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 information concerning an asset’s operation. If we interpret the info accurately, it can give us nice perception into the asset’s condition.
Frequent monitoring of the asset may find yourself in less maintenance required or more extended durations without any upkeep required.
It is essential to determine the key parameters which would possibly be needed to provide us a complete picture of the particular standing of the transformer and the action we need to take to ensure the continued reliability of the asset to achieve the maximum lifetime.
What is the data telling us?
Has the situation of the unit changed for the explanation that last maintenance period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate score for a selected period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit before we need to consider replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring outline
It is vitally important to determine clear objectives as part of your technique. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the result and what you wish to accomplish, it might be a lot simpler to establish the required parameters.
Health indexing of property is turning into a exceptional software in getting a clearer picture 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 standards for mineral oils, indicating the important values stipulated within the varied standards.
The scope of oil analysis, interpretation of the info, and important values
At the start of this part, it is essential to state that we cope with different size transformers within the trade. Transformers are divided into lessons in accordance with the kV rankings of the gear. It is up to the reliability or asset manager to use the guidelines for larger equipment, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or supervisor wants to discover out what kind of testing would benefit him in figuring out problem areas inside his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there’s an intensive range of checks that can help in figuring out particular problem criteria throughout the system, which could not be clear via the standard day-to-day analysis usually performed.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and how typically or beneath which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is among the most important and important influencers in the analysis end result. If a pattern just isn’t taken to the prescribed process, then there’s a vital risk that the evaluation carried out, and the outcomes acquired, are of no use.
Care ought to be taken to make sure quality management procedures are applied in every step of the sampling process as set out by worldwide standards. A good high quality sample taken by making use of the proper process is important. A pattern can be contaminated by numerous elements, all of which might influence the outcome of the leads to a unfavorable method.
All steps concerned in pattern taking must adhere to quality control procedures, including the container that’s used, the sampling equipment, the taking of the pattern, the marking of the sample, 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 pattern knowledge are handwritten, the human factor can result in incorrect interpretation of the data. The label must be stuck onto the container to make sure no mix-ups in pattern identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to put in writing on the label must be oil- and waterproof. Otherwise, some information may be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s discuss the kinds of exams to determine the transformer’s situation, the important values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visible inspection of the oil pattern in a clear vessel to determine the colour, turbidity, and possible particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there may be plenty of turbidity, it’d indicate a excessive water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles within the drain valve could be incorporated into the pattern. If particles are recognized as carbon, it might point out a possible electrical fault in the unit. The DGA evaluation of the oil will verify if that is so.
Clear oils without contamination will point out a great situation, and no action is beneficial.
When oils are dark or turbid, additional analysis will confirm any issues. The oil analysis outcomes will also determine the degree and type of motion.
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 indicate 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 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 motion plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If various checks point out severe getting older, the oil could be changed with new or reclaimed oil. Another possibility could be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar components are faraway from the oil. This course of removes acid and water in addition to another compounds. Another benefit is that the oil could be re-used, and in most conditions, this could be done without switching off the unit, which contributes to cost-saving. If doubtful – as an alternative switch off the unit during this remedy course of.
If the values are Poor, it’s advisable to take action as quickly as potential and never delay the maintenance course of. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content can cause flashover in the unit, resulting in lack of the asset.
c) Water content (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 take a look at
This is a routine take a look at for all lessons of electrical gear, except class G
The results of this take a look at ought to at all times be thought-about along side the breakdown power. If it’s found that the water content is excessive and the breakdown energy is low, further action must be taken. It is really helpful that a second pattern from the same unit is tested to verify the results.
In the case of switching gear, where there is not a paper present, the breakdown voltage is the determining factor.
It must be famous that the limits indicated by IEC 60422 Edition 4 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 is best to discuss with Annex A of the usual.
When the value obtained via analyses is GOOD, the traditional sampling interval could be maintained, requiring no further motion.
When the value returns a FAIR outcome, more frequent sampling is really helpful. It can additionally be helpful to contemplate other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR result will require immediate action from the asset manager. This would possibly embody taking another pattern to verify the results from the first analysis. If it is confirmed that the water content material is excessive, the oil can be filtered; this course of should take away a large portion of the moisture from the oil if applied accurately. Follow-up samples need to be taken to ensure that the moisture content remains to be throughout the required limits. The cause is that essentially the most good portion of the water is caught up within the paper system within the transformer. This moisture will transfer from the paper into the oil underneath conditions that favour this movement. It may be discovered later that the oil in the water has elevated once more with none obvious reason, however the source would be the paper within the transformer.
