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 related knowledge regarding an asset’s operation. If we interpret the information correctly, it may give us great insight into the asset’s condition.
Frequent monitoring of the asset may end up in much less upkeep required or extra extended intervals with none upkeep required.
It is crucial to establish the necessary thing parameters that are wanted to offer us a complete image of the actual status of the transformer and the action we have to take to ensure the continued reliability of the asset to achieve the utmost lifetime.
What is the info telling us?
Has the situation of the unit modified for the rationale that final upkeep 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 motion to make sure the continued reliability of the unit?
How long can we use the unit before we have to think about replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring define
It is vitally essential to determine clear objectives as a part of your strategy. What do you need to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it will be much easier to establish the required parameters.
Health indexing of property is becoming a outstanding tool in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the total value of the Health Index Value of the transformer. These parameter weight values had been calculated based mostly on the international standards for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the info, and significant values
At the beginning of this section, it’s essential to state that we take care of completely different size transformers within the industry. Transformers are divided into lessons in accordance with the kV rankings of the gear. It is up to the reliability or asset manager to make use of the rules for larger equipment, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or manager wants to determine what kind of testing would profit him in identifying drawback areas within his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there is an intensive vary of checks that may help in identifying particular downside criteria throughout the system, which could not be clear by way of the standard day-to-day analysis normally carried out.
Please see the score lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and the way usually or under which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil sample is one of the most vital and significant influencers in the analysis consequence. If a pattern just isn’t taken to the prescribed process, then there’s a significant possibility that the analysis performed, and the results acquired, are of no use.
Care ought to be taken to ensure quality control procedures are utilized in each step of the sampling process as set out by international requirements. A good quality sample taken by applying the correct procedure is essential. A sample may be contaminated by varied factors, all of which can affect the result of the ends in a adverse manner.
All steps involved in sample taking should adhere to high quality management procedures, including the container that is 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 sample, after which the delivery of the pattern to the laboratory.
If the sample identification and pattern data are handwritten, the human factor can lead to incorrect interpretation of the data. The label must be stuck onto the container to ensure no mix-ups in pattern identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. เครื่องมือที่ใช้วัดความดันโลหิต should be waterproof, and the pen used to put in writing on the label must be oil- and waterproof. Otherwise, some knowledge might be lost, making it extraordinarily tough for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s talk about the types of checks to discover out the transformer’s condition, the critical values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection applied to each oil sample.
When an oil pattern arrives at the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to find out the colour, turbidity, and attainable particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there is lots of turbidity, it might point out a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the filth particles in the drain valve could be incorporated into the pattern. If particles are recognized as carbon, it would indicate a attainable electrical fault in the unit. The DGA evaluation of the oil will affirm if so.
Clear oils without contamination will point out a good situation, and no motion is really helpful.
When oils are darkish or turbid, additional analysis will verify any problems. The oil evaluation outcomes will also decide the diploma and type 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 point application<40 kV for OLTC in delta or line-end application
G <30
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of international particles, or each in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage must be excessive.
If the values are Good, it is suggested to proceed with the current pattern interval action plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If various exams indicate extreme growing older, the oil may be changed with new or reclaimed oil. Another choice could be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar parts are removed from the oil. This process removes acid and water as nicely as another compounds. Another benefit is that the oil may be re-used, and in most conditions, this could be carried out with out switching off the unit, which contributes to cost-saving. If unsure – instead switch off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as soon as potential and not delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content could cause flashover within 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 courses of electrical equipment, besides class G
The results of this take a look at should all the time be thought-about in conjunction with the breakdown energy. If it is found that the water content material is high and the breakdown strength is low, additional motion must be taken. It is recommended that a second sample from the identical unit is tested to confirm the results.
In the case of switching tools, the place there isn’t any paper current, the breakdown voltage is the determining factor.
It should be noted that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it’s found that the unit’s working temperature is outdoors this temperature vary, it is best to refer to Annex A of the usual.
When the value obtained via analyses is GOOD, the normal sampling interval could be maintained, requiring no additional action.
When the worth returns a FAIR result, more frequent sampling is beneficial. It can be useful to consider other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR outcome would require immediate action from the asset supervisor. This might embody taking one other pattern to substantiate the outcomes from the primary analysis. If it’s confirmed that the water content is excessive, the oil may be filtered; this course of ought to take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples have to be taken to guarantee that the moisture content material is still inside the required limits. The cause is that the most good portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil underneath conditions that favour this movement. It could be found later that the oil in the water has increased again without any apparent cause, however the supply would be the paper within the transformer.
