PETROL CHEMICALS
TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
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Introduction
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the relevant knowledge relating to an asset’s operation. If we interpret the data accurately, it may give us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to less upkeep required or more extended durations with none upkeep required.
It is crucial to identify the important thing parameters which would possibly be needed to provide us an entire image of the actual standing of the transformer and the action we have to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the info telling us?
Has the condition of the unit changed because the final upkeep period?
Is it protected to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate ranking for a specific period?
Are we required to implement motion to make sure the continued reliability of the unit?
How long can we use the unit earlier than we need to think about replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring outline
It is vitally important to establish clear goals as part of your technique. What do you wish to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the outcome and what you want to accomplish, it would be a lot simpler to identify the required parameters.
Health indexing of property is turning into a exceptional tool in getting a clearer image of the condition 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 had been calculated primarily based on the international standards for mineral oils, indicating the important values stipulated within the various requirements.
The scope of oil analysis, interpretation of the information, and important values
At the start of this section, it is important to state that we take care of completely different dimension transformers in the industry. Transformers are divided into courses according to the kV scores of the equipment. It is as a lot as the reliability or asset manager to make use of the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager needs to determine what kind of testing would profit him in figuring out problem areas within his fleet. Some of the analysis has been identified as to routine type checks. Still, there’s an extensive vary of exams that may help in figuring out specific drawback criteria inside the system, which could not be clear via the standard day-to-day analysis often performed.
Please see the ranking courses 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
Oil sampling
The taking of the oil pattern is one of the most important and critical influencers in the evaluation end result. If a pattern is not taken to the prescribed process, then there’s a vital possibility that the evaluation carried out, and the outcomes acquired, are of no use.
Care should be taken to ensure quality management procedures are utilized in each step of the sampling process as set out by worldwide standards. A good high quality sample taken by applying the proper process is essential. A pattern can be contaminated by numerous components, all of which might influence the result of the ends in a adverse manner.
All steps involved in pattern taking must adhere to quality control procedures, including 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 dealing with of the pattern, and then the supply of the sample to the laboratory.
If the sample identification and sample data are handwritten, the human issue can result in incorrect interpretation of the data. 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 water-resistant. Otherwise, some information may be lost, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s focus on the kinds of checks to find out 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 pattern.
When an oil sample arrives at the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to determine the color, turbidity, and potential particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there may be plenty of turbidity, it might indicate a high water content in the oil.
If the drain valve was not cleaned by the sampler, the filth particles within the drain valve may be incorporated into the pattern. If particles are recognized as carbon, it’d point out a potential electrical fault in the unit. The DGA evaluation of the oil will affirm if so.
Clear oils with out contamination will indicate a good situation, and no motion is recommended.
When oils are dark or turbid, further analysis will verify any issues. The oil evaluation outcomes may even decide the degree 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 software
G <30
This is a routine inspection.
Breakdown voltage will point out the water content or the presence of foreign particles, or both in 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 should be high.
If the values are Good, it is recommended to proceed with the current sample interval motion plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If various tests indicate extreme getting older, the oil may be changed with new or reclaimed oil. Another possibility 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 elements are removed from the oil. This process removes acid and water in addition to some other compounds. Another benefit is that the oil could be re-used, and in most conditions, this can be accomplished without switching off the unit, which contributes to cost-saving. If in doubt – as a substitute switch off the unit during this therapy process.
If the values are Poor, it is advisable to take action as quickly as attainable and not delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content could cause flashover in the unit, leading to lack of the asset.
c) Water content material (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 check for all courses of electrical gear, besides class G
The outcomes of this test ought to always be thought of in conjunction with the breakdown power. If it’s discovered that the water content material is high and the breakdown power is low, further motion needs to be taken. It is beneficial that a second sample from the identical unit is tested to substantiate the results.
In the case of switching tools, where there isn’t any paper current, the breakdown voltage is the determining issue.
It ought to be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is outdoors 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 could be maintained, requiring no additional action.
When the value returns a FAIR outcome, extra frequent sampling is recommended. It can also be useful to contemplate different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the motion to be implemented.
A POOR result would require instant motion from the asset supervisor. This might embody taking one other sample to confirm the results from the first evaluation. If it’s confirmed that the water content is high, the oil may be filtered; this course of should take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples need to be taken to make sure that the moisture content material remains to be inside the required limits. The cause is that the most significant slice of the water is caught up within the paper system within the transformer. This moisture will transfer from the paper into the oil underneath circumstances that favour this motion. It could be discovered later that the oil within the water has increased once more with none obvious purpose, but the source could be the paper in the transformer.
