PETROL CHEMICALS
TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
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Introduction
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant knowledge relating to an asset’s operation. If we interpret the information accurately, it may give us great insight into the asset’s condition.
Frequent monitoring of the asset may end up in less upkeep required or extra extended intervals with none maintenance required.
It is essential to establish the key parameters which might be needed to give us a complete image of the actual standing of the transformer and the action we have to take to ensure the continued reliability of the asset to achieve the maximum lifetime.
What is the information telling us?
Has the condition of the unit modified for the rationale that last maintenance period?
Is it protected to function the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate ranking for a particular period?
Are we required to implement action to ensure the continued reliability of the unit?
How lengthy can we use the unit earlier than we have to contemplate replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring outline
It is vitally important to identify clear objectives as part of your technique. 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 result and what you want to accomplish, it might be a lot easier to identify the required parameters.
Health indexing of belongings is turning into a exceptional device in getting a clearer image 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 on the worldwide requirements for mineral oils, indicating the important values stipulated within the various requirements.
The scope of oil evaluation, interpretation of the info, and important values
At the start of this section, it’s essential to state that we take care of completely different size transformers in the industry. Transformers are divided into lessons in accordance with the kV ratings of the tools. It is up to the reliability or asset supervisor to make use of the guidelines for larger equipment, thereby implementing shorter increment oil analysis and electrical checks.
The maintenance engineer or supervisor wants to discover out what kind of testing would profit him in identifying problem areas inside his fleet. Some of the analysis has been identified as to routine type checks. Still, there’s an intensive vary of tests that may help in figuring out specific downside criteria inside the system, which could not be clear by way of the everyday day-to-day analysis normally performed.
Please see the rating lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and how typically or under which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil sample is likely considered one of the most important and important influencers within the analysis end result. If a pattern isn’t taken to the prescribed process, then there is a vital possibility that the analysis carried out, and the results acquired, are of no use.
Care ought to be taken to ensure 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 applying the proper process is important. A sample can be contaminated by various components, all of which might influence the finish result of the ends in a negative manner.
All steps involved in sample taking must adhere to quality management procedures, including the container that’s used, the sampling equipment, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and dealing with of the pattern, and then the supply of the pattern to the laboratory.
If the sample identification and pattern data are handwritten, the human issue can lead to incorrect interpretation of the data. The label should be caught 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 down on the label should be oil- and water resistant. Otherwise, some information might be lost, making it extraordinarily troublesome for the laboratory personnel to finish the report or allocate the sample to a selected transformer.
Now, let’s discuss the types of tests to discover out the transformer’s situation, the critical values, and the recommended actions in each case:
a) Colour and appearance
This is a routine inspection utilized to each oil pattern.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to determine the colour, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there could be a lot of turbidity, it would indicate a high water content material within the oil.
If the drain valve was not cleaned by the sampler, the filth particles within the drain valve might be included into the pattern. If particles are recognized as carbon, it might point out a possible electrical fault within the unit. The DGA analysis of the oil will verify if that is so.
Clear oils with out contamination will indicate a great situation, and no action is really helpful.
When oils are dark or turbid, additional analysis will confirm any issues. The oil evaluation outcomes may also determine the diploma and kind of motion.
b) Breakdown Voltage
PropertyCategoryGoodFairPoor
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
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of foreign particles, or both 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 have to be excessive.
If the values are Good, it is suggested to proceed with the current pattern interval motion plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with different parameter results just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If various exams point out extreme getting older, the oil could be changed with new or reclaimed oil. Another choice could be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar elements are removed from the oil. This process removes acid and water in addition to some other compounds. Another benefit is that the oil can be re-used, and in most conditions, this might be done with out switching off the unit, which contributes to cost-saving. If in doubt – as a substitute change off the unit during this therapy course of.
If the values are Poor, it’s advisable to take motion as quickly as possible and never delay the maintenance course of. ความหมายของเครื่องวัดความดัน within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content may cause flashover in the unit, leading to loss of the asset.
c) Water content (mg/kg at transformer working temperature)
PropertyCategoryGoodFairPoor
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 courses of electrical gear, besides class G
The results of this check should all the time be thought-about in conjunction with the breakdown energy. If it is found that the water content is high and the breakdown strength is low, additional action must be taken. It is beneficial that a second pattern from the same unit is tested to substantiate the outcomes.
In the case of switching gear, where there is not any paper present, the breakdown voltage is the determining factor.
It should be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between 40 and 70 °C. If it is discovered that the unit’s working temperature is exterior this temperature range, it is best to refer to Annex A of the usual.
When the value obtained via analyses is GOOD, the traditional sampling interval may be maintained, requiring no further motion.
When the value returns a FAIR outcome, more frequent sampling is beneficial. It is also useful to contemplate different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be implemented.
A POOR result will require instant action from the asset manager. This would possibly embrace taking another sample to verify the results from the first evaluation. If it’s confirmed that the water content material is high, the oil may be filtered; this course of should take away a big portion of the moisture from the oil if utilized correctly. Follow-up samples have to be taken to guarantee that the moisture content material remains to be within the required limits. The reason is that essentially the most significant 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 conditions that favour this movement. It could be discovered later that the oil in the water has elevated once more with none obvious reason, but the source can be the paper within the transformer.
