TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant data concerning an asset’s operation. If we interpret the information accurately, it may give us great perception into the asset’s situation.
Frequent monitoring of the asset can outcome in less upkeep required or more prolonged periods with none upkeep required.
It is essential to determine the key parameters which might be needed to offer us a whole image of the actual status of the transformer and the action we have to take to make sure the continued reliability of the asset to achieve the utmost lifetime.
What is the data telling us?
Has the situation of the unit modified because the final upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate score 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 have to think about replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally essential to identify clear objectives as a part of your technique. What do you need 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 wish to accomplish, it will be much simpler to identify the required parameters.
Health indexing of belongings is becoming a exceptional tool in getting a clearer picture of the condition of your transformer. Test parameters carry a numerical value-adding to the entire worth of the Health Index Value of the transformer. These parameter weight values were calculated primarily based on the worldwide requirements for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the data, and significant values
At the beginning of this part, it is essential to state that we cope with different dimension transformers in the business. Transformers are divided into courses in accordance with the kV rankings of the equipment. It is as much as the reliability or asset manager to use the rules for larger equipment, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or supervisor wants to find out what sort of testing would profit him in identifying drawback areas within his fleet. Some of the analysis has been identified as to routine type checks. Still, there might be an extensive range of checks that may assist in identifying particular problem standards throughout the system, which could not be clear by way of the standard day-to-day evaluation normally carried out.
Please see the ranking lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way usually or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is certainly one of the most vital and critical influencers within the analysis outcome. If a sample just isn’t taken to the prescribed process, then there’s a significant chance that the analysis performed, and the outcomes acquired, are of no use.
Care must be taken to ensure quality control procedures are utilized in each step of the sampling process as set out by international standards. A good quality pattern taken by applying the right procedure is essential. A pattern can be contaminated by various components, all of which may influence the result of the results in a unfavorable method.
All steps concerned in pattern taking must adhere to quality management procedures, together with the container that is used, the sampling kit, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and handling of the pattern, and then the supply of the pattern to the laboratory.
If the sample identification and pattern information are handwritten, the human issue can lead to incorrect interpretation of the info. The label should 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 should be waterproof, and the pen used to write on the label should be oil- and water-resistant. Otherwise, some information may be misplaced, making it extremely difficult for the laboratory personnel to finish the report or allocate the pattern to a specific transformer.
Now, let’s talk about the forms of checks to determine the transformer’s situation, the important values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil sample.
When an oil sample arrives at the laboratory, one of many “tests” is a visual inspection of the oil pattern in a clear vessel to determine the colour, turbidity, and attainable particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it might point out a high water content within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles in the drain valve might be integrated into the sample. If particles are identified as carbon, it’d point out a attainable electrical fault within the unit. The DGA analysis of the oil will verify if that is the case.
Clear oils with out contamination will indicate a great situation, and no action is beneficial.
When oils are dark or turbid, additional analysis will confirm any issues. The oil evaluation outcomes may even decide the diploma and type 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 software
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of overseas particles, or both in 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 should be high.
If the values are Good, it is suggested to proceed with the current sample interval motion plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter results just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil through oil reconditioning processes. If different tests point out extreme aging, the oil can be changed with new or reclaimed oil. Another possibility 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 parts are faraway 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 can be accomplished without switching off the unit, which contributes to cost-saving. If doubtful – instead switch off the unit during this remedy course of.
If the values are Poor, it’s advisable to take action as quickly as potential and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely excessive water content could cause flashover in the unit, resulting in 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 take a look at for all courses of electrical tools, besides class G
The results of this check should all the time be thought of at the facet of the breakdown strength. If it’s found that the water content material is excessive and the breakdown power is low, further motion needs to be taken. It is recommended that a second pattern from the same unit is tested to verify the results.
In the case of switching tools, where there isn’t any paper current, the breakdown voltage is the determining issue.
It should be noted that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is outside this temperature vary, it is best to refer to Annex A of the usual.
When the value obtained via analyses is GOOD, the normal sampling interval could be maintained, requiring no further motion.
When เกจวัดแรงดันอาร์กอน returns a FAIR result, extra frequent sampling is really helpful. It can additionally be helpful to think about other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be applied.
A POOR outcome would require quick motion from the asset supervisor. This would possibly include taking another pattern to substantiate the results from the primary evaluation. If it’s confirmed that the water content is excessive, the oil can be filtered; this course of ought to remove a big portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to ensure that the moisture content remains to be inside the required limits. The reason is that essentially the most significant slice of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil underneath circumstances that favour this movement. It may be discovered later that the oil in the water has elevated again with none obvious reason, but the source could be the paper in the transformer.
