TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the relevant information relating to an asset’s operation. If we interpret the info correctly, it can provide us great perception into the asset’s situation.
Frequent monitoring of the asset can end result in much less upkeep required or extra prolonged periods without any upkeep required.
It is essential to determine the key parameters which are needed to provide us an entire image of the particular standing of the transformer and the motion 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 condition of the unit changed since the last upkeep period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate score 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 before we have to think about replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring define
It is vitally important to determine clear goals as a part of your strategy. What do you wish to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it will be a lot simpler to identify the required parameters.
Health indexing of property is changing 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 value of the Health Index Value of the transformer. These parameter weight values had been calculated based on the international requirements for mineral oils, indicating the important values stipulated within the numerous requirements.
The scope of oil analysis, interpretation of the data, and significant values
At the start of this section, it’s essential to state that we take care of completely different measurement transformers in the trade. Transformers are divided into classes based on the kV scores of the tools. It is up to the reliability or asset manager to use the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or supervisor needs to find out what sort of testing would profit him in figuring out downside areas inside his fleet. Some of the evaluation has been recognized as to routine type tests. Still, there’s an extensive vary of tests that can help in figuring out specific drawback criteria within the system, which might not be clear via the everyday day-to-day analysis often carried out.
Please see the score lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way often or beneath which circumstances.
TABLE 2 FIG
The taking of the oil pattern is among the most vital and important influencers in the analysis end result. If a pattern just isn’t taken to the prescribed process, then there is a vital possibility that the analysis performed, and the outcomes acquired, are of no use.
Care should be taken to ensure high quality control procedures are applied in each step of the sampling process as set out by international standards. A good high quality pattern taken by applying the right process is important. A sample can be contaminated by varied components, all of which can affect the result of the ends in a negative manner.
All steps concerned in sample taking must adhere to high quality control procedures, together with the container that’s used, the sampling package, the taking of the pattern, the marking of the pattern, the closing of the sample, the storage and dealing with of the pattern, after which the supply of the sample to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human issue can result in incorrect interpretation of the info. The label should be caught onto the container to make sure no mix-ups in sample identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to write down on the label ought to be oil- and waterproof. Otherwise, some knowledge might be lost, making it extraordinarily tough for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s discuss the types of exams 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 utilized to every oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visual inspection of the oil pattern 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 might be plenty of turbidity, it’d point out a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the dirt particles within the drain valve may be integrated into the sample. If particles are recognized as carbon, it would indicate a attainable electrical fault within the unit. The DGA evaluation of the oil will verify if this is the case.
Clear oils with out contamination will indicate a great condition, and no action is beneficial.
When oils are darkish or turbid, further analysis will affirm any problems. The oil analysis results will also decide 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 level application<40 kV for OLTC in delta or line-end utility
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of international particles, or both in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage must be high.
If the values are Good, it is strongly recommended to proceed with the current pattern interval motion plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil by way of oil reconditioning processes. If different checks point out severe aging, the oil could be changed with new or reclaimed oil. Another option could be to carry out on-site oil reclamation utilizing 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 process removes acid and water as properly as some other compounds. Another benefit is that the oil can be re-used, and in most conditions, this might be accomplished with out switching off the unit, which contributes to cost-saving. If doubtful – as a substitute switch off the unit during this remedy process.
If the values are Poor, it’s advisable to take action as soon as attainable and never 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 within the unit, resulting in loss 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 take a look at
This is a routine check for all courses of electrical gear, besides class G
The outcomes of this check should at all times be thought of along side the breakdown strength. If it is discovered that the water content is excessive and the breakdown energy is low, additional action must be taken. It is beneficial that a second sample from the identical unit is tested to confirm the outcomes.
In the case of switching tools, where there isn’t a paper present, the breakdown voltage is the determining factor.
It should be famous that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is exterior this temperature range, it’s best to check with Annex A of the usual.
When the worth obtained via analyses is GOOD, the conventional sampling interval can be maintained, requiring no further action.
When the value returns a FAIR result, more frequent sampling is beneficial. It is also useful to assume about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be carried out.
