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 relevant knowledge concerning an asset’s operation. If we interpret the data accurately, it can give us great insight into the asset’s situation.
Frequent monitoring of the asset may find yourself in less maintenance required or more extended periods without any maintenance required.
It is crucial to identify the important thing parameters which may be wanted to provide us a complete image of the particular status of the transformer and the motion 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 for the rationale that final upkeep period?
Is it safe to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate ranking for a selected period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit before we need to contemplate replacement?
Are the recognized issues of a recurring nature?
Effective situation monitoring outline
It is vitally important to establish clear targets as part of your technique. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you want to accomplish, it would be a lot simpler to establish the required parameters.
Health indexing of property is turning into a exceptional device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the whole value of the Health Index Value of the transformer. These parameter weight values had been calculated primarily based on the worldwide standards for mineral oils, indicating the important values stipulated in the various standards.
The scope of oil evaluation, interpretation of the info, and critical values
At the start of this part, it is essential to state that we deal with completely different size transformers in the business. Transformers are divided into lessons according to the kV ratings of the tools. It is as a lot as the reliability or asset manager to use the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or supervisor wants to discover out what type of testing would profit him in figuring out drawback areas inside his fleet. Some of the analysis has been recognized as to routine kind checks. Still, there’s an intensive vary of exams that can help in figuring out particular downside criteria inside the system, which might not be clear through the everyday day-to-day analysis usually carried out.
Please see the score classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way often or beneath which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is among the most important and critical influencers in the analysis end result. If a sample is not taken to the prescribed process, then there is a significant risk that the analysis carried out, and the results acquired, are of no use.
Care must be taken to make sure high quality control procedures are utilized in every step of the sampling process as set out by international standards. A good quality sample taken by applying the proper procedure is essential. A sample could be contaminated by various components, all of which might influence the result of the ends in a unfavorable manner.
All steps involved in sample taking should adhere to high quality management procedures, together with the container that is used, the sampling equipment, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and handling of the sample, and then the supply of the pattern to the laboratory.
If the pattern identification and sample knowledge are handwritten, the human factor can lead to incorrect interpretation of the data. The label should be caught onto the container to ensure no mix-ups in pattern identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write on the label should be oil- and water resistant. Otherwise, some information may be misplaced, making it extremely troublesome for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s focus on the kinds of exams to discover out the transformer’s condition, the important values, and the beneficial actions in every case:
a) Colour and appearance
This is a routine inspection applied to each oil sample.
When an oil sample arrives at the laboratory, one of the “tests” is a visible inspection of the oil pattern in a transparent vessel to discover out the color, turbidity, and possible particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there could be a lot of turbidity, it’d point out a high water content material within the oil.
If the drain valve was not cleaned by the sampler, the filth particles in the drain valve may be included into the sample. If particles are identified as carbon, it’d indicate a possible electrical fault in the unit. The DGA evaluation of the oil will verify if this is the case.
Clear oils without contamination will indicate a good condition, and no action is recommended.
When oils are dark or turbid, further evaluation will confirm any issues. The oil evaluation results may 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 point 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 overseas particles, or each 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 current sample interval motion plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with different parameter outcomes just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If alternative exams indicate severe getting older, the oil may be changed with new or reclaimed oil. Another option can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar components 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 unsure – instead switch off the unit during this therapy course of.
If the values are Poor, it’s advisable to take action as quickly as possible and never delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content can cause flashover in the unit, leading to lack of the asset.
c) Water content (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 test
This is a routine take a look at for all courses of electrical tools, except class G
The results of this take a look at ought to always be thought of at the aspect of the breakdown strength. If it’s found that the water content is high and the breakdown power is low, further motion must be taken. It is really helpful that a second sample from the identical unit is tested to substantiate the results.
In the case of switching equipment, the place there isn’t a paper current, the breakdown voltage is the determining issue.
It must be famous that the boundaries 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 exterior this temperature range, it is best to check with Annex A of the standard.
When the value obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no further action.
When the worth returns a FAIR result, extra frequent sampling is recommended. It is also useful to assume about other parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the action to be applied.
A POOR end result would require immediate motion from the asset manager. This would possibly include taking another pattern to substantiate the results from the primary analysis. If it is confirmed that the water content is excessive, the oil could be filtered; this process ought to remove a big portion of the moisture from the oil if applied accurately. Follow-up samples need to be taken to ensure that the moisture content is still throughout the required limits. The reason is that essentially the most significant portion of the water is caught up within the paper system within the transformer. เกจวัดแรงดันปั๊มลม will transfer from the paper into the oil underneath circumstances that favour this movement. It might be discovered later that the oil in the water has increased again without any apparent cause, however the source would be the paper in the transformer.