A visual inspection is also beneficial to determine if any water might transfer into the transformer or electrical tools through leaks. This downside might be extra severe if the transformer or electrical tools is exterior and not in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.one hundred.10 to 0.25>0.20
C<0.one hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine check for all courses besides F and G
The acids in oils are shaped as a result of 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’ll adversely have an effect on the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, usually across the lower elements of the transformer core. The sludge will eventually form a semi-solid substance that is extremely troublesome to remove.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval must be decreased to fit the situation. Future analysis ought to embrace a visual inspection of the oil for sediment and sludge.
If the result’s POOR according to the prescribed values in IEC 60422 Edition four.0, the asset supervisor could decide to reclaim the oil or exchange it with new or reclaimed oil, whichever option may swimsuit their necessities the most effective.
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.a hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.one hundred.01 to zero.30>0.03
F, GNot a routine check
This is a routine test for all courses of electrical gear, besides F and G
The dielectric dissipation issue or tan delta of this take a look at 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’ve the formation of polar compounds, leading to phase displacement and dielectric losses. Other impurities which may affect the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking further parameters is beneficial.
When the result’s POOR, reclamation or an oil change is really helpful. The structure of the oil is damaged, in effect 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 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 take a look at
DC resistivity of the oil is one of the key parameters to assess the transformer insulation condition; this is primarily based on the fact that DC resistance is delicate to grease degradation.
When the result’s FAIR, more frequent sampling and checking extra parameters is beneficial.
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 unique value<40% of unique value
This test is restricted to oils with this additive.
It could be advisable to contact the oil supplier to verify the small print regarding 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 function of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the getting older course of in the oil and the solid insulation.
If the result’s FAIR, it’s suggested to high up the inhibitor stage to the prescribed stage per supplier directions. It is advised to make use of a field skilled educated within the procedure to perform this activity.
If the end result obtained is POOR, the recommendation for this scenario would counsel that the tip consumer continues to make use of the oil “uninhibited,” but this may lead to extra speedy degradation of both the liquid and stable insulation.
It should be famous that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges under 1000ppm. This can be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances just isn’t required, although it might add additional safety towards 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 lower of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and cut back their rate of reaction with compounds within 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 suggested use of passivators in transformer oil, of which the creator 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 would possibly deplete extra quickly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD results, regular sample intervals can be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it is suggested to remove the oil or remove the supply of corrosivity from the oil via special oil therapy.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this check is performed when the oil outcomes point out a excessive acid value and the dissipation factor is close to the unacceptable restrict.
The results must be less than 0.02% by mass to be negligible. If the outcomes return a price of more than 0.02% by mass, it is advised that it be reclaimed; in any other case, an oil change is beneficial.
j) Interfacial pressure
This isn’t 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 take a look at
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces in the course of the getting older course of. What this means in sensible terms is there is extra polar compound current within the oil, lowering the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial tension becomes a high quality criterion: the oil have to be modified below a predefined limit.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system can be significant. The extent of the corrosion harm brought on by the sulfur may be so severe that it’d trigger failure of the tools if not checked. The addition of a copper passivator can reduce the influence of this compound on the transformer system.
In a study by Doble, it has been found that rubber products used in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s threat assessment study. [4]
l) Particle counting and sizing
Table 3: 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 take a look at
If there’s a maximum lower in flashpoints by 10%, the tools might require further inspection. This value may differ in numerous nations.
It is suggested to carry out this check when an uncommon odour is noticed, the unit has been refilled, or an inner 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 security impact take a look at. PCB is hazardous to each people and the environment; it is important to check for PCBs after the retro fill of a transformer. It can additionally be required whenever any upkeep has been accomplished on the unit, and the potential of contamination is current. If PCB content material exceeds the beneficial limits, the appropriate action needs to be taken.
Units with a PCB content material of more than 50ppm require a hearth security plan, environmental protection plan, and additional precautionary measures when upkeep is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate 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 plenty of information and interpretation, we’ll talk about this phenomenon in part II of the article. The limits for the totally different gases and the interpretation of this information in accordance with worldwide requirements shall be discussed intimately, forming part of the general health rating willpower of the transformer.
Conclusion
Transformer situation monitoring is an interlaced, highly exciting area of examine. In this article, we targeted on the kinds of checks to determine the situation of the transformer, the important values, and the really helpful actions.
The Health Index indication makes it possible to see the supposed reliability of a particular unit at a specific date and time. This makes it possible to ensure greatest apply application and optimised maintenance. It additionally make it easier to draw up a maintenance plan and motion plan.
References:
1. IEC 60422 Edition 4.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 the way 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 published 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’ experience within the trade, having previously worked as laboratory supervisor for a major industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, significantly in the analysis of test information. Corné has huge practical and theoretical knowledge of reliability upkeep programmes.
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