A POOR result will require instant action from the asset supervisor. This may include taking another sample to substantiate the outcomes from the first evaluation. If it’s confirmed that the water content is excessive, the oil may be filtered; this process should remove a large portion of the moisture from the oil if utilized correctly. Follow-up samples need to be taken to make certain that the moisture content material is still inside the required limits. The cause is that probably the most good portion of the water is caught up in the paper system in the transformer. This moisture will transfer from the paper into the oil beneath circumstances that favour this movement. It may be found later that the oil within the water has increased once more without any apparent cause, however the source can be the paper in the transformer.
digital pressure gauge can additionally be recommended to find out if any water might move into the transformer or electrical gear by way of leaks. This drawback may be extra extreme if the transformer or electrical tools is outside and never in a lined area.
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.one hundred.10 to zero.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine check for all courses except F and G
The acids in oils are fashioned as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help in the formation of the acids. Because acids are polar compounds, it’s going to 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 finally form a semi-solid substance that’s 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 fit the scenario. Future evaluation should include a visual inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition four.0, the asset supervisor may resolve to reclaim the oil or replace it with new or reclaimed oil, whichever choice may suit 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.one hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to zero.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 check supplies information 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’ve the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that might affect the dissipation factor embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking additional parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful. The structure of the oil is broken, 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 take a look at
DC resistivity of the oil is probably one of the key parameters to assess the transformer insulation situation; that is primarily based on the reality that DC resistance is delicate to grease degradation.
When the result is FAIR, more frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of original worth
This check is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the small print regarding components.
The two most common 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 function of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the aging course of within the oil and the solid insulation.
If the result’s FAIR, it’s advised to top up the inhibitor degree to the prescribed degree per provider directions. It is suggested to use a field professional trained within the process to carry out this task.
If the outcome obtained is POOR, the recommendation for this state of affairs would suggest that the end consumer continues to use the oil “uninhibited,” however this will likely lead to more rapid degradation of each the liquid and strong insulation.
It must 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 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 under these circumstances just isn’t required, though it would add further safety 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 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 decreasing at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and cut back their fee of reaction with compounds in the oil. This consists of 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 aware, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.
As the oil ages, the passivator might deplete more quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR results, preserve common monitoring.
When POOR, it is advised to remove the oil or remove the source of corrosivity from the oil by way of particular oil remedy.
I) Sediment and sludge
This just isn’t a routine check.
It is advised that this check is performed when the oil results point out a high acid value and the dissipation factor is near the unacceptable limit.
The results need to be lower than 0.02% by mass to be negligible. If the outcomes return a value of more than zero.02% by mass, it is suggested that or not it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial pressure
This isn’t 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 in the course of the aging course of. What this implies in sensible phrases is there’s extra polar compound current in the oil, lowering the power of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial pressure becomes a quality criterion: the oil should be changed below a predefined restrict.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If outcomes are POOR, examine 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 may be vital. The extent of the corrosion harm brought on by the sulfur can be so severe that it would trigger failure of the gear if not checked. The addition of a copper passivator can scale back the influence of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber merchandise used in transformers may 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 based mostly on this institute’s threat assessment research. 
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (particles) encountered on power transformer insulating oil as measured utilizing IEC 60970 
m) Flashpoint ° C
Not a routine check
If there is a most decrease in flashpoints by 10%, the equipment may require further inspection. This value may differ in numerous countries.
It is advised to perform this test when an unusual odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not determine the condition of the transformer; it is a health and safety impression test. PCB is hazardous to each humans 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 upkeep has been accomplished on the unit, and the potential of contamination is current. If PCB content exceeds the really helpful limits, the appropriate action needs to be taken.
Units with a PCB content of more than 50ppm require a fireplace security plan, environmental safety plan, and additional precautionary measures when upkeep is completed. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear owner.
Local regulatory our bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of information and interpretation, we’ll focus on this phenomenon in part II of the article. The limits for the different gases and the interpretation of this information based on international standards will be discussed in detail, forming part of the overall well being ranking determination of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling field of research. In this text, we centered on the forms of checks to determine 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 specific unit at a selected date and time. This makes it potential to make sure best follow application and optimised maintenance. It additionally make it easier to draw up a upkeep plan and action plan.
1. IEC 60422 Edition 4.zero 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and upkeep guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
3. Passivators, what they are and how 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 printed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ experience in the business, having beforehand worked as laboratory supervisor for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her profession, notably within the evaluation of test knowledge. Corné has huge practical and theoretical information of reliability maintenance programmes.
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