A visual inspection can be really helpful to determine if any water would possibly transfer into the transformer or electrical tools by way of leaks. This drawback could be extra extreme if the transformer or electrical gear is outdoors 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.a hundred.10 to zero.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine take a look at for all classes except F and G
The acids in oils are formed 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 improve paper degradation. If left untreated in transformers, this will lead to sludge formation, normally across the lower components of the transformer core. The sludge will eventually type a semi-solid substance that’s extraordinarily tough to remove.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR result, the sampling interval must be decreased to fit the scenario. Future analysis ought to embody a visual inspection of the oil for sediment and sludge.
If the result is POOR in accordance with the prescribed values in IEC 60422 Edition four.0, the asset supervisor may resolve to reclaim the oil or replace it with new or reclaimed oil, whichever choice would possibly swimsuit their requirements the best.
e) Dielectric Dissipation issue 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 zero.03>0.03
E<0.one hundred.01 to zero.30>0.03
F, GNot a routine take a look at
This is a routine test for all lessons of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this check provides information concerning 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, resulting in phase displacement and dielectric losses. Other impurities that might affect the dissipation factor include water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is recommended. The construction of the oil is broken, 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 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 probably certainly one of the key parameters to assess the transformer insulation situation; that is based mostly on the fact that DC resistance is sensitive to oil degradation.
When the result is FAIR, more frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material %
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of authentic worth
This test is restricted to oils with this additive.
It could be advisable to contact the oil supplier to confirm the major points concerning components.
The two commonest 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 function of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the getting older course of in the oil and the solid insulation.
If the result is FAIR, it is advised to top up the inhibitor level to the prescribed degree per provider instructions. It is suggested to use a subject skilled trained within the process to carry out this process.
If the end result obtained is POOR, the advice for this state of affairs would suggest that the end consumer continues to make use of the oil “uninhibited,” but this will likely 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 maintain dissolved oxygen at ranges beneath 1000ppm. This can be within the type 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 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 steady, (rate of lower < 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 steel deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and reduce their price of response with compounds within the oil. This includes oxidation reactions with natural 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 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 may deplete extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals could be maintained.
With FAIR outcomes, maintain common monitoring.
When POOR, it is advised to remove the oil or remove the source of corrosivity from the oil by way of special oil treatment.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this check is performed when the oil results point out a excessive acid worth and the dissipation factor is near the unacceptable restrict.
The results must be less than 0.02% by mass to be negligible. If the results return a value of more than 0.02% by mass, it’s instructed that or not it’s reclaimed; in any other case, an oil change is beneficial.
j) Interfacial tension
This isn’t a routine test
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 getting older course of. What this implies in practical terms is there is more polar compound current within the oil, lowering the power of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial pressure turns into a high quality criterion: the oil should be modified below a predefined restrict.
If results are GOOD, proceed the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine test.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be important. The extent of the corrosion damage caused by the sulfur may be so severe that it would cause failure of the gear if not checked. The addition of a copper passivator can scale back the impact of this compound on the transformer system.
In a study by Doble, it has been discovered that rubber products utilized in transformers might 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 necessity of corrective actions based on this institute’s danger evaluation study. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine test
If there’s a maximum lower in flashpoints by 10%, the equipment would possibly require further inspection. This worth may differ in several countries.
It is advised to perform this take a look at when an unusual odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to determine the condition of the transformer; this is a well being and safety impact check. PCB is hazardous to each humans and the setting; it is important to test for PCBs after the retro fill of a transformer. It can be required whenever any maintenance has been accomplished on the unit, and the potential of contamination is current. If PCB content material exceeds the recommended limits, the appropriate action needs to be taken.
Units with a PCB content material of more than 50ppm require a hearth security plan, environmental safety plan, and extra precautionary measures when upkeep is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the equipment owner.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of knowledge and interpretation, we will focus on 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 standards will be mentioned in detail, forming a half of the overall well being rating willpower of the transformer.
Conclusion
Transformer situation monitoring is an interlaced, highly thrilling field of examine. In this article, we focused on the kinds of exams to discover out the situation of the transformer, the critical values, and the really helpful 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 possible to ensure greatest practice utility and optimised maintenance. It additionally make it easier to draw up a maintenance plan and motion 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
three. Passivators, what they’re 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 power.”
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 marketing consultant. She has 20+ years’ expertise in the business, having previously worked as laboratory manager for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, particularly within the analysis of check knowledge. Corné has vast sensible and theoretical information of reliability maintenance programmes.
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