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 analysis of the related knowledge relating to an asset’s operation. If we interpret the info accurately, it can provide us great insight into the asset’s situation.
Frequent monitoring of the asset may end up in less upkeep required or extra prolonged periods without any maintenance required.
It is essential to identify the key parameters which might be needed to give us an entire image of the actual standing of the transformer and the action we want to take to make sure 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 last maintenance period?
Is it secure to operate the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate rating for a particular period?
Are we required to implement action to make sure the continued reliability of the unit?
How long can we use the unit before we want to contemplate replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring define
It is vitally important to determine clear goals as a part of your strategy. What do you want to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the finish result and what you need to accomplish, it would be much simpler to determine the required parameters.
Health indexing of property is turning into a outstanding software 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 based mostly on the worldwide requirements for mineral oils, indicating the important values stipulated in the various requirements.
The scope of oil evaluation, interpretation of the info, and critical values
At the start of this part, it is important to state that we cope with completely different size transformers in the trade. Transformers are divided into classes according to the kV ratings of the tools. It is up to the reliability or asset manager to make use of the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or supervisor needs to determine what sort of testing would profit him in identifying downside areas inside his fleet. Some of the analysis has been identified as to routine kind exams. Still, there’s an intensive vary of tests that can assist in identifying particular problem criteria within the system, which could not be clear through the standard day-to-day evaluation usually carried out.
Please see the ranking lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and how often or underneath which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil sample is among the most vital and critical influencers within the analysis end result. If a pattern isn’t taken to the prescribed process, then there is a significant risk that the analysis performed, and the outcomes acquired, are of no use.
Care must be taken to make sure quality control procedures are utilized in every step of the sampling course of as set out by international standards. A good quality sample taken by applying the right procedure is important. A sample can be contaminated by various elements, all of which may affect the end result of the ends in a negative manner.
All steps involved in pattern taking must adhere to high quality management procedures, together with the container that is used, the sampling kit, the taking of the pattern, the marking of the pattern, the closing of the sample, the storage and handling of the pattern, and then the supply of the sample to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human issue can lead to incorrect interpretation of the info. The label must 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 should be waterproof, and the pen used to put in writing on the label should be oil- and water resistant. Otherwise, some knowledge may be lost, making it extremely troublesome for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s discuss the kinds of exams to determine the transformer’s situation, the crucial values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection applied to each oil sample.
When an oil pattern arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to discover out the color, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it would indicate a excessive water content material in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles within the drain valve might be integrated into the sample. If particles are identified as carbon, it would indicate a potential electrical fault in the unit. The DGA analysis of the oil will verify if so.
Clear oils without contamination will indicate a great condition, and no action is beneficial.
When oils are darkish or turbid, additional evaluation will confirm any problems. The oil evaluation results 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 application
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of overseas particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage should be excessive.
If the values are Good, it is recommended to continue with the present pattern interval motion plan.
If the values are Fair, more frequent sampling is recommended 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 by way of oil reconditioning processes. If alternative checks point out extreme aging, the oil can be replaced with new or reclaimed oil. Another possibility can be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar components are removed from the oil. This course of removes acid and water in addition to some other compounds. Another advantage is that the oil could be re-used, and in most situations, this can be done with out switching off the unit, which contributes to cost-saving. If doubtful – as an alternative switch off the unit throughout this therapy course of.
If the values are Poor, it is advisable to take motion as soon as attainable and never delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content could cause flashover in the unit, resulting in 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 check
This is a routine check for all classes of electrical gear, except class G
The results of this take a look at should always be considered along side the breakdown strength. If it is discovered that the water content material is excessive and the breakdown power is low, additional action must be taken. It is beneficial that a second sample from the identical unit is tested to confirm the outcomes.
In the case of switching tools, where there is not a paper present, the breakdown voltage is the figuring out factor.
It ought to be noted that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it’s found that the unit’s operating temperature is outside this temperature vary, it’s best to refer to Annex A of the standard.
When the value obtained through analyses is GOOD, the traditional sampling interval could be maintained, requiring no additional action.
When the value returns a FAIR outcome, more frequent sampling is beneficial. It is also useful to suppose about different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the motion to be implemented.
A POOR end result will require immediate motion from the asset manager. This would possibly embody taking one other pattern to verify the results from the primary evaluation. If it’s confirmed that the water content is excessive, the oil may be filtered; this course of should take away a large portion of the moisture from the oil if utilized appropriately. Follow-up samples need to be taken to ensure that the moisture content continues to be throughout the required limits. The cause is that probably the most significant slice of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under situations that favour this movement. It may be found later that the oil in the water has elevated again without any apparent purpose, but the source could be the paper in the transformer.
