TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related data regarding an asset’s operation. If we interpret the data appropriately, it can provide us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to much less maintenance required or more prolonged periods with none upkeep required.
It is essential to establish the important thing parameters which may be wanted to give us an entire image of the actual standing of the transformer and the motion we have to take to make sure the continued reliability of the asset to attain the utmost lifetime.
What is the information telling us?
Has the condition of the unit changed since the last maintenance period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate rating 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 earlier than we have to consider replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring define
It is vitally important to establish clear goals as a half of your technique. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you wish to accomplish, it might be a lot easier to establish the required parameters.
Health indexing of property is changing into a outstanding device in getting a clearer picture 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 requirements for mineral oils, indicating the important values stipulated in the various standards.
The scope of oil evaluation, interpretation of the data, and important values
At the start of this section, it is important to state that we take care of totally different dimension transformers in the business. Transformers are divided into lessons based on the kV scores of the tools. It is as a lot as the reliability or asset supervisor to make use of the guidelines for bigger tools, thereby implementing shorter increment oil evaluation and electrical checks.
The upkeep engineer or manager wants to determine what kind of testing would profit him in identifying problem areas inside his fleet. Some of the analysis has been identified as to routine sort tests. Still, there’s an extensive range of exams that may assist in identifying particular drawback criteria inside the system, which could not be clear by way of the standard day-to-day evaluation usually performed.
Please see the ranking classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way typically or beneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is one of the most important and critical influencers within the evaluation consequence. If a pattern is not taken to the prescribed process, then there’s a important chance that the analysis performed, and the outcomes acquired, are of no use.
Care must be taken to ensure quality control procedures are utilized in each step of the sampling course of as set out by international standards. A good high quality sample taken by applying the right procedure is essential. A sample could be contaminated by numerous components, all of which can influence the outcome of the leads to a negative manner.
All steps concerned in sample taking must adhere to high quality control procedures, together with the container that is 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 pattern to the laboratory.
If the sample identification and pattern information are handwritten, the human factor can result in incorrect interpretation of the info. The label must be stuck onto the container to ensure no mix-ups in sample 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 must be oil- and water-resistant. Otherwise, some data could be misplaced, making it extremely difficult for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s discuss the types of checks to find out the transformer’s condition, the important 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 many “tests” is a visible inspection of the oil sample in a transparent vessel to discover out the color, turbidity, and possible particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there could be a lot of turbidity, it’d point out a excessive water content within the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles within the drain valve might be included into the pattern. If particles are identified as carbon, it’d point out a possible electrical fault in the unit. The DGA analysis of the oil will affirm if that is the case.
Clear oils with out contamination will point out a good situation, and no motion is really helpful.
When oils are darkish or turbid, additional analysis will affirm any issues. The oil evaluation outcomes will also decide the diploma and sort 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 level 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 should be high.
If the values are Good, it is suggested to continue with the present sample interval action plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with different parameter results like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil through oil reconditioning processes. If alternative exams indicate extreme growing older, the oil can be changed with new or reclaimed oil. Another choice could be to perform on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar parts are faraway from the oil. This course of removes acid and water as nicely as another compounds. Another benefit is that the oil may be re-used, and in most situations, this might be carried out with out switching off the unit, which contributes to cost-saving. If in doubt – as a substitute switch off the unit during this remedy process.
If the values are Poor, it is advisable to take motion as soon as possible and never delay the upkeep process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content may cause flashover in the unit, resulting in lack of the asset.
c) Water content (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 check for all courses of electrical equipment, except class G
The outcomes of this take a look at should always be thought-about at the side of the breakdown power. If it is discovered that the water content material is high and the breakdown power is low, further motion must be taken. It is really helpful that a second pattern from the identical unit is examined to substantiate the results.
In the case of switching equipment, the place there is not a paper current, the breakdown voltage is the figuring out factor.
It should be famous that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it’s found that the unit’s working temperature is exterior this temperature range, it’s best to discuss with Annex A of the standard.
When the worth obtained through analyses is GOOD, the traditional sampling interval could be maintained, requiring no further motion.
When the worth returns a FAIR end result, more frequent sampling is recommended. It is also helpful to consider other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be carried out.
A POOR result would require quick action from the asset supervisor. This would possibly embrace taking another sample to substantiate the outcomes from the first evaluation. If it’s confirmed that the water content material is high, the oil could be filtered; this course of ought to remove a big portion of the moisture from the oil if applied correctly. Follow-up samples need to be taken to ensure that the moisture content material is still throughout the required limits. The purpose is that probably the most good portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil underneath situations that favour this movement. It could be discovered later that the oil within the water has increased once more with none apparent reason, however the source could be the paper within the transformer.
