by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the related data regarding an asset’s operation. If we interpret the information accurately, it can provide us nice insight into the asset’s condition.
Frequent monitoring of the asset can outcome in less maintenance required or extra prolonged durations with none upkeep required.
It is crucial to establish the important thing parameters which are wanted to provide us an entire picture of the actual status of the transformer and the motion we need to take to make sure the continued reliability of the asset to realize the maximum lifetime.
What is the info telling us?
Has the situation of the unit modified for the explanation that final maintenance period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate rating for a specific period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit before we have to think about replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally important to establish clear targets as part of your technique. What do you wish to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the finish result and what you wish to accomplish, it would be much easier to establish the required parameters.
Health indexing of belongings is turning into a remarkable device 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 were calculated based mostly on the worldwide requirements for mineral oils, indicating the critical values stipulated in the various requirements.
The scope of oil analysis, interpretation of the info, and critical values
At the start of this part, it’s essential to state that we take care of totally different dimension transformers in the business. Transformers are divided into lessons according to the kV ratings of the gear. It is as a lot as the reliability or asset supervisor to use the rules for larger equipment, thereby implementing shorter increment oil evaluation and electrical checks.
The maintenance engineer or manager needs to find out what sort of testing would benefit him in identifying drawback areas within his fleet. Some of the evaluation has been identified as to routine sort checks. Still, there’s an in depth vary of checks that can assist in figuring out specific problem standards inside the system, which might not be clear via the standard day-to-day analysis often performed.
Please see the ranking lessons in TABLE 1
TABLE 2 explains which oil analyses are beneficial and how usually or underneath which circumstances.
Oil sampling
The taking of the oil sample is probably certainly one of the most crucial and significant influencers within the evaluation outcome. If a sample just isn’t taken to the prescribed procedure, then there is a important chance that the evaluation carried out, and the results acquired, are of no use.
Care ought to be taken to make sure high quality management procedures are utilized in every step of the sampling course of as set out by international requirements. A good quality sample taken by applying the proper process is important. A pattern could be contaminated by varied components, all of which can affect the result of the leads to a negative manner.
All steps involved in pattern taking must adhere to quality control procedures, including the container that’s used, the sampling kit, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and dealing with of the sample, after which the supply of the sample to the laboratory.
If the sample identification and sample data are handwritten, the human factor can lead to incorrect interpretation of the information. The label ought to be caught onto the container to ensure no mix-ups in sample identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to write down on the label must be oil- and waterproof. Otherwise, some data might be misplaced, making it extremely difficult for the laboratory personnel to complete the report or allocate the sample to a specific transformer.
Now, let’s focus on the forms of exams to determine the transformer’s situation, the important values, and the beneficial actions in every case:
a) Colour and appearance
This is a routine inspection utilized to each oil pattern.
When an oil sample arrives on the laboratory, one of many “tests” is a visual inspection of the oil pattern in a clear vessel to find out the color, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there is a lot of turbidity, it might point out a high water content material in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles within the drain valve might be included into the sample. If particles are identified as carbon, it’d point out a attainable electrical fault in the unit. The DGA evaluation of the oil will confirm if that’s so.
Clear oils without contamination will point out an excellent condition, and no action is beneficial.
When oils are darkish or turbid, further evaluation will affirm any issues. The oil evaluation results will also determine the diploma and kind 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 point out the water content 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 within the unit, the breakdown voltage have to be excessive.
If the values are Good, it is strongly recommended to continue with the present pattern interval motion plan.
If the values are Fair, extra frequent sampling is really helpful 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 via oil reconditioning processes. If various exams point out extreme growing older, the oil can be replaced with new or reclaimed oil. Another possibility would be to perform 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 components are faraway from the oil. This process removes acid and water as nicely as another compounds. Another advantage is that the oil may be re-used, and in most situations, this might be accomplished with out switching off the unit, which contributes to cost-saving. If doubtful – as a substitute swap off the unit throughout this therapy process.
If the values are Poor, it is advisable to take motion as quickly as attainable and not delay the upkeep process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content material could cause flashover within the unit, leading to loss of the asset.
c) Water content (mg/kg at transformer operating 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 check for all courses of electrical gear, besides class G
The outcomes of this test ought to all the time be thought-about at the side of the breakdown power. If it is found that the water content is high and the breakdown power is low, additional motion must be taken. It is recommended that a second pattern from the identical unit is examined to substantiate the results.
In the case of switching equipment, the place there is not a paper current, the breakdown voltage is the figuring out factor.
It must be noted that the limits indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it’s found that the unit’s working temperature is exterior this temperature vary, it’s best to refer to Annex A of the usual.
When the worth obtained by way of analyses is GOOD, the normal sampling interval can be maintained, requiring no further action.
When the value returns a FAIR result, extra frequent sampling is beneficial. It is also helpful to suppose about other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be carried out.
