TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the related knowledge concerning an asset’s operation. If we interpret the information appropriately, it may give us great perception into the asset’s condition.
Frequent monitoring of the asset can lead to much less upkeep required or more extended intervals without any maintenance required.
It is essential to determine the key parameters which are wanted to provide us an entire picture of the actual status of the transformer and the action we have to take to make sure the continued reliability of the asset to attain the maximum lifetime.
What is the info telling us?
Has the condition of the unit changed because the final maintenance period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate rating for a specific period?
Are we required to implement action to ensure the continued reliability of the unit?
How lengthy can we use the unit before we have to think about replacement?
Are the identified problems of a recurring nature?
Effective situation monitoring define
It is vitally essential to identify clear targets as part of your strategy. What do you wish to achieve by implementing this condition 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 assets is becoming a remarkable device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the total worth of the Health Index Value of the transformer. These parameter weight values have been calculated based on the international standards for mineral oils, indicating the critical values stipulated in the varied requirements.
The scope of oil evaluation, interpretation of the data, and critical values
At the start of this section, it is important to state that we deal with totally different dimension transformers in the trade. Transformers are divided into courses according to the kV ratings of the gear. It is as much as the reliability or asset manager to use the rules for bigger gear, thereby implementing shorter increment oil analysis and electrical checks.
The upkeep engineer or manager wants to determine what type of testing would profit him in figuring out problem areas within his fleet. Some of the analysis has been recognized as to routine type tests. Still, there is an extensive range of checks that can help in identifying particular drawback standards inside the system, which might not be clear through the everyday day-to-day analysis normally performed.
Please see the score courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and how often or under which circumstances.
TABLE 2 FIG
The taking of the oil sample is probably one of the most vital and important influencers within the analysis end result. If a sample just isn’t taken to the prescribed procedure, then there’s a important possibility that the evaluation performed, and the results acquired, are of no use.
Care ought to be taken to ensure quality control procedures are utilized in each step of the sampling process as set out by worldwide requirements. A good high quality pattern taken by applying the correct procedure is crucial. A pattern can be contaminated by numerous factors, all of which might affect the result of the leads to a adverse manner.
All steps concerned in pattern taking must adhere to high 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 dealing with of the sample, and then the delivery of the pattern to the laboratory.
If the pattern identification and pattern information are handwritten, the human issue can result in incorrect interpretation of the info. The label should be stuck 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 on the label ought to be oil- and water resistant. Otherwise, some data could be misplaced, making it extraordinarily tough for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s discuss the forms of tests to determine the transformer’s situation, the important values, and the really helpful actions in every case:
a) Colour and appearance
This is a routine inspection applied to each oil pattern.
When an oil pattern arrives on the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to determine the color, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s a lot 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 sample. If particles are recognized as carbon, it might indicate a potential electrical fault in the unit. The DGA evaluation of the oil will verify if so.
Clear oils with out contamination will indicate a great situation, and no action is really helpful.
When oils are dark or turbid, further analysis will confirm any issues. The oil evaluation outcomes may even determine 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 point application<40 kV for OLTC in delta or line-end application
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of international particles, or both within 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 excessive.
If the values are Good, it is suggested to continue with the current pattern interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter results just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If different tests indicate severe getting older, the oil can be changed with new or reclaimed oil. Another possibility can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar components are faraway from the oil. This course of removes acid and water as well as another compounds. Another benefit is that the oil can be re-used, and in most conditions, this can be accomplished without switching off the unit, which contributes to cost-saving. If doubtful – as an alternative swap off the unit during this therapy process.
If the values are Poor, it is advisable to take action as quickly as attainable and not delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content material can cause flashover in the unit, leading to 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 check for all courses of electrical gear, except class G
The results of this test ought to at all times be thought-about in conjunction with the breakdown strength. If it is found that the water content is high and the breakdown energy is low, additional action needs to be taken. It is really helpful that a second sample from the identical 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 factor.
It ought to be noted that the limits indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it is found that the unit’s working temperature is outside this temperature vary, it’s best to check with Annex A of the standard.
When the worth obtained by way of analyses is GOOD, the traditional sampling interval can be maintained, requiring no further action.
When the worth returns a FAIR outcome, more frequent sampling is really helpful. It is also helpful to suppose about other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be applied.
