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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 related knowledge regarding an asset’s operation. If we interpret the information correctly, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset may end up in much less maintenance required or more extended intervals without any upkeep required.
It is crucial to determine the key parameters that are wanted to offer us an entire image of the particular status of the transformer and the motion we want to take to ensure the continued reliability of the asset to achieve the maximum lifetime.
What is the info telling us?
Has the situation of the unit modified because the final maintenance period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate score for a selected period?
Are we required to implement motion to make sure the continued reliability of the unit?
How lengthy can we use the unit earlier than we have to consider replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring define
It is vitally necessary to establish clear objectives as part of your technique. What do you need to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the end result and what you wish to accomplish, it might be a lot simpler to determine the required parameters.
Health indexing of property is changing 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 have been calculated primarily based on the worldwide requirements for mineral oils, indicating the important values stipulated within the various standards.
The scope of oil analysis, interpretation of the info, and critical values
At the start of this section, it is essential to state that we deal with different dimension transformers within the trade. Transformers are divided into lessons based on the kV ratings of the gear. It is up to the reliability or asset manager to use the rules for bigger tools, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or manager wants to discover out what type of testing would profit him in figuring out downside areas inside his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there is an extensive range of checks that may help in identifying particular drawback standards within the system, which might not be clear through the standard day-to-day evaluation normally 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 beneath which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil sample is likely considered one of the most vital and critical influencers in the analysis outcome. If a sample is not taken to the prescribed process, then there’s a vital possibility that the analysis carried out, and the outcomes acquired, are of no use.
Care must be taken to ensure high quality control procedures are utilized in each step of the sampling course of as set out by international standards. A good high quality sample taken by making use of the correct procedure is crucial. A pattern could be contaminated by varied components, all of which might influence the outcome of the leads to a adverse manner.
All steps involved in sample taking should adhere to high quality control procedures, together with the container that is used, the sampling package, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and handling of the sample, after which the delivery of the pattern to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human issue can result in incorrect interpretation of the info. The label should be stuck onto the container to ensure no mix-ups in sample identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to write down on the label must be oil- and waterproof. Otherwise, some information might be misplaced, making it extraordinarily difficult for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s focus on the forms of checks to determine the transformer’s situation, the important values, and the beneficial actions in each case:
a) Colour and look
This is a routine inspection applied to every oil pattern.
When an oil pattern arrives on the laboratory, one of the “tests” is a visual inspection of the oil sample in a clear vessel to find out the colour, turbidity, and attainable particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there might be เกวัดแรงดัน 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 dirt particles in the drain valve might be included into the pattern. If particles are identified as carbon, it might point out a potential electrical fault within the unit. The DGA analysis of the oil will affirm if so.
Clear oils without contamination will indicate a good condition, and no motion is beneficial.
When oils are dark or turbid, additional evaluation will affirm any issues. The oil analysis outcomes will also decide the degree and kind of action.
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 utility
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of international particles, or both in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage should be high.
If the values are Good, it is strongly recommended to continue with the current pattern interval motion plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with other parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil through oil reconditioning processes. If different exams indicate extreme aging, the oil could be changed with new or reclaimed oil. หลักการทํางานของpressuregauge can be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water as well as some other compounds. Another advantage is that the oil may be re-used, and in most situations, this may be accomplished with out switching off the unit, which contributes to cost-saving. If unsure – as a substitute switch off the unit throughout this remedy process.
If the values are Poor, it’s advisable to take action as soon as possible and not delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content may cause flashover within the unit, leading to lack of the asset.
c) Water content material (mg/kg at transformer operating 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 check
This is a routine test for all courses of electrical equipment, besides class G
The results of this check should all the time be thought of along side the breakdown power. If it’s discovered that the water content material is excessive and the breakdown strength is low, additional action must be taken. It is really helpful that a second pattern from the same unit is examined to verify the results.
In the case of switching equipment, the place there is not a paper present, the breakdown voltage is the figuring out issue.
It ought to be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is exterior this temperature vary, it’s best to discuss with Annex A of the usual.
When the worth obtained by way of analyses is GOOD, the traditional sampling interval can be maintained, requiring no additional motion.
When the worth returns a FAIR end result, extra frequent sampling is beneficial. It is also helpful to assume about other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the action to be applied.
