by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and analysis of the relevant knowledge regarding an asset’s operation. If we interpret the data correctly, it may give us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to much less upkeep required or more extended durations with none upkeep required.
It is essential to identify the key parameters which might be needed to give us a whole picture of the actual standing of the transformer and the action we want to take to ensure the continued reliability of the asset to attain the maximum lifetime.
What is the data telling us?
Has the condition of the unit modified since the last maintenance period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it safe to load the unit above the nameplate ranking 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 earlier than we need to think about replacement?
Are the recognized issues of a recurring nature?
Effective situation monitoring define
It is vitally important to determine clear objectives as a half of your strategy. What do you want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the end result and what you want to accomplish, it might be much easier to determine the required parameters.
Health indexing of property is becoming a exceptional software in getting a clearer picture 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 on the international standards for mineral oils, indicating the crucial values stipulated within the varied standards.
The scope of oil evaluation, interpretation of the info, and important values
At the start of this section, it is essential to state that we cope with completely different measurement transformers within the industry. Transformers are divided into classes in accordance with the kV scores of the tools. It is up to the reliability or asset supervisor to use the rules for larger tools, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or manager wants to find out what type of testing would benefit him in identifying downside areas inside his fleet. Some of the evaluation has been recognized as to routine kind tests. Still, there’s an extensive vary of exams that can help in identifying particular problem criteria within the system, which could not be clear via the typical day-to-day evaluation usually carried out.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are really helpful and how typically or under which circumstances.
Oil sampling
The taking of the oil sample is likely one of the most vital and critical influencers within the evaluation consequence. If a sample just isn’t taken to the prescribed process, then there is a significant possibility that the evaluation carried out, and the results acquired, are of no use.
Care ought to be taken to ensure high quality control procedures are utilized in every step of the sampling process as set out by worldwide standards. A good high quality pattern taken by making use of the proper procedure is important. A sample can be contaminated by varied factors, all of which might influence the result of the ends in a adverse method.
All steps concerned in pattern taking must adhere to quality control procedures, including the container that is used, the sampling equipment, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and dealing with of the pattern, after which the supply of the pattern to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human factor can lead to incorrect interpretation of the information. The label must be stuck onto the container to make sure 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 put in writing on the label ought to be oil- and water-resistant. Otherwise, some information may be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s focus on the forms of exams to find out the transformer’s situation, the crucial values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection applied to each oil pattern.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a transparent vessel to find out the colour, turbidity, and potential particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there’s lots of turbidity, it would indicate a high water content material within the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve might be incorporated into the sample. If particles are identified as carbon, it might indicate a potential electrical fault within the unit. The DGA analysis of the oil will verify if that’s so.
Clear oils without contamination will point out a great situation, and no action is recommended.
When oils are darkish or turbid, further evaluation will confirm any problems. The oil analysis results may even determine the degree 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 level application<40 kV for OLTC in delta or line-end utility
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 should be high.
If the values are Good, it is recommended to continue with the current sample interval motion plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If different checks point out severe getting older, the oil could be replaced with new or reclaimed oil. Another option can be to perform on-site oil reclamation using 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 can be accomplished with out switching off the unit, which contributes to cost-saving. If in doubt – as an alternative switch off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as soon as potential and never delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content may cause flashover within 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 take a look at for all courses of electrical equipment, except class G
The results of this check should always be considered in conjunction with the breakdown strength. If it’s discovered that the water content is high and the breakdown strength is low, additional action needs to be taken. It is recommended that a second pattern from the identical unit is tested to substantiate the outcomes.
In the case of switching tools, the place there is not a paper current, the breakdown voltage is the figuring out factor.
It must be famous that the boundaries indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it’s found that the unit’s working temperature is exterior this temperature vary, it is best to refer to Annex A of the usual.
When the value obtained through analyses is GOOD, the conventional sampling interval may be maintained, requiring no further action.
When the value returns a FAIR end result, extra frequent sampling is recommended. It is also useful to consider other parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the action to be applied.
A POOR outcome would require instant motion from the asset supervisor. This might include taking one other sample to confirm the outcomes from the primary analysis. If it is confirmed that the water content is excessive, the oil could be filtered; this process should take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples have to be taken to make certain that the moisture content remains to be within the required limits. The cause is that the most significant slice of the water is caught up within the paper system in the transformer. This moisture will transfer from the paper into the oil underneath circumstances that favour this motion. It could be discovered later that the oil in the water has increased again with none apparent cause, however the supply would be the paper within the transformer.
