TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the related knowledge relating to an asset’s operation. If we interpret the info accurately, it can provide us nice insight into the asset’s condition.
Frequent monitoring of the asset can lead to less maintenance required or extra prolonged intervals without any upkeep required.
It is crucial to determine the key parameters which are needed to give us an entire picture of the particular status of the transformer and the motion we have 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 modified for the rationale that final maintenance period?
Is it secure to operate the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate rating for a particular period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit before we want to consider replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally necessary to establish clear goals as a part of your strategy. What do เกจวัดแรงดันคือ want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the end result and what you wish to accomplish, it would be much simpler to establish the required parameters.
Health indexing of assets is becoming a exceptional device in getting a clearer image of the situation 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 were calculated based mostly on the worldwide standards for mineral oils, indicating the critical values stipulated within the various standards.
The scope of oil evaluation, interpretation of the data, and important values
At the start of this section, it’s important to state that we deal with completely different size transformers in the industry. Transformers are divided into classes based on the kV rankings of the tools. It is as much as the reliability or asset manager to make use of the guidelines for larger equipment, thereby implementing shorter increment oil analysis and electrical tests.
The maintenance engineer or manager needs to find out what kind of testing would benefit him in figuring out drawback areas within his fleet. Some of the analysis has been recognized as to routine kind exams. Still, there’s an in depth range of tests that can assist in figuring out particular drawback criteria throughout the system, which might not be clear by way of the standard day-to-day evaluation often 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 beneath which circumstances.
TABLE 2 FIG
The taking of the oil pattern is likely one of the most vital and critical influencers within the evaluation outcome. If a pattern just isn’t taken to the prescribed process, then there is a vital possibility that the evaluation carried out, and the results acquired, are of no use.
Care must be taken to make sure quality control procedures are applied in every step of the sampling course of as set out by international requirements. A good quality sample taken by making use of the proper procedure is essential. A sample may be contaminated by varied components, all of which can affect the result of the results in a unfavorable manner.
All steps concerned in sample taking must adhere to 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 handling of the sample, after which the delivery of the pattern to the laboratory.
If the sample identification and sample data are handwritten, the human factor can result in incorrect interpretation of the information. The label must be caught onto the container to ensure 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 water resistant. Otherwise, some knowledge could be lost, making it extremely difficult for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s talk about the forms of exams to discover out the transformer’s condition, the crucial values, and the beneficial actions in every case:
a) Colour and appearance
This is a routine inspection applied to each oil pattern.
When an oil sample arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a transparent vessel to find out the color, turbidity, and potential particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there is plenty of turbidity, it’d indicate a high water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles in the drain valve could be integrated into the pattern. If particles are identified as carbon, it would point out a attainable electrical fault within the unit. The DGA evaluation of the oil will verify if so.
Clear oils without contamination will point out a great condition, and no action is recommended.
When oils are darkish or turbid, additional evaluation will confirm any issues. The oil analysis results may even decide the degree 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 application
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 in the unit, the breakdown voltage must be high.
If the values are Good, it is suggested to continue with the current pattern interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with different parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil by way of oil reconditioning processes. If various checks point out severe getting older, the oil may be replaced with new or reclaimed oil. Another choice can be to perform 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 components are faraway from the oil. This process removes acid and water in addition to another compounds. Another advantage is that the oil could be re-used, and in most situations, this can be carried out with out switching off the unit, which contributes to cost-saving. If doubtful – instead swap off the unit throughout this therapy process.
If the values are Poor, it’s advisable to take motion as soon as attainable and not delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content material 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 check
This is a routine take a look at for all courses of electrical equipment, besides class G
The results of this check ought to at all times be thought-about in conjunction with the breakdown strength. If it’s discovered that the water content is high and the breakdown energy is low, additional motion needs to be taken. It is recommended that a second sample from the identical unit is examined to verify the results.
In the case of switching tools, where there is no paper present, the breakdown voltage is the determining issue.
It ought to be famous that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s operating temperature is exterior this temperature range, it is best to discuss with Annex A of the usual.
When the worth obtained through 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 really helpful. It can be helpful to contemplate different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be carried out.
A POOR end result will require immediate action from the asset manager. This may embrace taking another sample to confirm the results from the first evaluation. If it’s confirmed that the water content material is high, the oil can be filtered; this course of should take away a large portion of the moisture from the oil if utilized appropriately. Follow-up samples need to be taken to ensure that the moisture content is still throughout the required limits. The purpose is that probably the most significant portion of the water is caught up within the paper system within the transformer. This moisture will transfer from the paper into the oil under circumstances that favour this motion. It may be found later that the oil in the water has elevated again without any obvious purpose, however the supply could be the paper within the transformer.
