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 relevant data regarding an asset’s operation. If we interpret the info appropriately, it can provide us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to much less maintenance required or more prolonged intervals without any upkeep required.
It is crucial to establish the vital thing parameters which are needed to offer us a complete picture 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 data telling us?
Has the condition of the unit changed for the explanation that last upkeep period?
Is it safe to operate 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 lengthy can we use the unit earlier than we have to contemplate replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring outline
It is vitally essential to determine clear targets as a half of your strategy. What do you wish to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it will be much easier to determine the required parameters.
Health indexing of assets is changing into a exceptional device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the entire value of the Health Index Value of the transformer. These parameter weight values were calculated based on the worldwide standards for mineral oils, indicating the critical values stipulated in the various standards.
The scope of oil evaluation, interpretation of the info, and critical values
At the beginning of this section, it’s important to state that we take care of completely different size transformers within the industry. Transformers are divided into classes according to the kV ratings of the tools. It is up to the reliability or asset supervisor to make use of the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or manager needs to find out what type of testing would profit him in identifying downside areas inside his fleet. Some of the evaluation has been identified as to routine type checks. Still, there’s an intensive vary of tests that may assist in figuring out specific drawback criteria inside the system, which could not be clear through the standard day-to-day evaluation often carried out.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and the way often or beneath which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is likely certainly one of the most vital and critical influencers in the evaluation end result. If a sample is not taken to the prescribed process, then there’s a significant possibility that the analysis carried out, and the results acquired, are of no use.
Care should be taken to make sure high quality management procedures are applied in each step of the sampling process as set out by international requirements. A good quality pattern taken by making use of the correct procedure is crucial. A pattern may be contaminated by numerous factors, all of which can influence the result of the ends in a unfavorable method.
All steps concerned in sample taking must adhere to quality management procedures, together with the container that is used, the sampling kit, the taking of the sample, the marking of the sample, the closing of the pattern, the storage and handling of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and pattern information are handwritten, the human issue can result in incorrect interpretation of the information. The label should be caught 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 write down on the label should be oil- and water-resistant. Otherwise, some information could be misplaced, making it extremely tough for the laboratory personnel to finish the report or allocate the pattern to a particular transformer.
Now, let’s talk about the kinds of exams to determine the transformer’s condition, the critical values, and the beneficial actions in each case:
a) Colour and look
This is a routine inspection applied to each oil pattern.
When an oil pattern arrives at the laboratory, one of the “tests” is a visible inspection of the oil pattern in a clear vessel to discover out the color, turbidity, and potential particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there could be a lot of turbidity, it would indicate a excessive water content within the oil.
If the drain valve was not wiped clean by the sampler, the filth particles in the drain valve could be included into the sample. If particles are identified as carbon, it’d indicate a possible electrical fault within the unit. The DGA evaluation of the oil will affirm if that is the case.
Clear oils without contamination will point out an excellent situation, and no motion is recommended.
When oils are darkish or turbid, additional analysis will confirm any problems. The oil analysis results may also determine the degree 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 point out the water content or the presence of international particles, or each within 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 must be excessive.
If the values are Good, it is recommended to proceed with the current pattern interval action plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with different parameter outcomes like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil by way of oil reconditioning processes. If various tests indicate extreme getting older, the oil can be replaced with new or reclaimed oil. Another option can 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 faraway from the oil. This process removes acid and water in addition to another compounds. Another advantage is that the oil could be re-used, and in most conditions, this may be carried out without switching off the unit, which contributes to cost-saving. If pressure gauge ลม – as an alternative swap off the unit during this remedy course of.
If the values are Poor, it is advisable to take motion as quickly as attainable and never delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content may cause flashover within the unit, leading to 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 take a look at for all classes of electrical gear, except class G
The results of this take a look at ought to all the time be thought of in conjunction with the breakdown energy. If it is found that the water content is high and the breakdown energy is low, additional motion must be taken. It is really helpful that a second pattern from the same unit is tested to substantiate the results.
In the case of switching tools, where there is not any paper current, the breakdown voltage is the figuring out factor.
It must 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 outdoors this temperature range, it’s best to refer to Annex A of the usual.
When the worth obtained through analyses is GOOD, the conventional sampling interval could be maintained, requiring no further motion.
When the worth returns a FAIR result, more frequent sampling is really helpful. It can also be useful to suppose about other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR outcome would require immediate action from the asset manager. This would possibly embrace taking one other sample to verify the results from the primary evaluation. If it’s confirmed that the water content material is high, the oil may be filtered; this process ought to take away a big portion of the moisture from the oil if utilized appropriately. Follow-up samples need to be taken to guarantee that the moisture content material is still throughout the required limits. The cause is that essentially the most significant slice of the water is caught up within the paper system within the transformer. This moisture will transfer from the paper into the oil under circumstances that favour this motion. It might be discovered later that the oil within the water has increased once more without any apparent purpose, but the supply can be the paper within the transformer.
