TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant data concerning an asset’s operation. If we interpret the data accurately, it can give us nice insight into the asset’s condition.
Frequent monitoring of the asset can result in less upkeep required or extra extended periods with none maintenance required.
It is crucial to identify the vital thing parameters which might be needed to give us an entire picture of the actual standing of the transformer and the action we need to take to make sure the continued reliability of the asset to realize the maximum lifetime.
What is the information telling us?
Has pressure gauge วัด แรง ดัน น้ำ of the unit modified for the explanation that final upkeep period?
Is it safe to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate score for a specific period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit earlier than we want to think about replacement?
Are the identified problems of a recurring nature?
Effective situation monitoring outline
It is vitally necessary to establish clear objectives as a part of your technique. What do you need to obtain 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 need to accomplish, it would be a lot easier to identify the required parameters.
Health indexing of assets is becoming a remarkable software in getting a clearer picture of the condition 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 based on the international requirements for mineral oils, indicating the crucial values stipulated in the various requirements.
The scope of oil analysis, interpretation of the information, and critical values
At the start of this section, it’s essential to state that we take care of totally different size transformers in the business. Transformers are divided into classes in accordance with the kV ratings of the gear. It is as much as the reliability or asset supervisor to make use of the guidelines for larger equipment, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or supervisor needs to determine what kind of testing would profit him in figuring out problem areas within his fleet. Some of the evaluation has been recognized as to routine sort exams. Still, there might be an intensive vary of tests that can help in figuring out particular downside standards within the system, which might not be clear via the standard day-to-day analysis normally carried out.
Please see the rating lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and the way usually or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is one of the most important and critical influencers within the evaluation outcome. If a pattern is not taken to the prescribed process, then there is a vital possibility that the analysis carried out, and the outcomes acquired, are of no use.
Care should be taken to ensure high quality management procedures are utilized in every step of the sampling process as set out by international requirements. A good quality sample taken by making use of the correct procedure is essential. A pattern can be contaminated by various components, all of which may affect the outcome of the results in a adverse manner.
All steps concerned in sample taking should adhere to quality management procedures, together with the container that’s used, the sampling equipment, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and handling of the sample, and then the delivery of the sample to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human factor can result in incorrect interpretation of the info. The label ought to 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 should be waterproof, and the pen used to write on the label must be oil- and waterproof. Otherwise, some information may be lost, making it extremely difficult for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s focus on the kinds of checks to determine the transformer’s situation, the crucial values, and the really helpful actions in every case:
a) Colour and appearance
This is a routine inspection applied to every oil sample.
When an oil sample arrives on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to find out the color, turbidity, and attainable particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there’s lots of turbidity, it’d indicate a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve might be integrated into the sample. If particles are identified as carbon, it might point out a possible electrical fault in the unit. The DGA analysis of the oil will verify if that is so.
Clear oils without contamination will indicate a good condition, and no action is recommended.
When oils are dark or turbid, additional evaluation will verify any problems. The oil evaluation outcomes may also determine the degree and kind 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 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 or the presence of foreign particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to continue with the current pattern interval action plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with different parameter results 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 indicate severe aging, the oil may be changed with new or reclaimed oil. Another choice would 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 removed from the oil. This course of removes acid and water as properly as some other compounds. Another benefit is that the oil may be re-used, and in most situations, this could be carried out with out switching off the unit, which contributes to cost-saving. If doubtful – as an alternative change off the unit during this remedy process.
If the values are Poor, it’s advisable to take motion as quickly as attainable and not delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content material may cause flashover within the unit, resulting in loss of the asset.
c) Water content (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 test
This is a routine test for all classes of electrical tools, except class G
The outcomes of this check ought to all the time be considered at the side of the breakdown power. If it’s discovered that the water content is excessive and the breakdown strength is low, further motion needs to 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 not a paper current, the breakdown voltage is the determining factor.
It ought to be famous that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between 40 and 70 °C. If it is discovered that the unit’s operating temperature is outdoors this temperature vary, it is best to check with Annex A of the usual.
When the worth obtained via analyses is GOOD, the traditional sampling interval could be maintained, requiring no additional action.
When the value returns a FAIR result, extra frequent sampling is recommended. It can additionally be helpful to think about different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the motion to be carried out.
A POOR end result would require immediate motion from the asset manager. This may embody taking one other sample to confirm the results from the first analysis. If it’s confirmed that the water content is excessive, the oil could be filtered; this course of should remove a large portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to guarantee that the moisture content material continues to be inside the required limits. The cause is that probably the most significant slice of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil underneath situations that favour this movement. It might be found later that the oil in the water has increased once more without any obvious cause, but the source can be the paper in the transformer.