A visual inspection is also recommended to determine if any water might transfer into the transformer or electrical equipment by way of leaks. This downside could be more extreme if the transformer or electrical tools is exterior and never in a coated 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<0.a hundred.10 to 0.25>0.20
C<0.one hundred.15 to 0.30>0.30
F, G Not a routine check
This is a routine check for all lessons besides F and G
The acids in oils are formed because of 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’ll adversely have an effect on the insulation properties of the oil and can enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, usually around the lower elements of the transformer core. The sludge will ultimately type a semi-solid substance that is extremely troublesome to take away.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval ought to be decreased to suit the situation. Future evaluation ought to 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 4.zero, the asset manager could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever choice would possibly 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.one hundred.10 to 0.20>0.20
B, C<0.one hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.a hundred.01 to zero.30>0.03
F, GNot a routine check
This is a routine test for all lessons of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this check supplies information regarding the extent of the dielectric losses in transformer oil. This test 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 that may affect the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking further parameters is beneficial.
When the result’s POOR, reclamation or an oil change is recommended. The structure of the oil is damaged, in impact 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 take a look at
DC resistivity of the oil is probably one of the key parameters to evaluate the transformer insulation situation; that is primarily based on the fact that DC resistance is delicate to oil degradation.
When the result’s FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic 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 confirm the primary points concerning additives.
The two commonest 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 prevent oxygen from reacting with the oil. This significantly slows the growing older course of within the oil and the strong insulation.
If the result is FAIR, it’s suggested to top up the inhibitor stage to the prescribed degree per provider directions. It is advised to use a area professional educated within the procedure to carry out this process.
If the outcome obtained is POOR, the advice for this scenario would counsel that the end consumer continues to use the oil “uninhibited,” but this will likely lead to extra speedy degradation of each the liquid and stable insulation.
It ought to be famous that some transformers already have a built-in oil preservation system; this is designed to maintain 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 equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances isn’t required, although it’d add additional protection in opposition to oil oxidation if the preservation system ever fails. 
h) Passivator content material
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and secure, (rate of decrease < 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 identified as metal deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and cut back their price of response with compounds within the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental sorts, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the writer is aware, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.
As the oil ages, the passivator may deplete extra rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD results, common sample intervals could be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it’s advised to remove the oil or remove the source of corrosivity from the oil via special oil treatment.
I) Sediment and sludge
This just isn’t a routine check.
It is suggested that this check is performed when the oil results point out a excessive acid worth and the dissipation factor is near the unacceptable limit.
The results have to be lower than 0.02% by mass to be negligible. If the results return a price of greater than 0.02% by mass, it’s suggested that it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial pressure
This isn’t a routine take a look at
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 in the course of the aging process. What this means in practical phrases is there is more polar compound current within the oil, decreasing the ability of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial rigidity becomes a top quality criterion: the oil must be modified below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This just 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 may be vital. The extent of the corrosion damage caused by the sulfur can be so severe that it’d trigger failure of the tools if not checked. The addition of a copper passivator can cut back the impact of this compound on the transformer system.
In a study by Doble, it has been found that rubber products used in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s danger evaluation examine. 
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 
m) Flashpoint ° C
Not a routine check
If there’s a maximum decrease in flashpoints by 10%, the equipment may require further inspection. This value would possibly differ in several international locations.
It is suggested to perform this test when an unusual odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not decide the situation of the transformer; this is a well being and safety impression test. PCB is hazardous to each people and the surroundings; it is important to check for PCBs after the retro fill of a transformer. It is also required whenever any maintenance has been carried out on the unit, and the potential of contamination is present. If PCB content exceeds the really helpful limits, the suitable motion must be taken.
Units with a PCB content material of greater than 50ppm require a fireplace security plan, environmental safety plan, and additional precautionary measures when maintenance is completed. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment owner.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we are going to talk about this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this information according to international standards shall be mentioned intimately, forming a part of the general well being score dedication of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling subject of research. In this text, we centered on the types of exams to determine the situation of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a particular date and time. This makes it possible to make sure best apply application and optimised upkeep. It additionally make it easier to draw up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and maintenance guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
three. Passivators, what they are and the way 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 strength.”
6. Article initially published by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience in the business, having previously labored as laboratory supervisor for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the start of her career, significantly within the evaluation of check data. Corné has vast practical and theoretical data of reliability maintenance programmes.