TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant data regarding an asset’s operation. If we interpret the info appropriately, it can provide us nice insight into the asset’s condition.
Frequent monitoring of the asset may find yourself in less upkeep required or more extended periods with none maintenance required.
It is crucial to establish the key parameters which might be wanted to provide us a whole image of the particular status of the transformer and the motion we need to take to make sure the continued reliability of the asset to achieve the utmost lifetime.
What is the info telling us?
Has the situation of the unit changed since the final upkeep period?
Is it protected to operate the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate ranking for a selected period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit before we have to consider replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring define
It is vitally essential to determine clear targets as part of your technique. What do you need 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 might be much easier to establish the required parameters.
Health indexing of assets is becoming a outstanding software in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the whole worth of the Health Index Value of the transformer. These parameter weight values had been calculated based mostly on the international requirements for mineral oils, indicating the important values stipulated within the various requirements.
The scope of oil analysis, interpretation of the information, and significant values
At the start of this part, it is essential to state that we take care of different dimension transformers in the trade. Transformers are divided into courses based on the kV rankings of the gear. It is up to the reliability or asset supervisor to use the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical tests.
The maintenance engineer or manager needs to determine what sort of testing would profit him in figuring out problem areas within his fleet. Some of the analysis has been identified as to routine kind exams. Still, there might be an in depth vary of exams that can assist in figuring out particular drawback criteria throughout the system, which could not be clear via the standard day-to-day analysis normally performed.
Please see the rating courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and how usually or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is certainly one of the most crucial and important influencers in the analysis outcome. If a pattern just isn’t taken to the prescribed procedure, then there’s a vital possibility that the evaluation performed, and the results acquired, are of no use.
Care must be taken to make sure high quality control procedures are utilized in every step of the sampling process as set out by international standards. A good quality pattern taken by making use of the right procedure is important. A pattern can be contaminated by various components, all of which might affect the finish result of the ends in a adverse method.
All steps involved in sample taking must adhere to high quality control procedures, together with the container that is used, the sampling package, the taking of the sample, the marking of the sample, the closing of the pattern, the storage and handling of the pattern, after which the supply of the pattern to the laboratory.
If the sample identification and pattern data are handwritten, the human issue can result in incorrect interpretation of the info. The label must be caught 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 should be waterproof, and the pen used to write down on the label must be oil- and water-resistant. Otherwise, some data might be lost, making it extremely difficult for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s focus on the forms of exams to determine the transformer’s condition, the critical values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection utilized to every oil sample.
When an oil sample arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to discover out the colour, turbidity, and attainable particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there’s lots of turbidity, it might indicate a high water content material in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles within the drain valve could be integrated into the sample. If particles are recognized as carbon, it would point out a potential electrical fault in the unit. The DGA analysis of the oil will verify if that’s 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 verify any issues. The oil evaluation outcomes may also determine the degree 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 level application<40 kV for OLTC in delta or line-end utility
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of foreign particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to keep away from flashover within the unit, the breakdown voltage must be high.
If the values are Good, it is suggested to proceed with the present sample interval motion plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter outcomes like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If various tests indicate severe getting older, the oil may be replaced with new or reclaimed oil. Another option could be to perform 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 removed from the oil. This process removes acid and water as nicely as some other compounds. Another advantage is that the oil may 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 – as an alternative switch off the unit throughout this remedy process.
If the values are Poor, it is advisable to take action as quickly as potential and not delay the upkeep process. ตัววัดแรงดันน้ำ in the transformer system decreases the projected transformer lifetime considerably; extremely high water content material could cause flashover within the unit, leading to 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 take a look at
This is a routine test for all courses of electrical tools, except class G
The results of this test ought to all the time be thought of at the aspect of the breakdown power. If it is discovered that the water content is high and the breakdown strength is low, additional motion must be taken. It is really helpful that a second pattern from the same unit is tested to verify the outcomes.
In the case of switching tools, the place there is no paper current, the breakdown voltage is the determining issue.
It must be famous that the boundaries 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 operating temperature is exterior this temperature range, it is best to refer to Annex A of the standard.
When the value obtained via analyses is GOOD, the conventional 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 useful to contemplate different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the action to be implemented.