A visual inspection can be recommended to find out if any water might move into the transformer or electrical gear by way of leaks. This problem may be more extreme if the transformer or electrical gear is exterior and never in a lined 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 zero.25>0.20
C<0.one hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine take a look at for all courses except F and G
The acids in oils are formed 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 have an effect on the insulation properties of the oil and can enhance paper degradation. If left untreated in transformers, this will result in sludge formation, often across the lower parts of the transformer core. The sludge will finally form a semi-solid substance that is extremely troublesome to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR end result, the sampling interval should be decreased to suit the state of affairs. Future evaluation 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 may decide to reclaim the oil or substitute it with new or reclaimed oil, whichever choice might swimsuit 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.one hundred.10 to zero.20>0.20
B, C<0.a hundred.10 to zero.50>0.50
D<0.010.01 to zero.03>0.03
E<0.one hundred.01 to zero.30>0.03
F, GNot a routine test
This is a routine take a look at for all classes of electrical equipment, except F and G
The dielectric dissipation issue or tan delta of this take a look at offers data 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’ve the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that might affect the dissipation issue embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial. The construction of the oil is damaged, in impact that the chemical bonds between the molecules have damaged down, and even with filtration, the really helpful dielectric values can’t be achieved.
f) Resistivity (GΩm) at 20 °C or ninety °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 amongst the key parameters to evaluate the transformer insulation condition; that is primarily based on the reality that DC resistance is sensitive to oil degradation.
When the result’s FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content material percent
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of original worth
This test is restricted to oils with this additive.
It could be advisable to contact the oil supplier to confirm the details concerning components.
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 function of the inhibitor is to stop oxygen from reacting with the oil. This significantly slows the getting older process within the oil and the strong insulation.
If the result’s FAIR, it is suggested to prime up the inhibitor level to the prescribed stage per supplier instructions. It is suggested to make use of a field skilled trained within the process to perform this task.
If the result obtained is POOR, the advice for this scenario would suggest that the end user continues to make use of the oil “uninhibited,” but this will likely lead to more speedy degradation of both the liquid and strong insulation.
It ought to be noted that some transformers have already got a built-in oil preservation system; that is designed to keep dissolved oxygen at ranges under 1000ppm. This can be in the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances is not required, though it’d add further 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 secure, (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 referred to as metallic deactivators, react with reactive steel surfaces and dissolved metals corresponding to copper and silver and scale back their price of reaction with compounds in the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental varieties, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the author 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 might deplete more rapidly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, regular pattern intervals could be maintained.
With FAIR results, preserve common monitoring.
When POOR, it is advised to remove the oil or take away the supply of corrosivity from the oil by way of special oil treatment.
I) Sediment and sludge
This is not a routine test.
เกจ์วัดแรงดัน is suggested that this check is performed when the oil results point out a high acid value and the dissipation issue is close to the unacceptable limit.
The results must be less than zero.02% by mass to be negligible. If the outcomes return a worth of more than 0.02% by mass, it’s instructed that or not it’s reclaimed; otherwise, 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 tension between transformer oil and water reduces during the growing older course of. What this implies in practical terms is there is extra polar compound current in the oil, decreasing the power of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial pressure becomes a high quality criterion: the oil have to be changed under a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If results 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 can be significant. The extent of the corrosion injury brought on by the sulfur could be so severe that it would cause failure of the equipment 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 found that rubber products used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s danger assessment examine. [4]
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 [5]
m) Flashpoint ° C
Not a routine check
If there’s a maximum lower in flashpoints by 10%, the gear would possibly require further inspection. This value may differ in several countries.
It is suggested to perform this test when an unusual odour is observed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to determine the situation of the transformer; this is a well being and security impact check. PCB is hazardous to each humans and the surroundings; it’s critical to test for PCBs after the retro fill of a transformer. It can be required each time any upkeep has been carried out on the unit, and the potential for contamination is current. If PCB content material exceeds the beneficial limits, the suitable motion needs to be taken.
Units with a PCB content of more than 50ppm require a hearth safety plan, environmental protection plan, and additional precautionary measures when maintenance is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of protected disposal issued to the tools owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we are going to discuss this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this knowledge based on international requirements shall be mentioned intimately, forming part of the overall well being ranking determination of the transformer.
Conclusion
Transformer situation monitoring is an interlaced, highly exciting subject of research. In this text, we focused on the kinds of tests to discover out the condition 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 particular date and time. This makes it potential to make sure finest follow application and optimised upkeep. It also make it easier to attract up a upkeep plan and action plan.
References:
1. IEC 60422 Edition 4.0 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
3. Passivators, what they’re and how 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 strength.”
6. Article initially printed 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 within the trade, having previously worked as laboratory manager for a major industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry right from the beginning of her career, particularly within the evaluation of test data. Corné has vast practical and theoretical data of reliability maintenance programmes.
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