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TALKING TRANSFORMER TESTING

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 analysis of the related data concerning an asset’s operation. If we interpret the information correctly, it can provide us nice perception into the asset’s situation.
Frequent monitoring of the asset can lead to less upkeep required or extra extended durations with none maintenance required.
It is essential to determine the key parameters which are wanted to provide us an entire picture of the particular status 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 data telling us?
Has the situation of the unit changed since the last upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it safe 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 long can we use the unit before we have to contemplate replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring outline
It is vitally important to establish clear goals as part of your strategy. What do you need to achieve 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 need to accomplish, it might be a lot easier to establish the required parameters.
Health indexing of property is becoming a outstanding device in getting a clearer image of the condition 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 requirements for mineral oils, indicating the critical values stipulated in the numerous requirements.
The scope of oil analysis, interpretation of the info, and important values
At the start of this part, it is important to state that we deal with different measurement transformers in the industry. Transformers are divided into lessons based on the kV rankings of the equipment. It is up to the reliability or asset manager to make use of the rules for bigger tools, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager needs to find out what type of testing would profit him in figuring out drawback areas within his fleet. Some of the analysis has been identified as to routine sort exams. Still, there might be an extensive range of exams that may help in identifying specific drawback criteria inside the system, which could not be clear by way of the standard day-to-day analysis usually carried out.
Please see the ranking lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and how often or under which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is likely certainly one of the most vital and significant influencers within the evaluation end result. If a sample isn’t taken to the prescribed procedure, then there is a vital risk that the evaluation performed, and the outcomes acquired, are of no use.
Care should be taken to make sure quality management procedures are utilized in each step of the sampling course of as set out by international requirements. A good high quality pattern taken by making use of the proper procedure is crucial. A pattern could be contaminated by varied elements, all of which might affect the finish result of the results in a negative method.
All steps involved in pattern taking should adhere to quality management procedures, including the container that’s 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, and then the delivery of the sample to the laboratory.
If the pattern identification and sample 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 completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write on the label must be oil- and waterproof. Otherwise, some knowledge might be lost, making it extraordinarily difficult for the laboratory personnel to finish the report or allocate the pattern to a selected transformer.
Now, let’s focus on the types of checks to determine the transformer’s condition, the crucial 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 pattern in a clear vessel to determine the color, turbidity, and attainable particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it might indicate a high water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles in the drain valve may be integrated into the sample. If particles are recognized as carbon, it might point out a attainable electrical fault within the unit. The DGA evaluation of the oil will affirm if that’s so.
Clear oils without contamination will point out a great condition, and no motion is really helpful.
When oils are dark or turbid, further analysis will verify any issues. The oil analysis results may also determine 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 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 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 in the unit, the breakdown voltage should be excessive.
If the values are Good, it is strongly recommended to continue with the current sample interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with other parameter results like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil by way of oil reconditioning processes. If various tests indicate severe aging, the oil could be changed with new or reclaimed oil. Another choice could be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar components are removed from the oil. This process removes acid and water as well as some other compounds. Another benefit is that the oil can be re-used, and in most situations, this might be accomplished with out switching off the unit, which contributes to cost-saving. If unsure – instead switch off the unit during this treatment process.
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 significantly; extremely high water content may cause flashover within the unit, resulting in loss 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 take a look at
This is a routine check for all courses of electrical tools, except class G
The outcomes of this take a look at should at all times be thought of in conjunction with the breakdown power. If it is discovered that the water content is high and the breakdown strength is low, further action needs to be taken. It is beneficial that a second pattern from the identical unit is examined to confirm the results.
In the case of switching gear, where there isn’t any paper current, the breakdown voltage is the determining issue.
It must be noted that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it’s discovered that the unit’s operating temperature is outdoors this temperature range, it is best to refer to Annex A of the standard.
When the value obtained through analyses is GOOD, the conventional sampling interval may be maintained, requiring no further motion.
When the worth returns a FAIR end result, extra frequent sampling is recommended. It can be useful to contemplate other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be implemented.
A POOR result would require immediate action from the asset manager. This would possibly embrace taking one other pattern to verify the results from the first evaluation. If it’s confirmed that the water content is high, the oil can be filtered; this course of should take away a large portion of the moisture from the oil if applied correctly. Follow-up samples need to be taken to make sure that the moisture content remains to be throughout the required limits. The cause is that probably the most good portion of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under conditions that favour this movement. It may be discovered later that the oil in the water has elevated again with none obvious purpose, but the supply could be the paper within the transformer.