A POOR result will require immediate action from the asset manager. This might embrace taking one other sample to confirm the outcomes from the primary evaluation. If it’s confirmed that the water content is high, the oil could be filtered; this process ought to take away a large portion of the moisture from the oil if applied correctly. Follow-up samples need to be taken to ensure that the moisture content is still throughout the required limits. The cause is that the most significant portion of the water is caught up in the paper system in the transformer. This moisture will move from the paper into the oil beneath conditions that favour this motion. It may be discovered later that the oil within the water has elevated again with none obvious cause, but the source would be the paper within the transformer.
A visual inspection can be really helpful to determine if any water would possibly transfer into the transformer or electrical tools through leaks. This downside could be more severe if the transformer or electrical equipment is exterior and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D< hundred.10 to zero.15>0.15
B, E<0.100.10 to zero.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine check
This is a routine check for all lessons except F and G
The acids in oils are fashioned due to 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 will increase paper degradation. If left untreated in transformers, this can result in sludge formation, usually across the lower elements 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 regular sampling interval can continue.
In case of a FAIR end result, the sampling interval ought to be decreased to fit the state of affairs. Future analysis should include 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, the asset supervisor could decide to reclaim the oil or exchange it with new or reclaimed oil, whichever possibility would possibly go properly 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< hundred.10 to 0.20>0.20
B, C< hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E<0.a hundred.01 to zero.30>0.03
F, GNot a routine take a look at
This is a routine check for all classes of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this check provides data concerning the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities which may affect the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial. The structure of the oil is damaged, in effect that the chemical bonds between the molecules have broken 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 check
DC resistivity of the oil is doubtless certainly one of the key parameters to evaluate the transformer insulation situation; that is based mostly on the truth that DC resistance is delicate to grease degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
When the result’s 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 authentic value<40% of unique worth
This take a look at is restricted to oils with this additive.
It can be advisable to contact the oil provider to verify the small print regarding components.
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 getting older process in the oil and the strong insulation.
If the result is FAIR, it is advised to prime up the inhibitor level to the prescribed stage per provider instructions. It is suggested to make use of a subject skilled educated within the process to carry out this activity.
If the end result obtained is POOR, the advice for this situation would counsel that the top consumer continues to make use of the oil “uninhibited,” but this will likely result in more fast degradation of both the liquid and strong insulation.
It must be noted that some transformers have already got a built-in oil preservation system; that is designed to maintain dissolved oxygen at levels below 1000ppm. This would be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances is not required, though it’d add further safety towards 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 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 decreasing at >10mg/kg/year
Passivators, also identified as metallic deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and scale back their price of response with compounds within the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary sorts, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the author 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 may deplete more quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD results, common sample intervals could be maintained.
With FAIR outcomes, maintain 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.
Wacky ) Sediment and sludge
This isn’t a routine take a look at.
It is suggested that this test is performed when the oil results indicate a high acid value and the dissipation issue is near the unacceptable restrict.
The results need to be less than 0.02% by mass to be negligible. If the outcomes return a worth of greater than zero.02% by mass, it is suggested that it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial pressure
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 test
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the growing older course of. What this means in sensible terms is there may be more 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 quantity. Therefore, the interfacial pressure becomes a top quality criterion: the oil must be modified beneath a predefined limit.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t 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 can be vital. The extent of the corrosion injury caused 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 impression of this compound on the transformer system.
In a study by Doble, it has been discovered that rubber products utilized in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration would possibly 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 ranges (particles) encountered on energy transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine check
If there’s a most decrease in flashpoints by 10%, the gear would possibly require further inspection. This worth may differ in different international locations.
It is suggested to carry out this check when an uncommon odour is observed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not decide the condition of the transformer; this is a well being and safety impression test. PCB is hazardous to both people and the environment; it is important to test for PCBs after the retro fill of a transformer. It can also be required every time any maintenance has been accomplished on the unit, and the potential of contamination is current. If PCB content material exceeds the recommended limits, the suitable motion must be taken.
Units with a PCB content material of greater than 50ppm require a fire safety plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear proprietor.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of information and interpretation, we are going to discuss this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this knowledge according to worldwide standards might be discussed intimately, forming a part of the general health ranking willpower of the transformer.
Transformer condition monitoring is an interlaced, highly thrilling subject of research. In this article, we focused on the kinds of tests to find out the situation of the transformer, the critical values, and the recommended actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a particular date and time. This makes it attainable to ensure best follow software and optimised upkeep. It also make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 2013-01 International Standard (Mineral insulating oils in electrical equipment – 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 published by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience within the business, having beforehand labored as laboratory manager for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the beginning of her profession, notably within the evaluation of test knowledge. Corné has huge sensible and theoretical knowledge of reliability maintenance programmes.

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