A visual inspection can also be beneficial to determine if any water would possibly transfer into the transformer or electrical equipment through leaks. This problem may be extra extreme if the transformer or electrical gear is outdoors and never in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to zero.15>0.15
B, E<0.one hundred.10 to 0.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine check
This is a routine take a look at for all lessons except F and G
The acids in oils are fashioned as a end 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 result on the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this will lead to sludge formation, normally across the decrease components of the transformer core. The sludge will finally type a semi-solid substance that is extraordinarily troublesome to take away.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to fit the scenario. Future analysis should include a visual inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition 4.0, the asset manager may decide to reclaim the oil or replace it with new or reclaimed oil, whichever option would possibly swimsuit 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<0.100.10 to 0.20>0.20
B, C<0.a hundred.10 to 0.50>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 take a look at
This is a routine take a look at for all classes of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this test supplies data concerning the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, leading to phase 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 extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is beneficial. The structure 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 test
DC resistivity of the oil is certainly one of the key parameters to evaluate the transformer insulation condition; this is based mostly on the fact that DC resistance is delicate to grease degradation.
When the result is FAIR, more frequent sampling and checking further parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of unique value
This check is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the small print regarding additives.
The two most common 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 forestall oxygen from reacting with the oil. This significantly slows the getting older course of in the oil and the solid insulation.
If the result’s FAIR, it is advised to prime up the inhibitor stage to the prescribed degree per supplier instructions. It is suggested to use a field professional educated in the process to carry out this process.
If the result obtained is POOR, the recommendation for this situation would counsel that the top consumer continues to make use of the oil “uninhibited,” but this will likely lead to more rapid degradation of each the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges under 1000ppm. This could be within 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 is not required, though it might add additional protection towards oil oxidation if the preservation system ever fails. 
h) Passivator content
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 reducing at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and reduce their fee of reaction with compounds within the oil. This contains oxidation reactions with natural 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 may deplete more quickly; this depletion might accelerate when the oil is un-inhibited.
With GOOD results, common pattern intervals may be maintained.
With FAIR outcomes, maintain common monitoring.
When POOR, it is advised to remove the oil or remove the supply of corrosivity from the oil through special oil therapy.
I) Sediment and sludge
This just isn’t a routine check.
It is advised that this take a look at is performed when the oil results indicate a excessive 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 0.02% by mass, it is suggested that it’s reclaimed; in any other case, an oil change is recommended.
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 take a look at
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces during the getting older process. What this means in sensible phrases is there is more polar compound current within the oil, reducing the ability of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation number. Therefore, the interfacial rigidity turns into a quality criterion: the oil have to be modified below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes 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 both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be vital. The extent of the corrosion harm caused by the sulfur could be so severe that it would cause failure of the equipment if not checked. The addition of a copper passivator can reduce the impression of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber merchandise utilized 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 necessity of corrective actions based on this institute’s risk evaluation study. 
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 
m) Flashpoint ° C
Not a routine take a look at
If there is a most decrease in flashpoints by 10%, the gear may require additional inspection. This worth may differ in numerous countries.
It is suggested to carry out this test when an uncommon odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to decide the condition of the transformer; this is a health and safety impression take a look at. PCB is hazardous to each people and the surroundings; it’s critical to check for PCBs after the retro fill of a transformer. It can be required every time any upkeep has been done on the unit, and the chance of contamination is current. If PCB content exceeds the recommended limits, the appropriate action must be taken.
Units with a PCB content of greater than 50ppm require a fire security plan, environmental protection plan, and additional precautionary measures when upkeep is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the equipment owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we are going to focus on this phenomenon partially II of the article. The limits for the different gases and the interpretation of this data based on worldwide requirements shall be discussed in detail, forming a half of the overall well being ranking dedication of the transformer.
Transformer situation monitoring is an interlaced, highly exciting area of examine. In this article, we centered on the kinds of tests to determine the condition of the transformer, the crucial values, and the really helpful actions.
The Health Index indication makes it possible to see the supposed reliability of a specific unit at a specific date and time. This makes it possible to make sure greatest apply application and optimised maintenance. It additionally make it simpler to attract up a upkeep plan and motion plan.
1. IEC 60422 Edition four.0 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
3. 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 printed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the trade, having previously labored as laboratory manager for a major industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry right from the start of her career, particularly in the evaluation of test data. Corné has vast sensible and theoretical knowledge of reliability maintenance programmes.