A visual inspection can also be really helpful to determine if any water may transfer into the transformer or electrical gear through leaks. This downside may be more severe if the transformer or electrical tools is outdoors and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.100.10 to zero.25>0.20
C<0.one hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine test for all courses except F and G
The acids in oils are shaped as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist within the formation of the acids. Because acids are polar compounds, it will adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this could lead to sludge formation, often across the decrease parts of the transformer core. The sludge will eventually form a semi-solid substance that’s extraordinarily tough to remove.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR result, the sampling interval must be decreased to suit the scenario. Future evaluation should embody a visual inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition four.0, the asset supervisor might decide to reclaim the oil or exchange it with new or reclaimed oil, whichever choice may swimsuit their necessities the best.
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.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 check
This is a routine take a look at for all lessons of electrical tools, except F and G
The dielectric dissipation issue or tan delta of this test provides information regarding 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 phase displacement and dielectric losses. Other impurities that might affect the dissipation issue embody water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result’s POOR, reclamation or an oil change is really helpful. The construction of the oil is broken, 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 check
DC resistivity of the oil is doubtless certainly one of the key parameters to assess the transformer insulation situation; that is primarily based on the reality that DC resistance is delicate to grease degradation.
When the result is FAIR, extra frequent sampling and checking further parameters is really helpful.
When the result is 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 authentic worth
This check is restricted to oils with this additive.
It could be advisable to contact the oil provider to verify the small print relating to 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 purpose of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the getting older process in the oil and the strong insulation.
If the result is FAIR, it is suggested to top up the inhibitor level to the prescribed level per supplier instructions. It is suggested to use a area skilled trained within the procedure to perform this activity.
If the outcome obtained is POOR, the recommendation for this state of affairs would suggest that the top consumer continues to make use of the oil “uninhibited,” but this will lead to extra rapid degradation of both the liquid and strong insulation.
It ought to be famous that some transformers already have a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges beneath 1000ppm. This can be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances isn’t required, though it might add further protection 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 steady, (rate of decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and cut back their price of reaction with compounds within the oil. This consists of 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 author 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, common sample intervals could be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it is suggested to remove the oil or take away the source of corrosivity from the oil via special oil treatment.
I) Sediment and sludge
This just isn’t a routine check.
It is suggested that this take a look at is performed when the oil results indicate a high acid worth and the dissipation issue is close to the unacceptable restrict.
The outcomes need to be less than zero.02% by mass to be negligible. If the outcomes return a value of more than zero.02% by mass, it’s suggested that it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
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 rigidity between transformer oil and water reduces during the aging course of. What this implies in sensible terms is there may be extra polar compound present within the oil, lowering the power 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 tension becomes a prime quality criterion: the oil must be modified below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be vital. The extent of the corrosion damage attributable to 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 impact of this compound on the transformer system.
In a research by Doble, it has been found that rubber merchandise used in transformers might 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 threat evaluation research. 
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured using IEC 60970 
m) Flashpoint ° C
Not a routine test
If there’s a maximum decrease in flashpoints by 10%, the equipment might require further inspection. This value would possibly differ in numerous countries.
It is suggested to carry out this check when an uncommon odour is observed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to decide the situation of the transformer; this could be a health and safety impact check. PCB is hazardous to each people and the surroundings; it is vital to check for PCBs after the retro fill of a transformer. It can additionally be required every time any upkeep has been accomplished on the unit, and the potential of contamination is current. If PCB content material exceeds the recommended limits, the suitable motion must be taken.
Units with a PCB content material of more than 50ppm require a fire security 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 protected disposal issued to the gear owner.
Local regulatory our bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of information and interpretation, we will discuss this phenomenon in part II of the article. The limits for the different gases and the interpretation of this knowledge based on worldwide requirements shall be mentioned in detail, forming a half of the overall well being ranking determination of the transformer.
Transformer situation monitoring is an interlaced, extremely exciting subject of research. In this article, we centered on the kinds of tests to determine the condition of the transformer, the critical 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 finest apply utility and optimised maintenance. It also make it simpler to attract up a upkeep plan and action plan.
1. IEC 60422 Edition four.zero 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’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 sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise within the industry, having beforehand worked as laboratory manager for a major industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her profession, significantly within the evaluation of test knowledge. Corné has huge sensible and theoretical knowledge of reliability upkeep programmes.