by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the relevant information concerning an asset’s operation. If we interpret the info accurately, it can provide us great perception into the asset’s situation.
Frequent monitoring of the asset can result in much less maintenance required or extra prolonged periods without any maintenance required.
It is crucial to determine the vital thing parameters which are wanted to give us an entire picture of the particular standing of the transformer and the motion we need 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 changed because the last maintenance period?
Is it protected to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate ranking for a particular period?
Are we required to implement action to make sure the continued reliability of the unit?
How long can we use the unit earlier than we need to contemplate replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring outline
It is vitally important to establish clear objectives as part of your strategy. What do you want to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the finish result and what you want to accomplish, it might be a lot easier to determine the required parameters.
Health indexing of belongings is becoming a exceptional device in getting a clearer picture of the situation 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 had been calculated based mostly on the worldwide standards for mineral oils, indicating the important values stipulated within the various standards.
The scope of oil evaluation, interpretation of the data, and critical values
At the start of this part, it’s essential to state that we deal with completely different measurement transformers in the industry. Transformers are divided into lessons based on the kV scores of the tools. It is up to the reliability or asset manager to make use of the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or manager wants to discover out what sort of testing would profit him in figuring out downside areas inside his fleet. Some of the analysis has been identified as to routine type tests. Still, there is an in depth vary of tests that can help in figuring out specific problem criteria within the system, which could not be clear via the everyday day-to-day analysis usually carried out.
Please see the score lessons in TABLE 1
TABLE 2 explains which oil analyses are beneficial and how typically or under which circumstances.
Oil sampling
The taking of the oil sample is one of the most crucial and significant influencers within the evaluation outcome. If a sample is not taken to the prescribed procedure, then there’s a vital chance that the evaluation carried out, and the outcomes acquired, are of no use.
Care ought to be taken to ensure quality management procedures are utilized in each step of the sampling process as set out by worldwide standards. A good high quality pattern taken by applying the correct process is important. A sample can be contaminated by various components, all of which can influence the result of the results in a unfavorable manner.
All steps involved in pattern taking should adhere to quality control procedures, together with the container that is used, the sampling package, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the sample, and then the supply of the sample to the laboratory.
If the sample identification and pattern data are handwritten, the human factor can result in 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 accomplished in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write on the label ought to be oil- and water resistant. Otherwise, some information might 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 types of checks to find out the transformer’s situation, the critical values, and the really helpful actions in each case:
a) Colour and appearance
This is a routine inspection utilized to each oil sample.
When an oil pattern arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to determine the colour, turbidity, and possible particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there’s lots of turbidity, it’d point out a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles in the drain valve could be integrated into the pattern. If particles are recognized as carbon, it would point out a possible electrical fault within the unit. The DGA analysis of the oil will affirm if this is the case.
Clear oils without contamination will indicate an excellent condition, and no action is really helpful.
When oils are dark or turbid, additional analysis will confirm any problems. The oil evaluation outcomes may even determine the degree and sort 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 utility
G <30
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of international particles, or each in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage should be high.
If the values are Good, it is suggested to continue with the present pattern interval action plan.
If the values are Fair, extra 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 recommended to recondition the oil by way of oil reconditioning processes. If alternative checks indicate severe growing older, the oil may be changed with new or reclaimed oil. Another option would be to carry out 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 parts are faraway from the oil. This process removes acid and water as well as another compounds. Another advantage is that the oil may be re-used, and in most conditions, this can be accomplished with out switching off the unit, which contributes to cost-saving. If doubtful – as a substitute swap off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as quickly as attainable and not delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content material could 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 test
This is a routine check for all lessons of electrical equipment, except class G
The outcomes of this check should all the time be thought of along side the breakdown strength. If it’s discovered that the water content is excessive and the breakdown energy is low, further action needs to be taken. It is beneficial that a second sample from the identical unit is tested to confirm the results.
In the case of switching equipment, the place there is not a paper present, the breakdown voltage is the figuring out issue.
It must be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between 40 and 70 °C. If it’s found that the unit’s working temperature is outdoors this temperature vary, it is best to discuss with Annex A of the standard.
When the worth obtained via analyses is GOOD, the conventional sampling interval may be maintained, requiring no additional action.
When the worth returns a FAIR result, extra frequent sampling is beneficial. It can also be useful to suppose about different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the action to be carried out.
A POOR end result would require quick motion from the asset manager. This may include taking one other sample to confirm the outcomes from the first evaluation. If it is confirmed that the water content material is excessive, the oil could be filtered; this course of should take away a large portion of the moisture from the oil if utilized correctly. Follow-up Odd must be taken to make certain that the moisture content is still within the required limits. The reason is that the most significant portion of the water is caught up in the paper system in the transformer. This moisture will transfer from the paper into the oil underneath situations that favour this movement. It might be discovered later that the oil in the water has elevated once more with none obvious cause, but the source can be the paper within the transformer.