A visual inspection is also beneficial to determine if any water may move into the transformer or electrical tools via leaks. This problem could be more severe if the transformer or electrical tools is outside and never in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to 0.25>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 classes besides F and G
The acids in oils are fashioned because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist 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 increase paper degradation. If left untreated in transformers, this will result in sludge formation, normally around the lower elements of the transformer core. The sludge will finally type a semi-solid substance that is extraordinarily difficult to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR result, the sampling interval must be decreased to fit the state of affairs. Future evaluation ought to embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition four.zero, the asset supervisor might resolve 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 issue 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<0.100.10 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 test for all classes of electrical gear, except F and G
The dielectric dissipation factor or tan delta of this check supplies data 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, resulting in part displacement and dielectric losses. Other impurities that may affect the dissipation factor include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is recommended.
When the result is 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 broken down, and even with filtration, the beneficial 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 check
DC resistivity of the oil is among the key parameters to assess the transformer insulation condition; this is primarily based on the truth that DC resistance is sensitive to grease degradation.
When the result is FAIR, more frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content material %
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of authentic value
This take a look at is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the small print concerning components.
The two most common 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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the growing older course of within the oil and the stable insulation.
If the result’s FAIR, it is suggested to prime up the inhibitor degree to the prescribed degree per provider instructions. It is suggested to make use of a field professional educated within the process to perform this task.
If the outcome obtained is POOR, the recommendation for this scenario would recommend that the end person continues to use the oil “uninhibited,” but this will likely result in more fast degradation of both the liquid and strong insulation.
It must be famous that some transformers already have a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges beneath 1000ppm. This could be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances just isn’t required, though it might add extra safety in opposition to 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 lowering at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and scale back their price of reaction with compounds within the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental sorts, sulfur-based and nitrogen-based. The first advised 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 rapidly; this depletion may accelerate when the oil is un-inhibited.
With GOOD outcomes, common sample intervals may be maintained.
With FAIR results, keep common monitoring.
When POOR, it is advised to remove the oil or take away the supply of corrosivity from the oil via special oil therapy.
I) Sediment and sludge
This isn’t a routine take a look at.
It is suggested that this test is carried out when the oil outcomes indicate a high acid value and the dissipation issue is near the unacceptable restrict.
The outcomes need to 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 suggested that it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial rigidity
This just isn’t a routine take a look at
PropertyCategoryGoodFairPoor
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to twenty-eight 20 to 25<22 <20
ENot a routine test
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces in the course of the growing older process. What this means in sensible terms is there is more polar compound present 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 number. Therefore, the interfacial rigidity turns into a excessive quality criterion: the oil have to be modified beneath a predefined restrict.
If results are GOOD, proceed the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, check 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 effect on the transformer system can be significant. The extent of the corrosion injury brought on by the sulfur can be so severe that it might 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 merchandise 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 mostly on this institute’s risk 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 check
If there is a most decrease in flashpoints by 10%, the tools would possibly require additional inspection. This value would possibly differ in different nations.
It is advised to perform this check when an uncommon odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not decide the situation of the transformer; this is a well being and safety influence take a look at. PCB is hazardous to both humans and the environment; it is vital to check for PCBs after the retro fill of a transformer. It can be required every time any maintenance has been done on the unit, and the potential of contamination is current. If PCB content exceeds the really helpful limits, the appropriate action needs to be taken.
Units with a PCB content of more than 50ppm require a fireplace security plan, environmental protection plan, and extra precautionary measures when upkeep is completed. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the tools proprietor.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of data and interpretation, we’ll focus on this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this information based on international requirements will be discussed in detail, forming a half of the overall health score dedication of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, highly thrilling field of examine. In this text, we focused on the kinds of checks to determine the condition of the transformer, the important values, and the recommended actions.
The Health Index indication makes it attainable to see the supposed reliability of a specific unit at a particular date and time. This makes it potential to make sure finest apply utility and optimised upkeep. It also make it simpler to attract up a maintenance plan and motion plan.
References:
1. IEC 60422 Edition four.0 2013-01 International Standard (Mineral insulating oils in electrical gear – 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.
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’ experience within the industry, having previously worked as laboratory manager for a significant industrial laboratory group, focusing on transformer well being. เกจวัดแรงดันไทวัสดุ has been intrigued by transformer chemistry right from the start of her profession, notably in the analysis of check information. Corné has huge practical and theoretical knowledge of reliability maintenance programmes.
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