TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant data relating to an asset’s operation. If we interpret the information appropriately, it can give us great insight into the asset’s condition.
Frequent monitoring of the asset may end up in less maintenance required or extra extended durations with none upkeep required.
It is crucial to establish the important thing parameters that are wanted to offer us an entire image of the particular status of the transformer and the motion we want to take to make sure the continued reliability of the asset to realize the utmost lifetime.
What is the data telling us?
Has the condition of the unit changed because the final upkeep period?
Is it secure to operate the unit?
Are there signs of deterioration?
Is it protected 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 think about replacement?
Are the recognized issues of a recurring nature?
Effective condition monitoring define
It is vitally necessary to identify clear goals as part of your strategy. What do you want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the finish result and what you wish to accomplish, it will be a lot easier to determine the required parameters.
Health indexing of property is becoming a remarkable tool in getting a clearer picture of the condition 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 had been calculated based mostly on the international standards for mineral oils, indicating the crucial values stipulated in the varied requirements.
The scope of oil evaluation, interpretation of the info, and critical values
At the beginning of this section, it is important to state that we cope with totally different dimension transformers in the industry. Transformers are divided into classes based on the kV rankings of the tools. It is up to the reliability or asset manager to use the rules for larger tools, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or manager wants to discover out what sort of testing would benefit him in figuring out problem areas inside his fleet. Some of the analysis has been identified as to routine type exams. Still, there might be an extensive vary of checks that may help in figuring out particular problem criteria within the system, which could not be clear by way of the everyday day-to-day analysis normally carried out.
Please see the rating courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way typically or under which circumstances.
TABLE 2 FIG
The taking of the oil pattern is among the most vital and critical influencers within the analysis end result. If pressure gauge ไฮ ด รอ ลิ ค isn’t taken to the prescribed process, then there is a vital possibility that the evaluation performed, and the results acquired, are of no use.
Care ought to be taken to make sure high quality management procedures are utilized in each step of the sampling course of as set out by worldwide requirements. A good quality pattern taken by applying the right procedure is essential. A pattern could be contaminated by varied components, all of which can affect the result of the results in a unfavorable manner.
All steps involved in pattern taking must adhere to quality management procedures, including the container that’s used, the sampling equipment, the taking of the sample, the marking of the pattern, the closing of the pattern, the storage and handling of the pattern, after which the delivery of the sample to the laboratory.
If the sample identification and pattern knowledge are handwritten, the human factor can result in incorrect interpretation of the information. The label must be stuck 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 ought to be waterproof, and the pen used to write on the label ought to be oil- and waterproof. Otherwise, some information might be misplaced, making it extremely troublesome for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s discuss the kinds of tests to discover out the transformer’s condition, the critical values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection applied to every oil pattern.
When an oil pattern arrives on the laboratory, one of many “tests” is a visual inspection of the oil pattern in a transparent vessel to determine the color, turbidity, and potential particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there is plenty of turbidity, it would point out 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 may be integrated into the pattern. If particles are identified as carbon, it might indicate a potential electrical fault in the unit. The DGA evaluation of the oil will confirm if that is so.
Clear oils with out contamination will indicate an excellent condition, and no motion is really helpful.
When oils are darkish or turbid, further evaluation will verify any issues. The oil evaluation results will also 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 software
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of foreign particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage should be excessive.
If the values are Good, it is recommended to continue with the current sample interval action plan.
If the values are Fair, more frequent sampling is beneficial 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 alternative exams point out severe growing older, the oil can be changed with new or reclaimed oil. Another possibility could be to perform on-site oil reclamation using 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 benefit is that the oil can be re-used, and in most conditions, this might be carried out without switching off the unit, which contributes to cost-saving. If unsure – as an alternative switch off the unit during this treatment process.
If the values are Poor, it’s advisable to take motion as soon as potential and never delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content may cause flashover within the unit, resulting in lack 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 test
This is a routine test for all lessons of electrical tools, except class G
The outcomes of this test ought to all the time be considered at the facet of the breakdown energy. If it’s discovered that the water content material is high and the breakdown strength is low, further motion needs to be taken. It is really helpful that a second pattern from the same unit is tested to verify the results.
In the case of switching equipment, the place there is not a paper current, the breakdown voltage is the figuring out issue.
It should be noted that the boundaries 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 outdoors this temperature range, it is best to discuss with Annex A of the usual.
When the value obtained by way of analyses is GOOD, the conventional sampling interval may be maintained, requiring no further motion.