A visual inspection is also beneficial to determine if any water might transfer into the transformer or electrical gear via leaks. This drawback could be more extreme if the transformer or electrical equipment is outside and never 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 zero.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine check
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 increases will assist within the formation of the acids. Because acids are polar compounds, it’s going to adversely affect the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this will result in sludge formation, usually across the lower parts of the transformer core. The sludge will ultimately kind a semi-solid substance that is extraordinarily troublesome to remove.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval should be decreased to fit the scenario. Future analysis should embody 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.0, the asset manager might determine to reclaim the oil or replace it with new or reclaimed oil, whichever option might swimsuit their necessities the most effective.
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 zero.20>0.20
B, C<0.one hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E<0.100.01 to zero.30>0.03
F, GNot a routine check
This is a routine take a look at for all lessons of electrical equipment, except F and G
The dielectric dissipation issue or tan delta of this check offers info 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 phase displacement and dielectric losses. Other impurities that might affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking further parameters is really helpful.
When the result’s POOR, reclamation or an oil change is recommended. The construction of the oil is broken, in effect that the chemical bonds between the molecules have broken 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 check
DC resistivity of the oil is probably certainly one of the key parameters to assess the transformer insulation situation; this is primarily based on the truth that DC resistance is delicate to oil degradation.
When the result is FAIR, extra frequent sampling and checking extra parameters is recommended.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content 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 would be advisable to contact the oil supplier to confirm the details relating to additives.
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 function of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the aging process in the oil and the solid insulation.
If the result is FAIR, it’s advised to prime up the inhibitor stage to the prescribed stage per supplier instructions. It is suggested to use a field professional trained in the process to perform this process.
If the end result obtained is POOR, the recommendation for this state of affairs would suggest that the top person continues to make use of the oil “uninhibited,” however this will likely result in extra speedy degradation of each the liquid and stable insulation.
It ought to be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges beneath 1000ppm. This could be in the type 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 is not required, though it’d add extra 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 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 identified as metal deactivators, react with reactive metal surfaces and dissolved metals such as copper and silver and scale back their rate of reaction with compounds in the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental sorts, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the author 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 would possibly deplete extra quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals could be maintained.
With FAIR results, keep common monitoring.
When POOR, it’s advised to take away the oil or remove the source of corrosivity from the oil via special oil remedy.
I) Sediment and sludge
This isn’t a routine test.
It is suggested that this check is performed when the oil outcomes indicate a excessive acid worth and the dissipation issue is close to the unacceptable limit.
The outcomes need to be lower than 0.02% by mass to be negligible. If the outcomes return a price of more than 0.02% by mass, it is instructed that it be reclaimed; otherwise, an oil change is really helpful.
j) Interfacial pressure
This just 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 rigidity between transformer oil and water reduces through the getting older process. What this implies in practical terms is there’s more polar compound current in the oil, reducing the ability of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial pressure becomes a quality criterion: the oil must be modified below a predefined restrict.
If outcomes are GOOD, continue the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, verify 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 impact on the transformer system could be vital. The extent of the corrosion damage brought on by the sulfur could be so severe that it’d cause failure of the gear if not checked. The addition of a copper passivator can reduce the influence of this compound on the transformer system.
In a research by Doble, it has been found that rubber merchandise used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions primarily based on this institute’s risk assessment examine. [4]
l) Particle counting and sizing
Table 3: 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 check
If there is a maximum lower in flashpoints by 10%, the equipment might require additional inspection. This value might differ in different nations.
It is suggested to perform this take a look at when an unusual odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the situation of the transformer; it is a well being and security impression test. PCB is hazardous to both people and the environment; it’s critical to test for PCBs after the retro fill of a transformer. It can be required every time any upkeep has been accomplished on the unit, and the potential of contamination is present. If PCB content material exceeds the recommended limits, the suitable motion needs to be taken.
Units with a PCB content material of greater than 50ppm require a hearth security plan, environmental protection plan, and extra precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the gear owner.
Local regulatory our bodies define 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 partly II of the article. The limits for the totally different gases and the interpretation of this knowledge in accordance with worldwide standards shall be mentioned in detail, forming part of the general health ranking dedication of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, extremely exciting subject of examine. In this text, we targeted on the types of tests 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 selected unit at a particular date and time. This makes it possible to ensure finest apply utility and optimised upkeep. It additionally make it simpler to attract up a maintenance plan and action plan.
References:
1. IEC 60422 Edition four.0 2013-01 International Standard (Mineral insulating oils in electrical tools – supervision and upkeep guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
3. Passivators, what they’re 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 power.”
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 marketing consultant. She has 20+ years’ expertise in the trade, having beforehand labored as laboratory supervisor for a significant industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry right from the start of her profession, significantly within the analysis of take a look at data. Corné has huge practical and theoretical data of reliability upkeep programmes.
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