A visual inspection is also recommended to find out if any water would possibly transfer into the transformer or electrical tools via leaks. This drawback could be extra extreme if the transformer or electrical equipment is outside and never in a lined area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.100.10 to 0.25>0.20
C<0.100.15 to 0.30>0.30
F, G Not a routine check
This is a routine take a look at for all courses besides F and G
The acids in oils are formed as a end result 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 will adversely have an result on the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this will result in sludge formation, usually across the lower parts of the transformer core. The sludge will finally kind a semi-solid substance that is extremely troublesome to take away.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR result, the sampling interval ought to be decreased to fit the situation. Future evaluation ought to embody a visual inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset manager might determine to reclaim the oil or exchange it with new or reclaimed oil, whichever possibility would possibly suit 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.one hundred.10 to 0.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to 0.30>0.03
F, GNot a routine check
This is a routine check for all classes of electrical gear, besides F and G
The dielectric dissipation factor or tan delta of this test offers info 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 section displacement and dielectric losses. Other impurities which may affect the dissipation factor embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is beneficial. The structure of the oil is broken, in impact 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 amongst the key parameters to evaluate 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 extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is beneficial.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of unique worth
This check is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the details 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 forestall oxygen from reacting with the oil. This considerably slows the growing older process in the oil and the strong insulation.
If the result’s FAIR, it is advised to top up the inhibitor level to the prescribed degree per supplier directions. It is advised to make use of a area professional skilled in the process to perform this process.
If the outcome obtained is POOR, the advice for this state of affairs would suggest that the end person continues to use the oil “uninhibited,” however this will result in extra fast degradation of each the liquid and solid insulation.
It ought to be noted 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 within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, though it would add further safety against oil oxidation if the preservation system ever fails. 
h) Passivator content
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 recognized as metal deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and scale back their price of reaction with compounds within the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental varieties, 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 might deplete extra rapidly; this depletion may accelerate when the oil is un-inhibited.
With GOOD results, regular sample intervals may be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it is suggested to take away the oil or take away the supply of corrosivity from the oil via particular oil remedy.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is suggested that this check is performed when the oil outcomes point out a high acid worth and the dissipation issue is near the unacceptable limit.
The outcomes have to be lower than zero.02% by mass to be negligible. If the outcomes return a value of more than 0.02% by mass, it is suggested that or not it’s reclaimed; in any other case, an oil change is beneficial.
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 tension between transformer oil and water reduces during the aging process. What this means in practical phrases is there may be more polar compound present within the oil, lowering the ability of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, the interfacial pressure becomes a high quality criterion: the oil have to be modified under a predefined limit.
If results are GOOD, proceed the common sampling interval.
If outcomes are FAIR, lower the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine test.
Oil is either 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 injury brought on by the sulfur could be so severe that it might cause failure of the gear 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 found that rubber products utilized 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 necessity of corrective actions based mostly on this institute’s risk evaluation research. 
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination levels (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 gear would possibly require further inspection. This worth might differ in several countries.
It is advised to perform this check when an uncommon odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not decide the condition of the transformer; this could be a health and safety influence check. PCB is hazardous to both people and the setting; it is important to check for PCBs after the retro fill of a transformer. It can also be required each time any maintenance has been accomplished on the unit, and the potential of contamination is present. If PCB content exceeds the beneficial limits, the appropriate motion needs to be taken.
Units with a PCB content of more than 50ppm require a hearth safety plan, environmental protection plan, and additional precautionary measures when maintenance is completed. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the tools proprietor.
Local regulatory our bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we will talk about this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this knowledge according to international requirements might be discussed intimately, forming part of the overall well being ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, extremely exciting subject of research. In this article, we targeted on the kinds of checks to determine the situation of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it possible to see the supposed reliability of a particular unit at a particular date and time. This makes it potential to ensure best follow application and optimised maintenance. It additionally make it simpler to attract up a maintenance plan and action plan.
1. IEC 60422 Edition four.0 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’re 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 marketing consultant. เกจวัดแรงดันน้ำ10บาร์ has 20+ years’ experience in the trade, having beforehand labored as laboratory supervisor for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the start of her profession, particularly within the evaluation of check knowledge. Corné has huge practical and theoretical information of reliability upkeep programmes.