When the worth returns a FAIR end result, extra frequent sampling is really helpful. It is also useful to suppose about other parameters 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 will require quick motion from the asset manager. This would possibly embrace taking another pattern to verify the results from the primary evaluation. If it is confirmed that the water content is excessive, the oil may be filtered; this course of should take away a large portion of the moisture from the oil if applied correctly. Follow-up samples have to be taken to ensure that the moisture content is still within the required limits. The cause is that the most significant portion of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil beneath conditions that favour this motion. It could be discovered later that the oil in the water has elevated once more without any obvious reason, but the source can be the paper within the transformer.
A visual inspection can be recommended to discover out if any water would possibly move into the transformer or electrical equipment via leaks. This problem might be more extreme if the transformer or electrical equipment is outdoors and not in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to zero.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 courses except F and G
The acids in oils are shaped as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist in the formation of the acids. Because acids are polar compounds, it will adversely affect the insulation properties of the oil and can enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, often across the lower parts of the transformer core. The sludge will eventually type a semi-solid substance that is extremely tough to take away.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR result, the sampling interval must be decreased to suit the situation. Future evaluation should include a visible inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition 4.zero, the asset manager might resolve to reclaim the oil or replace it with new or reclaimed oil, whichever option may go nicely with 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<0.100.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.one hundred.01 to 0.30>0.03
F, GNot a routine test
This is a routine take a look at for all classes of electrical tools, besides F and G
The dielectric dissipation issue or tan delta of this check supplies information concerning the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we have the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities that may influence the dissipation factor include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended. The construction of the oil is damaged, in effect that the chemical bonds between the molecules have broken 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 test
DC resistivity of the oil is likely certainly one of the key parameters to assess the transformer insulation condition; that is primarily based on the truth that DC resistance is sensitive to oil degradation.
When the result is FAIR, extra frequent sampling and checking further parameters is recommended.
When the result is 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 original value<40% of original worth
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil provider to confirm the main points concerning additives.
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 purpose of the inhibitor is to stop oxygen from reacting with the oil. This considerably slows the growing older course of within the oil and the solid insulation.
If the result’s FAIR, it is suggested to prime up the inhibitor degree to the prescribed stage per provider directions. It is suggested to make use of a area skilled educated in the procedure to perform this process.
If the result obtained is POOR, the recommendation for this state of affairs would recommend that the tip consumer continues to make use of the oil “uninhibited,” however this will result in more speedy degradation of both the liquid and stable insulation.
It ought to be noted that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This can 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 beneath these circumstances isn’t required, though it might add further protection towards 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 stable, (rate of lower < 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 identified 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 includes 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 writer is conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.
As the oil ages, the passivator might deplete extra quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it is advised to take away the oil or remove the source of corrosivity from the oil through particular oil treatment.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is suggested that this take a look at is carried out when the oil outcomes point out a excessive acid value and the dissipation factor is near the unacceptable restrict.
The results have to be lower than 0.02% by mass to be negligible. If the results return a worth of more than 0.02% by mass, it’s advised that or not it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial pressure
This isn’t a routine check
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 pressure between transformer oil and water reduces in the course of the aging process. What this means in sensible phrases is there’s more polar compound present within the oil, reducing the power of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial tension turns into a high quality criterion: the oil should be modified under a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine take a look at.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system can be significant. The extent of the corrosion damage caused by the sulfur may be so extreme that it would cause failure of the equipment if not checked. The addition of a copper passivator can scale back the impression of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber products utilized in transformers might 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 danger assessment research. 
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on power transformer insulating oil as measured using IEC 60970 
m) Flashpoint ° C
Not a routine check
If there is a most lower in flashpoints by 10%, the gear might require additional inspection. This worth would possibly differ in numerous nations.
It is suggested to carry out this check when an unusual odour is noticed, 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 health and safety impression test. PCB is hazardous to both humans and the environment; it is important to check for PCBs after the retro fill of a transformer. It can additionally be required every time any upkeep has been done on the unit, and the potential for contamination is present. If PCB content material exceeds the beneficial limits, the suitable action 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 upkeep is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the gear owner.
Local regulatory our bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we’ll talk about this phenomenon in part II of the article. The limits for the different gases and the interpretation of this information based on international standards might be mentioned intimately, forming part of the general well being ranking determination of the transformer.
Transformer situation monitoring is an interlaced, extremely thrilling subject of research. In this article, we focused on the forms of tests to determine the situation of the transformer, the important values, and the beneficial actions.
The Health Index indication makes it attainable to see the supposed reliability of a particular unit at a specific date and time. This makes it potential to ensure finest follow software and optimised maintenance. It additionally make it easier to attract up a maintenance plan and motion 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
3. Passivators, what they are and the way 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 energy.”
6. Article initially published by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise within the industry, having beforehand worked as laboratory supervisor for a significant industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the start of her career, notably within the analysis of take a look at information. Corné has vast sensible and theoretical knowledge of reliability maintenance programmes.
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