by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related data regarding an asset’s operation. If we interpret the info accurately, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset may find yourself in much less maintenance required or extra extended durations with none upkeep required.
It is essential to determine the key parameters which would possibly be wanted to give us a complete image of the particular status of the transformer and the motion we need to take to make sure the continued reliability of the asset to achieve the utmost lifetime.
What is the info telling us?
Has the situation of the unit changed because the final maintenance period?
Is it secure to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate score for a specific 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 consider replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally essential to establish clear goals as part of your strategy. What do you wish to achieve by implementing this situation 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 identify the required parameters.
Health indexing of property is changing into a exceptional software in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the total value of the Health Index Value of the transformer. These parameter weight values have been calculated based on the international requirements for mineral oils, indicating the crucial values stipulated in the numerous requirements.
The scope of oil analysis, interpretation of the information, and significant values
At the beginning of this section, it is important to state that we cope with totally different dimension transformers within the business. Transformers are divided into courses in accordance with the kV rankings of the equipment. It is up to the reliability or asset manager to make use of the rules for bigger tools, thereby implementing shorter increment oil analysis and electrical checks.
The upkeep engineer or supervisor wants to determine what type of testing would benefit him in figuring out downside areas within his fleet. Some of the evaluation has been recognized as to routine type tests. Still, there’s an in depth vary of checks that can assist in identifying particular downside standards inside the system, which might not be clear by way of the typical day-to-day evaluation usually performed.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way often or underneath which circumstances.
Oil sampling
The taking of the oil sample is probably certainly one of the most crucial and critical influencers in the evaluation outcome. If a pattern is not taken to the prescribed process, then there is a significant risk that the evaluation carried out, and the outcomes acquired, are of no use.
Care should be taken to make sure high 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 making use of the correct procedure is essential. A pattern could be contaminated by various elements, all of which might influence the outcome of the ends in a negative manner.
All steps concerned in sample taking must adhere to high quality control procedures, together with the container that is used, the sampling kit, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the sample, after which the delivery of the sample to the laboratory.
If the pattern identification and sample information are handwritten, the human issue can result in incorrect interpretation of the data. The label ought to be stuck 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 should be waterproof, and the pen used to write down on the label must be oil- and water resistant. Otherwise, some knowledge could be misplaced, making it extraordinarily difficult for the laboratory personnel to finish the report or allocate the sample to a particular transformer.
Now, let’s talk about the forms of tests to determine the transformer’s situation, the critical values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection utilized to every oil sample.
When an oil sample arrives at the laboratory, one of the “tests” is a visible inspection of the oil sample in a transparent vessel to determine the color, turbidity, and potential particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there might be plenty of turbidity, it’d indicate a high water content material 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 sample. If particles are recognized as carbon, it’d indicate a attainable electrical fault in the unit. The DGA evaluation of the oil will verify if so.
Clear oils without contamination will point out a good condition, and no action is recommended.
When oils are darkish or turbid, further analysis will confirm any problems. The oil analysis outcomes may also decide the degree 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 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 or the presence of foreign particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to continue with the present pattern interval action plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter outcomes 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 tests indicate severe getting older, the oil could be changed with new or reclaimed oil. Another choice can be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water as well as another 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 – as a substitute switch off the unit throughout this treatment course of.
If the values are Poor, it is advisable to take motion as soon as potential and not delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content material can cause flashover in the unit, leading to 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 test
This is a routine check for all classes of electrical tools, besides class G
The outcomes of this test ought to all the time be thought-about in conjunction with the breakdown power. If it’s discovered that the water content is high and the breakdown power is low, further action needs to be taken. It is beneficial that a second pattern from the same unit is tested to confirm the outcomes.
In the case of switching tools, the place there isn’t a paper current, the breakdown voltage is the figuring out factor.
It must be noted that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it’s discovered that the unit’s working temperature is outside this temperature vary, it is best to check with Annex A of the standard.
When the worth obtained through analyses is GOOD, the traditional sampling interval can be maintained, requiring no further action.
When the worth returns a FAIR end result, more frequent sampling is beneficial. It is also helpful to think about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be applied.
A POOR result will require quick motion from the asset manager. This would possibly embody taking one other pattern to substantiate the results from the first analysis. If it’s confirmed that the water content is excessive, the oil can be filtered; this course of should take away a big portion of the moisture from the oil if applied accurately. Follow-up samples must be taken to make sure that the moisture content remains to be inside the required limits. The reason is that probably the most significant slice of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under situations that favour this movement. It might be found later that the oil in the water has elevated once more with none obvious purpose, however the source can be the paper in the transformer.