A POOR result would require instant action from the asset manager. This may include taking another sample to substantiate the results from the first analysis. If it’s 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 utilized accurately. Follow-up samples have to be taken to guarantee that the moisture content remains to be within the required limits. The cause is that essentially the most good portion of the water is caught up in the paper system within the transformer. This moisture will transfer from the paper into the oil underneath situations that favour this motion. It could be discovered later that the oil within the water has increased again without any obvious reason, but the source would be the paper in the transformer.
A visual inspection can be recommended to determine if any water may transfer into the transformer or electrical equipment via leaks. This downside may be more extreme if the transformer or electrical tools is outdoors and not in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to zero.15>0.15
B, E<0.100.10 to 0.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine test
This is a routine check 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 within the formation of the acids. Because acids are polar compounds, it’ll adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this could result in sludge formation, usually around the lower elements of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily difficult to remove.
If the result is GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval should be decreased to fit the situation. Future analysis should embrace a visual inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset supervisor might decide to reclaim the oil or replace it with new or reclaimed oil, whichever option may go properly with their necessities the most effective.
e) Dielectric Dissipation issue 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.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 take a look at
This is a routine test for all classes of electrical tools, besides F and G
The dielectric dissipation factor or tan delta of this check provides info 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, leading to section displacement and dielectric losses. Other impurities that may affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended. 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 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 check
DC resistivity of the oil is certainly one of the key parameters to evaluate the transformer insulation situation; this is based on the reality that DC resistance is delicate to grease degradation.
When the result’s FAIR, more frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of authentic worth
This check is restricted to oils with this additive.
It can be advisable to contact the oil supplier to confirm the small print relating to components.
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 prevent oxygen from reacting with the oil. This considerably slows the aging course of in the oil and the stable insulation.
If the result is FAIR, it is advised to high up the inhibitor degree to the prescribed stage per provider instructions. It is advised to use a area skilled educated in the process to carry out this process.
If the end result obtained is POOR, the advice for this situation would suggest that the tip person continues to use the oil “uninhibited,” but this will result in extra speedy degradation of each the liquid and strong insulation.
It should be noted that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at levels under 1000ppm. This could be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, though it’d add additional protection towards 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 steady, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also recognized as metal deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and scale back their price of response with compounds within the oil. This includes 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 conscious, 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 quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it is suggested to remove the oil or remove the source of corrosivity from the oil by way of particular oil treatment.
I) Sediment and sludge
This isn’t a routine take a look at.
It is suggested that this take a look at is carried out when the oil outcomes point out a high acid value and the dissipation issue is close to the unacceptable limit.
The outcomes have 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 suggested that or not it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial pressure
This just 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 rigidity between transformer oil and water reduces during the getting older process. What this implies in practical phrases is there could be more polar compound present within 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 tension becomes a top quality criterion: the oil have to be modified below a predefined limit.
If outcomes are GOOD, proceed the common sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not 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 damage brought on by the sulfur could be so extreme that it would cause failure of the tools if not checked. The addition of a copper passivator can cut back the impact of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber merchandise utilized in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted 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 assessment study. 
l) Particle counting and sizing
Table three: 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 test
If there is a most lower in flashpoints by 10%, the gear may require further inspection. This worth may differ in several international locations.
It is suggested to perform this test when an uncommon odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the condition of the transformer; this is a health and security impression check. PCB is hazardous to each humans and the environment; it is vital to test 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 risk of contamination is current. If PCB content exceeds the really helpful limits, the appropriate action must be taken.
Units with a PCB content material of greater than 50ppm require a fire security plan, environmental protection plan, and extra precautionary measures when maintenance is completed. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment proprietor.
Local regulatory bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we’ll focus on this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this information in accordance with international requirements shall be discussed in detail, forming a part of the general well being score determination of the transformer.
Transformer situation monitoring is an interlaced, highly exciting area of research. In this article, we centered on the forms of exams to discover out the situation of the transformer, the important values, and the recommended actions.
The Health Index indication makes it attainable to see the supposed reliability of a particular unit at a selected date and time. This makes it attainable to make sure best follow application and optimised maintenance. It also make it simpler to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and maintenance guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
3. 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 power.”
6. Article initially revealed by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the industry, having beforehand labored as laboratory manager for a significant 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 check data. Corné has vast sensible and theoretical information of reliability maintenance programmes.