A visual inspection is also recommended to determine if any water may transfer into the transformer or electrical gear through leaks. This downside might be more extreme if the transformer or electrical equipment is outdoors and never in a coated 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<0.100.10 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 formed as a outcome 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 will adversely have an effect on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this can result in sludge formation, often around the lower parts of the transformer core. The sludge will finally type a semi-solid substance that is extremely troublesome to take away.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR result, the sampling interval must be decreased to suit the situation. Future analysis should include a visible 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 could resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever choice would possibly go nicely with their necessities one of the best.
e) Dielectric Dissipation issue 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<0.100.10 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 check
This is a routine take a look at for all classes of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this take a look at supplies info 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, leading to section displacement and dielectric losses. Other impurities that may influence the dissipation factor include water, dissolved insulating resin, and paper.
When the result is 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 damaged, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the recommended 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 test
DC resistivity of the oil is certainly one of the key parameters to assess the transformer insulation situation; this is based mostly on the fact that DC resistance is delicate to oil degradation.
When the result is FAIR, more frequent sampling and checking extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original value
This test is restricted to oils with this additive.
It can be advisable to contact the oil provider to verify the small print 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 function of the inhibitor is to stop oxygen from reacting with the oil. This significantly slows the growing older course of within the oil and the strong insulation.
If the result is FAIR, it is suggested to high up the inhibitor degree to the prescribed level per provider directions. It is suggested to use a subject professional skilled within the procedure to perform this task.
If the end result obtained is POOR, the advice for this scenario would suggest that the top consumer continues to use the oil “uninhibited,” however this will likely result in more rapid degradation of each the liquid and strong insulation.
It ought to be noted that some transformers already have a built-in oil preservation system; this is designed to keep 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 outfitted with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, though it would add extra 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 steady, (rate of decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called steel deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and cut back their price of response with compounds in the oil. This consists of oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two basic varieties, sulfur-based and nitrogen-based. The first advised 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 might deplete more rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD results, regular pattern intervals could be maintained.
With FAIR outcomes, maintain regular monitoring.
When POOR, it is advised to take away the oil or take away the supply of corrosivity from the oil through special oil therapy.
I) Sediment and sludge
This is not a routine check.
It is advised that this take a look at is carried out when the oil outcomes indicate a excessive acid value and the dissipation issue is near the unacceptable limit.
The outcomes must be less than 0.02% by mass to be negligible. If the outcomes return a worth of more than zero.02% by mass, it is advised that it be reclaimed; otherwise, an oil change is really helpful.
j) Interfacial tension
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 test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces through the getting older process. What this implies in sensible phrases is there might be more polar compound present within the oil, decreasing the power of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, เกจวัดแรงดันน้ำราคา turns into a high quality criterion: the oil should be changed beneath a predefined restrict.
If results are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system may be vital. The extent of the corrosion damage brought on by the sulfur may be so severe that it’d trigger failure of the tools if not checked. The addition of a copper passivator can reduce the impact of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber merchandise utilized in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted 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 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 utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there’s a maximum decrease in flashpoints by 10%, the gear might require further inspection. This value may differ in several international locations.
It is suggested to carry out this test when an uncommon odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not determine the situation of the transformer; this can be a health and safety influence test. PCB is hazardous to both people and the environment; it’s critical to test for PCBs after the retro fill of a transformer. It is also required each time any maintenance has been done on the unit, and the potential of contamination is present. If PCB content exceeds the beneficial limits, the appropriate action must be taken.
Units with a PCB content material of more than 50ppm require a fire safety plan, environmental safety plan, and additional precautionary measures when upkeep is finished. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment owner.
Local regulatory our bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots 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 information based on international standards shall be mentioned intimately, forming a part of the general health rating willpower of the transformer.
Transformer situation monitoring is an interlaced, highly exciting subject of research. In this article, we targeted on the types of tests to determine the situation of the transformer, the crucial 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 possible to make sure greatest apply utility and optimised maintenance. It additionally make it simpler to attract up a upkeep plan and motion plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical equipment – supervision and upkeep guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
3. 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 revealed by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise in the trade, having previously labored as laboratory supervisor for a significant industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her career, notably in the evaluation of take a look at data. Corné has huge practical and theoretical knowledge of reliability maintenance programmes.

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