TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant information concerning an asset’s operation. If we interpret the info correctly, it can provide us nice insight into the asset’s situation.
Frequent monitoring of the asset can lead to much less maintenance required or more extended durations with none maintenance required.
It is essential to identify the important thing parameters which might be wanted to provide us an entire picture of the particular status of the transformer and the action we have to take to ensure the continued reliability of the asset to attain the maximum lifetime.
What is the information telling us?
Has the situation of the unit changed because the last upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate rating for a selected period?
Are we required to implement action 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 identified issues of a recurring nature?
Effective condition monitoring define
It is vitally necessary to establish clear goals as a half of your strategy. What do you want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the finish result and what you wish to accomplish, it might be a lot easier to determine the required parameters.
Health indexing of property is turning into a remarkable device in getting a clearer image 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 primarily based on the worldwide standards 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 deal with totally different size transformers within the business. Transformers are divided into classes in accordance with the kV scores of the tools. It is up to the reliability or asset manager to use the rules for bigger tools, thereby implementing shorter increment oil analysis and electrical checks.
The maintenance engineer or manager wants to discover out what type of testing would profit him in identifying drawback areas within his fleet. ร้านซ่อมเครื่องวัดความดันomron of the analysis has been recognized as to routine kind checks. Still, there’s an in depth vary of checks that can help in figuring out particular drawback criteria throughout the system, which could not be clear through the typical day-to-day evaluation usually performed.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and how usually or under which circumstances.
TABLE 2 FIG
The taking of the oil sample is doubtless one of the most important and critical influencers within the analysis end result. If a sample is not taken to the prescribed procedure, then there’s a significant possibility that the analysis carried out, and the outcomes acquired, are of no use.
Care must be taken to make sure quality management procedures are utilized in every step of the sampling process as set out by international standards. A good high quality pattern taken by applying the correct process is important. A sample may be contaminated by various factors, all of which can affect the outcome of the ends in a unfavorable method.
All steps concerned in pattern taking should adhere to high quality management procedures, together with the container that’s used, the sampling package, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and handling of the pattern, and then the delivery of the sample to the laboratory.
If the pattern identification and sample knowledge are handwritten, the human issue can lead to 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 should be waterproof, and the pen used to write down on the label should be oil- and water-resistant. Otherwise, some data could be lost, making it extraordinarily difficult for the laboratory personnel to finish the report or allocate the pattern to a specific transformer.
Now, let’s focus on the forms of checks to determine the transformer’s condition, the crucial values, and the really helpful actions in each case:
a) Colour and look
This is a routine inspection applied to every oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to discover out the color, 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’d point out a high water content within the oil.
If the drain valve was not cleaned by the sampler, the filth particles in the drain valve may be integrated into the pattern. If particles are recognized as carbon, it might point out a potential electrical fault within the unit. The DGA evaluation of the oil will confirm if this is the case.
Clear oils without contamination will indicate an excellent situation, and no action is really helpful.
When oils are dark or turbid, further analysis will verify any issues. The oil analysis 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 point application<40 kV for OLTC in delta or line-end utility
This is a routine inspection.
Breakdown voltage will indicate 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 avoid flashover in the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to proceed with the current pattern interval motion plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If alternative tests indicate severe aging, the oil could be replaced with new or reclaimed oil. Another possibility would be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water in addition to another 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 – as a substitute switch off the unit throughout this therapy process.
If the values are Poor, it’s advisable to take motion as soon as potential and never delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content can 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 take a look at
This is a routine test for all courses of electrical tools, besides class G
The results of this take a look at should at all times be thought-about at the side of the breakdown energy. If it is discovered that the water content material is high and the breakdown energy is low, additional motion needs to be taken. It is really helpful that a second pattern from the same unit is examined to verify the outcomes.
In the case of switching equipment, where there is no paper current, the breakdown voltage is the determining issue.
It must be famous that the bounds 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 outside this temperature range, it is best to check with Annex A of the standard.
When the worth obtained through analyses is GOOD, the conventional sampling interval could be maintained, requiring no further motion.
When the value returns a FAIR result, more frequent sampling is really helpful. It can additionally be helpful to think about different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the motion to be implemented.
A POOR end result will require immediate action from the asset manager. This might include taking another pattern to confirm the outcomes from the primary evaluation. If it’s confirmed that the water content material is excessive, the oil could be filtered; this process ought to remove a big portion of the moisture from the oil if applied accurately. Follow-up samples have to be taken to make certain that the moisture content material is still inside the required limits. The purpose is that the most significant portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil beneath situations that favour this movement. It could be discovered later that the oil within the water has elevated once more without any apparent cause, however the supply can be the paper within the transformer.