A visible inspection can also be beneficial to find out if any water would possibly transfer into the transformer or electrical equipment through leaks. This problem might be extra severe if the transformer or electrical equipment is outdoors and not in a covered area.
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 zero.25>0.20
C< hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine test for all lessons except F and G
The acids in oils are shaped because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist within the formation of the acids. Because ตัววัดแรงดัน are polar compounds, it’ll adversely have an result on the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this can lead to sludge formation, usually around the decrease parts of the transformer core. The sludge will ultimately type a semi-solid substance that’s extremely troublesome to take away.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR end result, the sampling interval must be decreased to suit the situation. Future analysis should embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR in accordance with the prescribed values in IEC 60422 Edition 4.0, the asset manager may resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever choice might swimsuit 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 0.50>0.50
D<0.010.01 to 0.03>0.03
E< hundred.01 to zero.30>0.03
F, GNot a routine test
This is a routine test for all classes of electrical tools, besides F and G
The dielectric dissipation factor or tan delta of this take a look at offers data 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, resulting in section displacement and dielectric losses. Other impurities which may affect the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is really helpful.
When the result’s POOR, reclamation or an oil change is beneficial. The construction of the oil is damaged, 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 check
DC resistivity of the oil is among the key parameters to evaluate the transformer insulation condition; that is based on the reality that DC resistance is sensitive to oil degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of unique worth
This check is restricted to oils with this additive.
It can be advisable to contact the oil provider to confirm the primary points concerning additives.
The two most common oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a couple of,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to stop oxygen from reacting with the oil. This considerably slows the getting older process within the oil and the solid insulation.
If the result’s FAIR, it is advised to prime up the inhibitor stage to the prescribed level per provider directions. It is advised to make use of a field professional educated in the procedure to perform this activity.
If the outcome obtained is POOR, the recommendation for this situation would suggest that the end user continues to use the oil “uninhibited,” but this will result in extra speedy degradation of each the liquid and strong insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges under 1000ppm. This can be in the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils under these circumstances isn’t required, although it’d add further safety 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 decrease < 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 known as steel deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and cut back their rate of response with compounds within the oil. This contains oxidation reactions with organic 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 author 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 more quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD outcomes, common sample intervals may be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it’s suggested to take away the oil or remove the source of corrosivity from the oil through particular oil therapy.
I) Sediment and sludge
This isn’t a routine take a look at.
It is advised that this test is performed when the oil results indicate a high acid worth and the dissipation factor is close to the unacceptable restrict.
The results have to be less than 0.02% by mass to be negligible. If the outcomes return a worth of greater than 0.02% by mass, it is advised that it’s reclaimed; otherwise, an oil change is really helpful.
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 check
F, GNot Applicable
The interfacial tension between transformer oil and water reduces in the course of the aging process. What this means in practical phrases is there is more polar compound present within the oil, reducing the ability of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure turns into a high quality criterion: the oil have to be changed below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine take a look at.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system may be significant. The extent of the corrosion injury attributable to the sulfur can be so severe that it’d cause 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 used in transformers may 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 necessity of corrective actions based mostly on this institute’s threat assessment examine. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine check
If there is a most decrease in flashpoints by 10%, the equipment would possibly require additional inspection. This value would possibly differ in different nations.
It is suggested to carry out this test when an unusual 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 situation of the transformer; it is a well being and safety impression take a look at. PCB is hazardous to both humans and the environment; it is vital to test for PCBs after the retro fill of a transformer. It can be required every time any upkeep has been done on the unit, and the potential of contamination is present. If PCB content material exceeds the beneficial limits, the suitable motion must be taken.
Units with a PCB content material of more than 50ppm require a hearth safety plan, environmental safety plan, and further precautionary measures when maintenance is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear proprietor.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of knowledge and interpretation, we’ll focus on this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this data based on worldwide requirements will be mentioned in detail, forming a part of the general health score willpower of the transformer.
Transformer condition monitoring is an interlaced, highly exciting subject of examine. In this text, we focused on the kinds of tests to determine the situation of the transformer, the critical values, and the beneficial actions.
The Health Index indication makes it potential to see the supposed reliability of a particular unit at a specific date and time. This makes it potential to make sure greatest practice software and optimised upkeep. It additionally make it easier to attract up a upkeep plan and action plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical tools – supervision and upkeep 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 published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise in the industry, having previously labored as laboratory manager for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her profession, particularly within the analysis of test data. Corné has vast sensible and theoretical information of reliability upkeep programmes.

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