by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related knowledge regarding an asset’s operation. If we interpret the information appropriately, it may give us great perception into the asset’s situation.
Frequent monitoring of the asset can lead to less upkeep required or more prolonged intervals without any upkeep required.
It is essential to determine the key parameters that are needed to offer us an entire picture of the particular standing of the transformer and the motion we have to take to ensure the continued reliability of the asset to achieve the utmost lifetime.
What is the info telling us?
Has the condition of the unit modified for the rationale that last maintenance period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate ranking for a particular period?
Are we required to implement motion to make sure the continued reliability of the unit?
How lengthy can we use the unit earlier than we want to contemplate replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring outline
It is vitally necessary to establish clear objectives as part of your technique. What do you want to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it might be a lot easier to determine the required parameters.
Health indexing of belongings is turning into a remarkable tool in getting a clearer picture of the condition 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 were calculated primarily based on the international standards for mineral oils, indicating the important values stipulated in the various standards.
The scope of oil evaluation, interpretation of the information, and significant values
At the start of this part, it’s essential to state that we take care of totally different dimension transformers in the industry. Transformers are divided into lessons according to the kV rankings of the tools. It is up to the reliability or asset supervisor to make use of the rules for larger gear, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or manager needs to discover out what type of testing would benefit him in identifying drawback areas within his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there is an intensive vary of checks that can help in identifying particular drawback standards throughout the system, which might not be clear by way of the typical day-to-day evaluation often carried out.
Please see the rating lessons 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 among the most vital and significant influencers within the evaluation outcome. If a pattern isn’t taken to the prescribed process, then there’s a significant chance that the evaluation performed, and the results acquired, are of no use.
Care must be taken to make sure quality management procedures are applied in each step of the sampling process as set out by international requirements. A good high quality sample taken by applying the right process is essential. A pattern could be contaminated by numerous components, all of which may affect the outcome of the ends in a unfavorable manner.
All steps involved in sample taking should adhere to high quality control procedures, including the container that’s used, the sampling equipment, the taking of the sample, the marking of the sample, the closing of the pattern, the storage and handling of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and sample knowledge are handwritten, the human factor can result in incorrect interpretation of the information. The label ought to be caught 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 must be waterproof, and the pen used to write on the label should be oil- and waterproof. Otherwise, some knowledge may be misplaced, making it extremely difficult for the laboratory personnel to finish the report or allocate the pattern to a selected transformer.
Now, let’s focus on the forms of checks to determine the transformer’s situation, the crucial values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection applied to every oil sample.
When an oil pattern arrives on the laboratory, one of many “tests” is a visual inspection of the oil sample in a transparent vessel to discover out the colour, turbidity, and attainable particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it would indicate a high water content in the oil.
If the drain valve was not cleaned by the sampler, the filth particles within the drain valve may be included into the sample. If particles are recognized as carbon, it’d indicate a possible electrical fault within the unit. The DGA evaluation of the oil will confirm if so.
Clear oils with out contamination will indicate a great condition, and no motion is beneficial.
When oils are darkish or turbid, further evaluation will affirm any issues. The oil analysis outcomes may even determine the diploma and type 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 application
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of international 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 should 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 different parameter outcomes 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 various tests point out extreme growing older, the oil can be replaced with new or reclaimed oil. Another choice could be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar elements are removed from the oil. This process removes acid and water as properly as some other compounds. Another benefit is that the oil could be re-used, and in most conditions, this can be done with out switching off the unit, which contributes to cost-saving. If in doubt – as a substitute change off the unit during this remedy process.
If the values are Poor, it is advisable to take action as quickly as possible and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content material could cause flashover in the unit, resulting in loss of the asset.
c) Water content material (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 tools, except class G
The outcomes of this test ought to all the time be considered in conjunction with the breakdown power. If it is found that the water content material is high and the breakdown power is low, additional action needs to be taken. It is really helpful that a second sample from the same unit is tested to substantiate the results.
In the case of switching tools, where there is not a paper present, the breakdown voltage is the determining factor.
It should be famous that the limits indicated by IEC 60422 Edition four 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 check with Annex A of the standard.
When the value obtained through analyses is GOOD, the conventional sampling interval may be maintained, requiring no further action.
When the worth returns a FAIR end result, more frequent sampling is really helpful. It is also useful to think about different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the action to be applied.