A visual inspection can be really helpful to determine if any water might move into the transformer or electrical tools by way of leaks. This drawback may be extra severe if the transformer or electrical gear is outside and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to 0.15>0.15
B, E<0.one hundred.10 to zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine take a look at for all courses except F and G
The acids in oils are fashioned as a result of chemical reactions between the oil, water, and paper. Higher temperatures or load increases 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 enhance paper degradation. If left untreated in transformers, this could lead to sludge formation, often around the decrease elements of the transformer core. The sludge will finally kind a semi-solid substance that is extremely tough to remove.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to fit the situation. Future analysis should include a visual 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 may resolve to reclaim the oil or exchange it with new or reclaimed oil, whichever option might go properly with their necessities 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 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 0.30>0.03
F, GNot a routine test
This is a routine test for all lessons of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this take a look at supplies information concerning 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 part displacement and dielectric losses. Other impurities which may affect the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial. The construction of the oil is damaged, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the beneficial 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 considered one of the key parameters to evaluate the transformer insulation condition; that is based on the fact that DC resistance is sensitive to oil degradation.
When the result’s FAIR, more frequent sampling and checking further parameters is really helpful.
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 original value<40% of original 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 small print regarding additives.
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 purpose 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 stable insulation.
If the result’s FAIR, it’s suggested to prime up the inhibitor level to the prescribed stage per supplier directions. It is suggested to use a subject skilled skilled within the process to carry out this task.
If the end result obtained is POOR, the recommendation for this situation would counsel that the end user continues to make use of the oil “uninhibited,” but this will lead to extra speedy degradation of both the liquid and stable insulation.
It must be famous that some transformers have already got a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This could 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 underneath these circumstances just isn’t required, although it’d add extra protection in opposition to oil oxidation if the preservation system ever fails. 
h) Passivator content
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and secure, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive steel surfaces and dissolved metals corresponding to copper and silver and scale back their rate of reaction with compounds in the oil. This contains oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two basic 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.
As the oil ages, the passivator may deplete more quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals could be maintained.
With FAIR results, maintain regular monitoring.
When POOR, it’s advised to take away the oil or take away the source of corrosivity from the oil through special oil remedy.
I) Sediment and sludge
This is not a routine test.
It is advised that this take a look at is performed when the oil results point out a high acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes need to be lower than 0.02% by mass to be negligible. If the outcomes return a price of more than zero.02% by mass, it’s suggested that or not it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial pressure
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 tension between transformer oil and water reduces during the aging course of. What this implies in sensible terms is there’s extra polar compound present within the oil, lowering the flexibility of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial tension turns into a quality criterion: the oil have to be modified beneath a predefined restrict.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system can be important. The extent of the corrosion harm attributable to the sulfur can be so extreme that it might cause failure of the equipment if not checked. The addition of a copper passivator can scale back the influence of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber products used in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved 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 research. 
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured utilizing IEC 60970 
m) Flashpoint ° C
Not a routine check
If there is a maximum decrease in flashpoints by 10%, the equipment would possibly require further inspection. This value might differ in different nations.
It is advised to carry out this test when an unusual odour is noticed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to decide the condition of the transformer; this may be a well being and safety influence take a look at. PCB is hazardous to each people and the setting; it’s critical to test for PCBs after the retro fill of a transformer. It can additionally be required every time any maintenance has been accomplished on the unit, and the chance of contamination is current. If PCB content material exceeds the beneficial limits, the suitable motion needs to be taken.
Units with a PCB content material of greater than 50ppm require a fire safety plan, environmental safety plan, and extra precautionary measures when upkeep is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the gear proprietor.
Local regulatory our bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we’ll talk about this phenomenon partially II of the article. The limits for the different gases and the interpretation of this data based on worldwide standards might be mentioned in detail, forming a half of the overall well being score willpower of the transformer.
Transformer condition monitoring is an interlaced, highly thrilling subject of research. In this text, we targeted on the forms of tests to determine the situation of the transformer, the critical values, and the recommended actions.
The Health Index indication makes it attainable to see the supposed reliability of a specific unit at a selected date and time. This makes it possible to make sure greatest follow utility and optimised maintenance. It additionally make it easier to draw up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.zero 2013-01 International Standard (Mineral insulating oils in electrical equipment – supervision and upkeep 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 power.”
6. Article initially published by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise in the trade, having beforehand labored as laboratory manager for a serious industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry right from the beginning of her profession, notably in the evaluation of test information. Corné has vast sensible and theoretical knowledge of reliability upkeep programmes.