A visual inspection can additionally be really helpful to determine if any water would possibly move into the transformer or electrical tools via leaks. This downside might be more extreme if the transformer or electrical tools is outside and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to zero.15>0.15
B, E<0.100.10 to 0.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine test for all classes except F and G
The acids in oils are formed because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help within the formation of the acids. Because acids are polar compounds, it will adversely have an result on the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this can lead to sludge formation, normally across the lower components of the transformer core. The sludge will ultimately form a semi-solid substance that’s extremely tough to remove.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval should be decreased to fit the state of affairs. Future evaluation should 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 4.zero, the asset supervisor might decide to reclaim the oil or substitute it with new or reclaimed oil, whichever option might suit their necessities one of 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 0.50>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 test
This is a routine test for all classes of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this take a look at offers data concerning the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating materials.
When oil ages, we have the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities that may influence the dissipation factor include 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 recommended. The structure of the oil is damaged, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the beneficial 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 take a look at
DC resistivity of the oil is amongst the key parameters to evaluate the transformer insulation condition; this is primarily based on the fact that DC resistance is delicate to oil degradation.
When the result is FAIR, more frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material percent
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of unique value
This test is restricted to oils with this additive.
It can be advisable to contact the oil provider to confirm the details concerning components.
The two most common oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The function of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the aging course of within the oil and the solid insulation.
If the result is FAIR, it’s advised to prime up the inhibitor degree to the prescribed level per provider instructions. It is advised to use a field skilled skilled in the procedure to carry out this task.
If the outcome obtained is POOR, the recommendation for this situation would suggest that the end user continues to use the oil “uninhibited,” however this will lead to more rapid degradation of both the liquid and solid insulation.
It must be famous that some transformers already have a built-in oil preservation system; that is designed to keep dissolved oxygen at levels under 1000ppm. This can be in the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, although it would add extra protection towards oil oxidation if the preservation system ever fails. [2]
h) Passivator content
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 reducing at >10mg/kg/year
Passivators, also known as metal deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and reduce 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 basic varieties, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the creator is conscious, was in 1967 by J.J. เพรสเชอร์เกจลม and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator might deplete extra quickly; this depletion might accelerate when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals may be maintained.
With FAIR outcomes, maintain regular monitoring.
When POOR, it is suggested to take away the oil or remove the supply of corrosivity from the oil via particular oil remedy.
I) Sediment and sludge
This isn’t a routine take a look at.
It is advised that this test is carried out when the oil outcomes point out a high acid value and the dissipation factor is near the unacceptable limit.
The results must be lower than 0.02% by mass to be negligible. If the outcomes return a price of greater than 0.02% by mass, it’s advised that it’s reclaimed; in any other case, an oil change is beneficial.
j) Interfacial rigidity
This isn’t a routine take a look at
PropertyCategoryGoodFairPoor
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to 28 20 to 25<22 <20
ENot a routine take a look at
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the aging process. What this implies in sensible phrases is there’s more polar compound present within the oil, lowering the power of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial pressure becomes a quality criterion: the oil have to be changed under a predefined restrict.
If results are GOOD, continue the common sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine test.
Oil is either 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 damage brought on by the sulfur can be so severe that it might cause failure of the tools if not checked. The addition of a copper passivator can scale back the impact of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber products used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted 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 threat assessment research. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (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 decrease in flashpoints by 10%, the equipment may require further inspection. This value may differ in several international locations.
It is suggested to perform this take a look at when an unusual odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to decide the situation of the transformer; this is a health and security impact take a look at. PCB is hazardous to both humans and the surroundings; it’s critical to test for PCBs after the retro fill of a transformer. It can also be required each time any maintenance has been done on the unit, and the risk of contamination is current. If PCB content material exceeds the recommended limits, the appropriate action must be taken.
Units with a PCB content of more than 50ppm require a fire security plan, environmental safety plan, and additional precautionary measures when upkeep is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the equipment proprietor.
Local regulatory bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we will talk about this phenomenon in part II of the article. The limits for the different gases and the interpretation of this information in accordance with worldwide requirements will be mentioned intimately, forming part of the overall health score willpower of the transformer.
Conclusion
Transformer situation monitoring is an interlaced, highly exciting subject of study. In this article, we centered on the types of checks to determine the condition of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a specific date and time. This makes it possible to ensure greatest practice software and optimised upkeep. It also make it easier to attract 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 maintenance 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 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 labored as laboratory supervisor for a major industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry right from the start of her career, notably within the evaluation of check knowledge. Corné has vast sensible and theoretical data of reliability maintenance programmes.
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