TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the related data regarding an asset’s operation. If we interpret the info correctly, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset can lead to much less upkeep required or extra extended periods without any upkeep required.
It is essential to establish the necessary thing parameters which would possibly be wanted to offer us an entire image of the actual status of the transformer and the motion we have to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is เครื่องมือวัดความดัน telling us?
Has the condition of the unit changed since the final maintenance period?
Is it protected to function the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate rating for a selected period?
Are we required to implement motion 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 recognized problems of a recurring nature?
Effective condition monitoring outline
It is vitally essential to identify clear goals as a half of your strategy. What do you wish to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the finish result and what you need to accomplish, it will be a lot easier to establish the required parameters.
Health indexing of belongings is becoming a outstanding 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 crucial values stipulated in the varied requirements.
The scope of oil evaluation, interpretation of the data, and important values
At the start of this section, it is essential to state that we cope with different dimension transformers in the business. Transformers are divided into classes according to the kV rankings of the tools. It is up to the reliability or asset supervisor to make use of the rules for bigger tools, thereby implementing shorter increment oil evaluation and electrical tests.
The upkeep engineer or supervisor wants to discover out what type of testing would benefit him in identifying downside areas inside his fleet. Some of the evaluation has been recognized as to routine type exams. Still, there might be an extensive range of exams that can assist in figuring out specific downside criteria inside the system, which could not be clear via the standard day-to-day evaluation often carried out.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and the way usually or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is likely considered one of the most important and significant influencers in the evaluation outcome. If a pattern isn’t taken to the prescribed process, then there is a important risk that the evaluation carried out, and the outcomes acquired, are of no use.
Care ought to be taken to make sure high quality management procedures are utilized in each step of the sampling process as set out by worldwide standards. A good quality sample taken by making use of the right procedure is crucial. A sample could be contaminated by varied components, all of which may influence the outcome of the ends in a adverse manner.
All steps concerned in pattern taking should adhere to quality control procedures, including 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 sample, and then the supply 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 should be stuck onto the container to make sure no mix-ups in pattern identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write on the label ought to be oil- and water-resistant. Otherwise, some information could be lost, making it extremely tough for the laboratory personnel to finish the report or allocate the sample to a selected transformer.
Now, let’s discuss the types of checks to determine the transformer’s condition, the important values, and the really helpful actions in each case:
a) Colour and appearance
This is a routine inspection utilized to every oil sample.
When ที่วัดแรงดันน้ำ arrives on the laboratory, one of the “tests” is a visual inspection of the oil sample in a clear vessel to discover out the color, turbidity, and attainable particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it’d point out a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the filth particles within the drain valve might be integrated into the sample. If particles are identified as carbon, it might point out a potential electrical fault within the unit. The DGA analysis of the oil will confirm if so.
Clear oils with out contamination will indicate an excellent situation, and no motion is recommended.
When oils are darkish or turbid, further evaluation will affirm any problems. The oil evaluation outcomes will also determine the degree 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 utility
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of overseas particles, or each within 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 continue with the present pattern interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with different parameter outcomes just like the water content material, 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 alternative tests indicate extreme aging, the oil can be replaced with new or reclaimed oil. Another option 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 parts 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 situations, this can be done without switching off the unit, which contributes to cost-saving. If doubtful – as a substitute swap off the unit during this treatment course of.
If the values are Poor, it is advisable to take action as soon as potential and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely excessive water content may cause flashover within the unit, leading to loss 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 check
This is a routine test for all lessons of electrical equipment, except class G
The outcomes of this test should all the time be considered at the aspect of the breakdown power. If it is found that the water content is excessive and the breakdown power is low, additional motion must be taken. It is beneficial that a second pattern from the identical unit is examined to verify the outcomes.
In the case of switching tools, where there is no paper current, the breakdown voltage is the determining issue.
It should be famous that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it’s found that the unit’s working temperature is outside this temperature range, it’s best to discuss with Annex A of the usual.
When the value obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no further action.
When the value returns a FAIR end result, more frequent sampling is really helpful. It can be helpful to contemplate other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the action to be implemented.
A POOR outcome would require immediate action from the asset manager. This might embody taking another pattern to confirm the outcomes from the first analysis. If it is confirmed that the water content material is high, the oil may be filtered; this course of ought to take away a large portion of the moisture from the oil if applied appropriately. 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 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 underneath situations that favour this motion. It might be found later that the oil in the water has increased again without any apparent cause, but the source would be the paper within the transformer.