A POOR result will require immediate motion from the asset manager. This might embody taking another pattern to verify the outcomes from the primary analysis. If it is confirmed that the water content material is excessive, the oil may be filtered; this process ought to remove a big portion of the moisture from the oil if applied correctly. Follow-up samples need to be taken to ensure that the moisture content material remains to be inside 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 move from the paper into the oil under circumstances that favour this movement. It may be discovered later that the oil within the water has increased once more without any apparent purpose, but the supply would be the paper in the transformer.
A visual inspection can be beneficial to find out if any water may transfer into the transformer or electrical tools through leaks. This drawback could be more extreme if the transformer or electrical tools is outside and not in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine take a look at for all lessons except F and G
The acids in oils are formed due to chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist within the formation of the acids. Because acids are polar compounds, it’ll adversely have an result 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 decrease elements of the transformer core. The sludge will ultimately kind 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 result, the sampling interval ought to be decreased to suit the scenario. Future evaluation should embrace a visual inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition four.0, the asset manager may decide to reclaim the oil or exchange it with new or reclaimed oil, whichever option would possibly swimsuit their necessities the best.
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 zero.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 courses of electrical tools, besides F and G
The dielectric dissipation issue or tan delta of this test provides data relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we now have the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities that might influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking extra parameters is recommended.
When the result’s 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 broken down, and even with filtration, the beneficial 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 one of the key parameters to evaluate the transformer insulation condition; that is primarily based on the reality that DC resistance is delicate to grease degradation.
When pressure gauge น้ำ , more frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material percent
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 supplier to confirm the details regarding components.
The two commonest 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 function of the inhibitor is to forestall 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’s suggested to top up the inhibitor stage to the prescribed degree per provider directions. It is advised to use a subject skilled educated within the procedure to perform this process.
If the result obtained is POOR, the advice for this scenario would suggest that the top consumer continues to make use of the oil “uninhibited,” but this will lead to extra speedy degradation of both the liquid and stable insulation.
It ought to be noted that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This could be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances isn’t required, although it’d add extra safety in opposition to 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 decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also referred to as metallic deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and scale back their price of response with compounds within the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental types, 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 extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, common sample intervals may be maintained.
With FAIR outcomes, maintain regular monitoring.
When POOR, it’s advised to take away the oil or take away the source of corrosivity from the oil by way of special oil remedy.
I) Sediment and sludge
This just isn’t a routine check.
It is advised that this test is performed when the oil results point out a high acid worth and the dissipation factor is close to the unacceptable restrict.
The results have to be lower than zero.02% by mass to be negligible. If the outcomes return a value of more than zero.02% by mass, it is suggested that it’s reclaimed; in any other case, an oil change is really helpful.
j) Interfacial rigidity
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 test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces during the getting older course of. What this means in practical terms is there’s more polar compound present within the oil, lowering the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial pressure turns into a high quality criterion: the oil must be changed beneath a predefined restrict.
If outcomes are GOOD, continue the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not 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 may be important. The extent of the corrosion injury brought on by the sulfur could be so severe that it might trigger failure of the equipment if not checked. The addition of a copper passivator can cut back the impression of this compound on the transformer system.
In a study by Doble, it has been discovered that rubber products utilized 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 need of corrective actions based mostly on this institute’s threat assessment examine. [4]
l) Particle counting and sizing
Table three: 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’s a most lower in flashpoints by 10%, the equipment may require additional inspection. This value may differ in numerous international locations.
It is suggested to perform this test when an uncommon odour is observed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not decide the condition of the transformer; this could be a health and security influence 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 is also required each time any maintenance has been accomplished on the unit, and the potential for contamination is current. If PCB content material exceeds the beneficial limits, the appropriate motion must be taken.
Units with a PCB content material of more than 50ppm require a fire safety plan, environmental protection plan, and further precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of protected disposal issued to the gear 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 will discuss this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this information based on worldwide requirements might be mentioned intimately, forming part of the general well being score determination of the transformer.
Transformer situation monitoring is an interlaced, extremely exciting field of examine. In this text, we targeted on the types of tests to find out the situation of the transformer, the critical values, and the really helpful actions.
The Health Index indication makes it attainable to see the supposed reliability of a selected unit at a specific date and time. This makes it attainable to ensure finest apply software and optimised upkeep. It also make it simpler to attract up a upkeep plan and action plan.
1. IEC 60422 Edition four.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
three. Passivators, what they’re 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 published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer marketing 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 proper from the beginning of her career, particularly within the evaluation of check knowledge. Corné has huge practical and theoretical knowledge of reliability maintenance programmes.

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