by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the related information regarding an asset’s operation. If we interpret the information appropriately, it can provide us great perception into the asset’s situation.
Frequent monitoring of the asset can lead to less upkeep required or more extended periods without any upkeep required.
It is crucial to identify the key parameters that are wanted to provide us a whole image of the particular status of the transformer and the action we have to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the data telling us?
Has the situation of the unit changed since the final maintenance period?
Is it protected to operate the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate rating for a selected period?
Are we required to implement motion to make sure the continued reliability of the unit?
How long can we use the unit earlier than we need to think about replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring define
It is vitally important to establish clear targets as part of your technique. What do you wish to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the result and what you wish to accomplish, it might be a lot simpler to determine the required parameters.
Health indexing of property is changing into a remarkable 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 have been calculated based mostly on the worldwide requirements for mineral oils, indicating the crucial values stipulated in the numerous standards.
The scope of oil evaluation, interpretation of the data, and critical values
At the beginning of this part, it’s essential to state that we deal with totally different size transformers within the business. Transformers are divided into courses based on the kV rankings of the tools. It is as a lot as the reliability or asset supervisor to make use of the rules for larger equipment, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or manager needs to find out what kind of testing would profit him in figuring out drawback areas inside his fleet. Some of the analysis has been recognized as to routine kind exams. Still, there’s an intensive vary of tests that may help in figuring out specific drawback criteria throughout the system, which could not be clear by way of the typical day-to-day evaluation often carried out.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way usually or underneath which circumstances.
Oil sampling
The taking of the oil pattern is certainly one of the most important and important influencers within the analysis consequence. If a sample is not taken to the prescribed procedure, then there’s a vital chance that the evaluation performed, and the results acquired, are of no use.
Care ought to be taken to ensure quality control procedures are utilized in each step of the sampling process as set out by worldwide standards. A good high quality sample taken by applying the right procedure is essential. A sample may be contaminated by numerous factors, all of which may affect the outcome of the results in a negative method.
All steps involved 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 sample, 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 data are handwritten, the human factor can result in incorrect interpretation of the information. 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 should be waterproof, and the pen used to write on the label ought to be oil- and water resistant. Otherwise, some knowledge might be lost, making it extremely difficult for the laboratory personnel to finish the report or allocate the pattern to a selected transformer.
Now, let’s talk about the forms of exams to find out the transformer’s condition, the important values, and the recommended actions in each case:
a) Colour and look
This is a routine inspection utilized to every 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 discover out the colour, turbidity, and potential particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there may 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 in the drain valve could be included into the pattern. If particles are recognized as carbon, it would point out a attainable electrical fault in the unit. The DGA evaluation of the oil will affirm if so.
Clear oils with out contamination will point out a good situation, and no action is really helpful.
When oils are dark or turbid, additional analysis will confirm any issues. 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 application
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of international particles, or both 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 must be high.
If the values are Good, it is strongly recommended to proceed with the present pattern interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with different parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil by way of oil reconditioning processes. If alternative exams indicate extreme getting older, the oil could be changed with new or reclaimed oil. Another possibility can be to carry out 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 parts are removed from the oil. This course of removes acid and water in addition to some other 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 doubtful – instead swap off the unit during this treatment process.
If the values are Poor, it is 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 excessive water content material may cause flashover in the unit, resulting in loss 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 equipment, except class G
The results of this check should all the time be considered at the side of the breakdown strength. If it is found that the water content is high and the breakdown energy is low, additional motion needs to be taken. It is beneficial that a second sample from the identical unit is tested to verify the results.
In the case of switching tools, the place there is no paper current, the breakdown voltage is the determining factor.
It ought to be famous that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it is found that the unit’s operating temperature is outdoors this temperature range, it is best to refer to Annex A of the standard.
When the value obtained via analyses is GOOD, the traditional sampling interval may be maintained, requiring no additional action.
When the value returns a FAIR end result, extra frequent sampling is recommended. It can also be useful to contemplate other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the motion to be carried out.
A POOR end result will require immediate action from the asset supervisor. This may embody taking another sample to substantiate the outcomes from the first analysis. If it is confirmed that the water content is excessive, the oil can be filtered; this process should take away a big portion of the moisture from the oil if utilized appropriately. Follow-up samples must be taken to make sure that the moisture content material continues to be 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 transfer from the paper into the oil under conditions that favour this motion. It might be discovered later that the oil within the water has increased once more with none apparent reason, however the source can be the paper within the transformer.