A visual inspection can be really helpful to determine if any water would possibly transfer into the transformer or electrical equipment by way of leaks. This problem may be extra extreme if the transformer or electrical tools is exterior and never in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.100.10 to zero.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine check
This is a routine test for all classes except F and G
The acids in oils are shaped due to 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’s going to adversely affect the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this could result in sludge formation, usually across the decrease components of the transformer core. The sludge will ultimately kind a semi-solid substance that’s extremely tough 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 suit the situation. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result is POOR in accordance with the prescribed values in IEC 60422 Edition four.zero, the asset manager could resolve to reclaim the oil or replace it with new or reclaimed oil, whichever choice would possibly go properly with their necessities the best.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C
PropertyCategoryGoodFairPoor
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.a hundred.10 to zero.20>0.20
B, C<0.one hundred.10 to zero.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 courses of electrical tools, except F and G
The dielectric dissipation issue or tan delta of this check supplies data relating to the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating material.
When oil ages, we now have the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that might influence the dissipation factor include 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 really helpful. The construction of the oil is broken, in impact that the chemical bonds between the molecules have damaged down, and even with filtration, the recommended dielectric values can’t be achieved.
f) Resistivity (GΩm) at 20 °C or 90 °C
PropertyCategoryGoodFairPoor
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 certainly one of the key parameters to evaluate the transformer insulation situation; that is based on the truth that DC resistance is sensitive to oil degradation.
When the result’s FAIR, more frequent sampling and checking extra parameters is really helpful.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material percent
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of original value
This check is restricted to oils with this additive.
It could be advisable to contact the oil supplier to confirm the small print concerning components.
The two most common oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a couple of,6-di-tertiary butyl-phenol (DBP). The objective of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the aging process in the oil and the stable insulation.
If the result is FAIR, it is advised to high up the inhibitor level to the prescribed level per supplier directions. It is suggested to use a area skilled skilled within the procedure to carry out this task.
If the end result obtained is POOR, the advice for this situation would suggest that the tip consumer continues to use the oil “uninhibited,” but this may lead to more rapid degradation of each the liquid and strong insulation.
It must be noted that some transformers already have a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges beneath 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 beneath these circumstances is not required, although it might add extra safety in opposition to oil oxidation if the preservation system ever fails. [2]
h) Passivator content material
PropertyCategoryGoodFairPoor
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and secure, (rate of decrease < 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 referred to as metal deactivators, react with reactive metallic surfaces and dissolved metals similar to copper and silver and cut 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 primary sorts, sulfur-based and nitrogen-based. The first instructed 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.[3]
As the oil ages, the passivator may deplete more rapidly; this depletion may accelerate when the oil is un-inhibited.
With GOOD results, common sample intervals can be maintained.
With FAIR outcomes, maintain common monitoring.
When POOR, it’s advised to remove the oil or take away the source of corrosivity from the oil by way of particular oil remedy.
I) Sediment and sludge
This is not a routine check.
It is suggested that this test is performed when the oil outcomes point out a excessive acid value and the dissipation factor is close to the unacceptable limit.
The outcomes need to be less than 0.02% by mass to be negligible. If the results return a price of greater than 0.02% by mass, it’s advised that it’s reclaimed; in any other case, an oil change is really helpful.
j) Interfacial rigidity
This just isn’t a routine check
PropertyCategoryGoodFairPoor
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to 28 20 to 25<22 <20
ENot a routine check
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces in the course of the growing older process. What this means in practical phrases is there is more polar compound current within the oil, decreasing the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial tension becomes a high quality criterion: the oil must be modified below a predefined restrict.
If results are GOOD, proceed 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 isn’t 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 can be significant. The extent of the corrosion injury attributable to the sulfur can be so extreme that it’d trigger failure of the equipment if not checked. The addition of a copper passivator can cut back the impression of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber products used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s risk assessment examine. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine check
If there’s a maximum lower in flashpoints by 10%, the equipment might require further inspection. This worth might differ in numerous nations.
It is advised to carry out this take a look at when an uncommon odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is not to determine the situation of the transformer; this may be a health and safety influence check. PCB is hazardous to each people and the surroundings; it is important to check for PCBs after the retro fill of a transformer. It can additionally be required each time any upkeep has been accomplished on the unit, and the risk of contamination is present. If PCB content material exceeds the beneficial limits, the suitable motion must be taken.
Units with a PCB content of greater than 50ppm require a fire security plan, environmental protection plan, and further precautionary measures when upkeep is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the equipment owner.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we are going to discuss this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this data according to international requirements might be discussed intimately, forming a half of the overall health rating dedication of the transformer.
Conclusion
Transformer situation monitoring is an interlaced, extremely thrilling area of research. In this article, we centered on the forms of tests to determine the situation of the transformer, the critical values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a specific unit at a specific date and time. This makes it attainable to ensure finest practice utility and optimised upkeep. It additionally make it simpler to attract up a maintenance plan and action plan.
References:
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 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 published by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience in the trade, having previously labored as laboratory supervisor for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry proper from the beginning of her career, significantly in the analysis of test data. Corné has vast sensible and theoretical information of reliability maintenance programmes.
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