A visual inspection can additionally be really helpful to find out if any water would possibly move into the transformer or electrical gear by way of leaks. This drawback may be more severe if the transformer or electrical tools is exterior and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.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 check for all courses besides F and G
The acids in oils are fashioned as a end result of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist in the formation of the acids. Because acids are polar compounds, it’ll adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this can result in sludge formation, normally around the decrease components of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily difficult to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to suit the scenario. Future evaluation should embody a visible inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition 4.0, the asset manager might decide to reclaim the oil or substitute it with new or reclaimed oil, whichever option may go nicely with their requirements 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<0.a hundred.10 to zero.20>0.20
B, C<0.100.10 to zero.50>0.50
D<0.010.01 to zero.03>0.03
E< hundred.01 to 0.30>0.03
F, GNot a routine take a look at
This is a routine take a look at for all courses of electrical tools, besides F and G
The dielectric dissipation factor or tan delta of this take a look at supplies info concerning the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating material.
When oil ages, we have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that might influence the dissipation issue include water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking extra parameters is recommended.
When Uncomplicated is POOR, reclamation or an oil change is really helpful. The construction 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 check
DC resistivity of the oil is doubtless considered one of the key parameters to evaluate the transformer insulation situation; this is primarily based on the truth that DC resistance is delicate to oil degradation.
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 beneficial.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of authentic value
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil provider to confirm the small print relating to components.
The two most typical 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 prevent oxygen from reacting with the oil. This significantly slows the getting older course of within the oil and the solid insulation.
If the result’s FAIR, it’s suggested to high up the inhibitor stage to the prescribed level per supplier directions. It is suggested to make use of a area professional skilled within the procedure to perform this process.
If the end result obtained is POOR, the recommendation for this scenario would counsel that the tip consumer continues to use the oil “uninhibited,” but this will lead to more fast degradation of each the liquid and stable 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 levels below 1000ppm. This could 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 beneath these circumstances isn’t required, although it would add extra 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 lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also referred to as steel deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and cut back their rate of reaction with compounds within the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary sorts, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the creator 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 quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD results, regular pattern intervals may be maintained.
With FAIR results, maintain regular monitoring.
When POOR, it’s suggested to remove the oil or take away the source of corrosivity from the oil through particular oil therapy.
I) Sediment and sludge
This isn’t a routine check.
It is advised that this test is carried out when the oil outcomes indicate a excessive acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes must be lower than zero.02% by mass to be negligible. If the results return a worth of more than zero.02% by mass, it’s instructed that it be reclaimed; otherwise, an oil change is recommended.
j) Interfacial tension
This is not a routine test
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 through the growing older course of. What this implies in sensible phrases is there is more polar compound present within the oil, lowering the ability of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial rigidity turns into a high quality criterion: the oil must be changed below a predefined limit.
If outcomes are GOOD, continue the common sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine test.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be significant. The extent of the corrosion damage attributable to the sulfur could be so severe that it might trigger failure of the gear if not checked. The addition of a copper passivator can scale back the impact of this compound on the transformer system.
In a examine by Doble, it has been found that rubber merchandise 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 necessity of corrective actions primarily based on this institute’s risk evaluation research. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine test
If there is a maximum lower in flashpoints by 10%, the equipment might require further inspection. This value would possibly differ in different international locations.
It is advised to perform this take a look at when an uncommon odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not determine the situation of the transformer; this is a well being and security influence take a look at. PCB is hazardous to each humans and the environment; it is important to test for PCBs after the retro fill of a transformer. It is also required every time any maintenance has been done on the unit, and the chance of contamination is current. If PCB content exceeds the recommended limits, the appropriate motion must be taken.
Units with a PCB content of more than 50ppm require a fireplace safety plan, environmental protection plan, and further 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 tools proprietor.
Local regulatory our bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of knowledge and interpretation, we are going to discuss this phenomenon in part II of the article. The limits for the totally different gases and the interpretation of this data in accordance with international requirements will be mentioned intimately, forming a part of the general health rating dedication of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling subject of study. In this text, we targeted on the forms of checks to discover 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 particular unit at a specific date and time. This makes it possible to make sure finest practice utility and optimised maintenance. It additionally make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 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 energy.”
6. Article initially published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience in the business, having beforehand labored as laboratory manager for a significant industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the beginning of her career, notably within the analysis of take a look at information. Corné has huge sensible and theoretical information of reliability maintenance programmes.

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