A POOR result would require immediate action from the asset manager. This might embody taking one other sample to substantiate the outcomes from the primary evaluation. If it is confirmed that the water content is excessive, the oil could be filtered; this course of ought to take away a big portion of the moisture from the oil if utilized accurately. Follow-up samples need to be taken to make certain that the moisture content material continues to be throughout the required limits. The reason is that 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 under circumstances that favour this motion. It could be discovered later that the oil in the water has increased once more with none obvious reason, but the source can be the paper in the transformer.
A visual inspection is also beneficial to determine if any water might transfer into the transformer or electrical gear through leaks. This downside might be more severe if the transformer or electrical gear is outside and not in a coated space.
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<0.100.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine take a look at for all classes besides F and G
The acids in oils are formed due to chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist within the formation of the acids. Because acids are polar compounds, it’s going to adversely affect the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this will lead to sludge formation, usually across the decrease elements of the transformer core. The sludge will finally type a semi-solid substance that’s extraordinarily tough to remove.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, 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 according to the prescribed values in IEC 60422 Edition 4.zero, the asset manager might resolve to reclaim the oil or replace it with new or reclaimed oil, whichever choice might go nicely with 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 zero.20>0.20
B, C<0.one hundred.10 to zero.50>0.50
D<0.010.01 to zero.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 courses of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this test supplies info relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that might influence the dissipation issue embody water, dissolved insulating resin, and paper.
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 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 doubtless certainly one of the key parameters to evaluate the transformer insulation situation; that is primarily based on the reality that DC resistance is delicate to oil degradation.
When the result is FAIR, more frequent sampling and checking extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material %
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of authentic worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to confirm the primary points concerning additives.
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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the aging process within the oil and the strong insulation.
If the result’s FAIR, it’s advised to top up the inhibitor stage to the prescribed degree per provider directions. It is advised to make use of a subject professional skilled within the process to carry out this task.
If the result obtained is POOR, the recommendation for this scenario would recommend that the top person continues to make use of the oil “uninhibited,” however this will result in more speedy degradation of both the liquid and solid insulation.
It ought to be famous that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at levels below 1000ppm. This would be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, although it’d add extra safety 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 stable, (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 metallic deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and reduce their fee of reaction with compounds in the oil. This includes oxidation reactions with organic 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 conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator might deplete more quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals may be maintained.
With FAIR results, maintain common monitoring.
When POOR, it’s suggested to remove the oil or take away the source of corrosivity from the oil via particular oil treatment.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is advised that this check is carried out when the oil outcomes point out a excessive acid worth and the dissipation factor is near the unacceptable limit.
The outcomes must be less than zero.02% by mass to be negligible. If the results return a worth of more than 0.02% by mass, it’s suggested that or not it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial tension
This isn’t a routine test
PropertyCategoryGoodFairPoor
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 growing older course of. What this implies in sensible phrases is there’s more polar compound current in 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 pressure turns into a quality criterion: the oil must be modified beneath a predefined limit.
If outcomes are GOOD, continue the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This 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 impact on the transformer system could be vital. The extent of the corrosion harm caused by the sulfur could be so extreme that it might cause failure of the gear if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a examine by Doble, it has been found that rubber products used in transformers may 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 need of corrective actions based on this institute’s danger evaluation examine. [4]
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 [5]
m) Flashpoint ° C
Not a routine test
If there’s a most decrease in flashpoints by 10%, the gear would possibly require additional inspection. This value may differ in numerous countries.
It is advised to perform this check when an unusual odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not determine the situation of the transformer; it is a well being and security influence take a look at. PCB is hazardous to both people and the environment; it is important to test for PCBs after the retro fill of a transformer. It is also required each time any upkeep has been accomplished on the unit, and the potential for contamination is present. If PCB content exceeds the beneficial limits, the suitable motion needs to be taken.
Units with a PCB content of more than 50ppm require a hearth security plan, environmental protection plan, and additional precautionary measures when upkeep is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment owner.
Local regulatory our bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we’ll focus on this phenomenon in part II of the article. The limits for the totally different gases and the interpretation of this data based on worldwide requirements might be discussed intimately, forming part of the general health ranking dedication of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, highly thrilling field of examine. In this text, we focused on the forms of exams to determine the condition of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a selected date and time. This makes it attainable to ensure best follow software and optimised maintenance. It also make it easier to attract up a upkeep plan and action plan.
References:
1. IEC 60422 Edition four.zero 2013-01 International Standard (Mineral insulating oils in electrical equipment – supervision and upkeep 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 energy.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise in the trade, having beforehand worked as laboratory manager for a major industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry right from the beginning of her profession, particularly within the evaluation of check knowledge. Corné has huge sensible and theoretical data of reliability maintenance programmes.
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