A visible inspection is also really helpful to discover out if any water would possibly transfer into the transformer or electrical gear through leaks. This drawback might be more 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<0.100.15 to zero.30>0.30
F, G Not a routine test
This is a routine test 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 will adversely have an effect on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this could result in sludge formation, normally across the lower parts of the transformer core. The sludge will eventually type a semi-solid substance that is extraordinarily tough to take away.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval ought to be decreased to suit the state of affairs. Future evaluation ought to embrace a visible 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 could resolve to reclaim the oil or exchange it with new or reclaimed oil, whichever choice would possibly go nicely with 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 0.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to 0.30>0.03
F, GNot a routine check
This is a routine test for all lessons of electrical gear, besides F and G
The dielectric dissipation factor or tan delta of this test supplies info regarding the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities which may influence the dissipation factor embody water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking additional parameters is beneficial.
When the result’s POOR, reclamation or an oil change is really helpful. The structure of the oil is damaged, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the really helpful 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 considered one of the key parameters to evaluate the transformer insulation situation; this is based mostly on the fact that DC resistance is sensitive to oil degradation.
When the result’s FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of unique value
This take a look at is restricted to oils with this additive.
It would be advisable to contact the oil provider 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 a pair of,6-di-tertiary butyl-phenol (DBP). The objective of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the getting older process within the oil and the strong insulation.
If the result’s FAIR, it’s advised to prime up the inhibitor stage to the prescribed stage per provider instructions. It is advised to make use of a area skilled educated within the process to perform this process.
If the outcome obtained is POOR, the recommendation for this scenario would suggest that the end consumer continues to use the oil “uninhibited,” but this will likely lead to more speedy degradation of both the liquid and strong insulation.
It should be noted that some transformers have already got a built-in oil preservation system; that is designed to maintain dissolved oxygen at levels under 1000ppm. This would 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 underneath these circumstances is not required, though it might add extra protection 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 lower of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and scale back their price of response with compounds in the oil. This consists of oxidation reactions with organic compounds and reactions with corrosive sulfur. เกจวัดแรงดันถังออกซิเจน are composed of two fundamental types, 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 may deplete more quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals can be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it is advised to remove the oil or remove the source of corrosivity from the oil by way of particular oil therapy.
I) Sediment and sludge
This is not a routine take a look at.
It is suggested that this test is carried out when the oil results indicate a high acid value and the dissipation issue is near the unacceptable limit.
The results must be less than zero.02% by mass to be negligible. If the results return a value of greater than zero.02% by mass, it is advised that or not it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial tension
This is not a routine take a look at
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to twenty-eight 20 to 25<22 <20
ENot a routine check
F, GNot Applicable
The interfacial tension between transformer oil and water reduces through the aging course of. What this implies in practical terms is there’s extra polar compound current within the oil, decreasing the power of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, the interfacial tension becomes a 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 results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine test.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be important. The extent of the corrosion damage caused by the sulfur may be so severe that it might trigger failure of the gear if not checked. The addition of a copper passivator can reduce the influence of this compound on the transformer system.
In a research 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 accredited for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based mostly on this institute’s threat evaluation examine. [4]
l) Particle counting and sizing
Table 3: 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 take a look at
If there’s a maximum decrease in flashpoints by 10%, the gear may require additional inspection. This value would possibly differ in several nations.
It is advised to perform this test when an unusual odour is noticed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not determine the condition of the transformer; it is a health and security impression check. 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 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 beneficial limits, the appropriate motion must be taken.
Units with a PCB content material of more than 50ppm require a fireplace safety plan, environmental safety plan, and additional precautionary measures when maintenance is completed. 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 bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we’ll focus on this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data in accordance with worldwide requirements will be mentioned intimately, forming part of the general well being rating dedication of the transformer.
Transformer condition monitoring is an interlaced, extremely exciting subject of study. In this text, we centered on the forms of exams to determine the condition of the transformer, the crucial values, and the beneficial 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 finest apply utility and optimised upkeep. It also make it easier to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition four.0 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
three. Passivators, what they are 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 marketing consultant. She has 20+ years’ experience within the industry, having beforehand labored as laboratory supervisor for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the start of her career, particularly within the analysis of test information. Corné has huge practical and theoretical information of reliability upkeep programmes.

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