A visual inspection can additionally be recommended to determine if any water might move into the transformer or electrical gear by way of leaks. This downside might be extra severe if the transformer or electrical gear is exterior and not in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.15>0.15
B, E<0.one hundred.10 to 0.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine check for all classes except F and G
The acids in oils are formed as a outcome 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’s going to adversely affect the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this can lead to sludge formation, normally around the decrease parts of the transformer core. The sludge will ultimately form a semi-solid substance that is extraordinarily tough to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR end result, the sampling interval must be decreased to suit the scenario. Future evaluation should include a visible 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 may decide to reclaim the oil or substitute it with new or reclaimed oil, whichever choice may suit their requirements one of 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 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 take a look at for all lessons of electrical gear, besides F and G
The dielectric dissipation issue or tan delta of this take a look at offers info regarding the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities which may affect the dissipation issue embody water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result’s POOR, reclamation or an oil change is recommended. The construction of the oil is broken, in effect 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
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 doubtless considered one of the key parameters to assess the transformer insulation situation; this is based on the truth that DC resistance is delicate to oil degradation.
When the result’s FAIR, extra frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content %
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of unique worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to verify the primary points concerning additives.
The two commonest 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 process in the oil and the solid insulation.
If the result’s FAIR, it’s suggested to top up the inhibitor degree to the prescribed level per provider directions. ที่วัดแรงดัน is suggested to make use of a area skilled educated within the process to carry out this process.
If the outcome obtained is POOR, the recommendation for this situation would counsel that the tip person continues to make use of the oil “uninhibited,” but this may result in extra rapid degradation of each the liquid and strong insulation.
It ought to be noted that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels below 1000ppm. This can 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 underneath these circumstances isn’t required, though it’d add additional protection against 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 steady, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive metal surfaces and dissolved metals similar to 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 basic sorts, 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. ร้านซ่อมเครื่องวัดความดันโลหิต and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator may deplete extra rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD outcomes, common sample intervals may be maintained.
With FAIR outcomes, keep regular monitoring.
When POOR, it’s advised to take away the oil or take away the source of corrosivity from the oil by way of special oil treatment.
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 results indicate a excessive acid value and the dissipation issue is near the unacceptable limit.
The results have to be less than 0.02% by mass to be negligible. If the outcomes return a price of more than zero.02% by mass, it is instructed that it’s reclaimed; in any other case, an oil change is beneficial.
j) Interfacial rigidity
This is not a routine check
PropertyCategoryGoodFairPoor
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 tension between transformer oil and water reduces during the aging course of. What this implies in practical phrases is there’s extra polar compound current in the oil, reducing the ability of an oil to serve as an insulator in 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 beneath a predefined restrict.
If outcomes are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine take a look at.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system can be important. The extent of the corrosion injury brought on by the sulfur could 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 study by Doble, it has been discovered that rubber products utilized in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based mostly on this institute’s risk assessment examine. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (particles) encountered on power transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine check
If there is a most decrease in flashpoints by 10%, the tools would possibly require further inspection. This value might differ in different international locations.
It is suggested to carry out this take a look at when an unusual odour is noticed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not determine the condition of the transformer; this is a health and security impact take a look at. PCB is hazardous to each humans and the setting; it is vital to check for PCBs after the retro fill of a transformer. It can also be required whenever any upkeep has been done on the unit, and the potential of contamination is present. If PCB content material exceeds the beneficial limits, the suitable motion must be taken.
Units with a PCB content material of greater than 50ppm require a fire safety plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of safe 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 lots of data and interpretation, we are going to talk about this phenomenon in part II of the article. The limits for the totally different gases and the interpretation of this data according to worldwide standards will be discussed intimately, forming part of the overall well being rating dedication of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, highly thrilling field of study. In this article, we centered on the kinds of tests to discover out 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 particular unit at a specific date and time. This makes it attainable to ensure finest follow application and optimised maintenance. It also make it simpler to attract up a maintenance plan and action plan.
References:
1. IEC 60422 Edition four.zero 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 how they work, by Lance Lewand, Doble Engineering Company.
four. 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 printed by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise within the business, having previously worked as laboratory supervisor for a major industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her career, notably in the evaluation of take a look at data. Corné has huge practical and theoretical knowledge of reliability upkeep programmes.
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