vital to check for PCBs after the retro fill of a transformer. It can additionally be required every time any maintenance has been done on the unit, and the possibility of contamination is current. If PCB content material exceeds the beneficial limits, the suitable motion needs to be taken.
Units with a PCB content of greater than 50ppm require a fireplace safety plan, environmental safety plan, and extra precautionary measures when upkeep is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the gear owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we are going to discuss this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this information according to international requirements might be mentioned intimately, forming a half of the overall health rating determination of the transformer.
Transformer condition monitoring is an interlaced, highly thrilling field of research. In this article, we focused on the kinds of checks to determine the condition of the transformer, the important values, and the really helpful actions.
The Health Index indication makes it attainable to see the supposed reliability of a specific unit at a particular date and time. This makes it potential to make sure greatest apply application and optimised upkeep. It additionally make it simpler to draw up a maintenance plan and action 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’re and how 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 power.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the industry, having beforehand worked as laboratory manager for a major industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the start of her profession, significantly in the evaluation of test information. Corné has huge sensible and theoretical information of reliability maintenance programmes.

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