A POOR end result would require quick motion from the asset manager. This may embrace taking one other pattern to confirm the outcomes from the primary analysis. If it is confirmed that the water content is excessive, the oil can be filtered; this course of should remove a big portion of the moisture from the oil if applied accurately. Follow-up samples need to be taken to guarantee that the moisture content material continues to be throughout the required limits. The purpose is that essentially the most significant slice of the water is caught up within the paper system in the transformer. This moisture will transfer from the paper into the oil under circumstances that favour this movement. It may be found later that the oil in the water has elevated again with none apparent reason, but the source could be the paper within the transformer.
A visual inspection is also recommended to find out if any water might move into the transformer or electrical tools by way of leaks. This drawback could be extra severe if the transformer or electrical equipment is outdoors and never in a coated 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.a hundred.10 to zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine test for all classes besides 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 assist in 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 could lead to sludge formation, often around the lower elements of the transformer core. The sludge will eventually form a semi-solid substance that is extraordinarily difficult to take away.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR end result, the sampling interval ought to be decreased to fit the situation. Future analysis ought to embody a visible 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 supervisor could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever possibility would possibly swimsuit 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.one hundred.10 to zero.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.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 equipment, except F and G
The dielectric dissipation issue or tan delta of this take a look at offers information relating to 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 section displacement and dielectric losses. Other impurities that might influence the dissipation issue embrace water, dissolved insulating resin, and paper.
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 really helpful. The construction of the oil is damaged, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the recommended 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 test
DC resistivity of the oil is one of the key parameters to evaluate the transformer insulation condition; this is based mostly on the reality that DC resistance is delicate to oil degradation.
When the result is FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s 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 worth
This test is restricted to oils with this additive.
It can be advisable to contact the oil supplier to verify 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 function of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the aging course of within the oil and the strong insulation.
If the result is 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 trained within the procedure to carry out this activity.
If the result obtained is POOR, the recommendation for this situation would suggest that the end user continues to make use of the oil “uninhibited,” but this will likely lead to more speedy degradation of each the liquid and stable insulation.
It should be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges below 1000ppm. This could be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, although it would add further safety 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 secure, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and cut 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 sorts, sulfur-based and nitrogen-based. The first suggested 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 may accelerate when the oil is un-inhibited.
With GOOD results, common pattern intervals could be maintained.
With FAIR outcomes, preserve common monitoring.
When POOR, it’s advised to remove the oil or remove the supply of corrosivity from the oil via particular oil treatment.
I) Sediment and sludge
This just isn’t a routine test.
It is suggested that this check is carried out when the oil outcomes point out a high acid worth and the dissipation factor is near the unacceptable limit.
The outcomes need to be less than zero.02% by mass to be negligible. If the results return a worth of greater than zero.02% by mass, it is instructed that it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial pressure
This isn’t a routine test
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 tension between transformer oil and water reduces in the course of the growing older course of. What this means in sensible phrases is there is more polar compound present in the oil, reducing the ability of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial tension turns into a excessive quality criterion: the oil should be modified beneath a predefined restrict.
If outcomes are GOOD, continue 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 is not 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 may be vital. The extent of the corrosion harm attributable to the sulfur may be so extreme that it’d trigger failure of the equipment 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 merchandise utilized in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based on this institute’s risk assessment examine. 
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 test
If there’s a maximum lower in flashpoints by 10%, the gear would possibly require additional inspection. This value would possibly differ in numerous international locations.
It is suggested to carry out this check when an uncommon odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to determine the situation of the transformer; it is a well being and security impression test. PCB is hazardous to each humans and the environment; it is important to check for PCBs after the retro fill of a transformer. It can be required each time any upkeep has been done on the unit, and the potential of contamination is present. If PCB content exceeds the really helpful limits, the suitable action must be taken.
Units with a PCB content material of more than 50ppm require a fireplace safety plan, environmental protection plan, and extra precautionary measures when maintenance is finished. This oil must 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 define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we will talk about this phenomenon partially II of the article. The limits for the completely different gases and the interpretation of this data according to international standards will be discussed intimately, forming part of the general health score determination of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling area of study. In this text, we targeted on the kinds of tests to determine the condition of the transformer, the important values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a specific date and time. This makes it potential to make sure best practice application and optimised maintenance. 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 tools – 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.
4. 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 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer advisor. เกจวัดความดันน้ำ has 20+ years’ expertise in the industry, having previously worked as laboratory manager for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the start of her profession, significantly within the evaluation of check information. Corné has huge practical and theoretical data of reliability upkeep programmes.