A visible inspection is also recommended to determine if any water would possibly transfer into the transformer or electrical equipment through leaks. This problem could be extra extreme if the transformer or electrical tools is exterior and not in a coated 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.a 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 test for all classes besides F and G
The acids in oils are shaped because 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’ll adversely have an effect on the insulation properties of the oil and can enhance paper degradation. If left untreated in transformers, this can lead to sludge formation, often across the lower parts of the transformer core. The sludge will eventually type a semi-solid substance that is extraordinarily tough to remove.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR end result, the sampling interval should be decreased to suit the scenario. Future evaluation ought to embody a visible inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition 4.zero, the asset manager might resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever option may swimsuit their requirements the most effective.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to zero.20>0.20
B, C<0.100.10 to 0.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to zero.30>0.03
F, GNot a routine test
This is a routine take a look at for all classes of electrical gear, except F and G
The dielectric dissipation factor or tan delta of this take a look at provides information regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities which may influence 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 recommended. The structure of the oil is broken, 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 amongst the key parameters to assess the transformer insulation condition; that is based on the reality that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
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 original worth
This take a look at is restricted to oils with this additive.
It would be advisable to contact the oil supplier to confirm the small print concerning components.
The two most typical 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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the getting older process within the oil and the strong insulation.
If the result is FAIR, it’s suggested to prime up the inhibitor degree to the prescribed degree per supplier directions. It is advised to make use of a area professional trained within the procedure to perform this task.
If the outcome obtained is POOR, the advice for this state of affairs would recommend that the tip person continues to use the oil “uninhibited,” but this will lead to extra rapid degradation of both the liquid and stable insulation.
It ought to be noted that some transformers already have a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges under 1000ppm. This would 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 underneath these circumstances isn’t required, though it would add further 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 decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also known as metallic deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and reduce their fee of reaction with compounds within the oil. This includes 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 creator 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 extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals can be maintained.
With FAIR results, maintain common monitoring.
When POOR, it is advised to take away the oil or take away the source of corrosivity from the oil via particular oil treatment.
I) Sediment and sludge
This isn’t a routine check.
It is suggested that this test is performed when the oil results point out a high acid value and the dissipation factor is near the unacceptable restrict.
The outcomes must be lower than zero.02% by mass to be negligible. If the results return a price of greater than zero.02% by mass, it’s instructed that it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial pressure
This is not a routine take a look at
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to twenty-eight 20 to 25<22 <20
ENot a routine check
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the growing older process. What this implies in practical phrases is there is more polar compound present in the oil, reducing the flexibility of an oil to function an insulator in 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 must be changed beneath a predefined restrict.
If outcomes are GOOD, continue the common sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be important. The extent of the corrosion injury caused by the sulfur may be so severe that it would trigger failure of the tools if not checked. The addition of a copper passivator can cut 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 might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions primarily based on this institute’s threat evaluation study. 
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured using IEC 60970 
m) Flashpoint ° C
Not a routine check
If there’s a most lower in flashpoints by 10%, the gear may require further inspection. This worth might differ in numerous countries.
It is advised to carry out this take a look at when an unusual 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 situation of the transformer; it is a health and security impact take a look at. PCB is hazardous to both people and the setting; it is important to check for PCBs after the retro fill of a transformer. It is also required every time any upkeep has been carried out on the unit, and the possibility of contamination is current. If PCB content exceeds the beneficial limits, the appropriate action must be taken.
Units with a PCB content of greater than 50ppm require a fireplace safety plan, environmental safety plan, and additional precautionary measures when upkeep is done. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of safe 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 information 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 based on international requirements shall be mentioned in detail, forming part of the general health score willpower of the transformer.
Transformer condition monitoring is an interlaced, highly exciting field of study. In this article, we centered on the forms of checks to determine the condition of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it possible to see the supposed reliability of a specific unit at a particular date and time. This makes it attainable to ensure best practice software and optimised upkeep. It also make it easier to draw up a maintenance plan and motion plan.
1. IEC 60422 Edition four.0 2013-01 International Standard (Mineral insulating oils in electrical equipment – supervision and maintenance 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 power.”
6. Article initially published by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience within the industry, having beforehand labored as laboratory manager for a major industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry proper from the beginning of her career, particularly in the evaluation of test information. Corné has huge practical and theoretical information of reliability maintenance programmes.