by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the related data relating to an asset’s operation. If we interpret the info appropriately, it may give us nice insight into the asset’s situation.
Frequent monitoring of the asset may end up in much less maintenance required or extra extended durations with none maintenance required.
It is essential to determine the vital thing parameters which might be wanted to offer us a complete image of the particular 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 changed since the last upkeep period?
Is it protected to operate 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 action to make sure the continued reliability of the unit?
How lengthy can we use the unit before we have to consider replacement?
Are the recognized issues of a recurring nature?
Effective situation monitoring define
It is vitally necessary to determine clear goals as a half of your strategy. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the finish result and what you need to accomplish, it would be much easier to determine the required parameters.
Health indexing of belongings is changing into a remarkable device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the whole worth of the Health Index Value of the transformer. These parameter weight values were calculated primarily based on the worldwide standards for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the information, and critical values
At the beginning of this section, it’s essential to state that we deal with completely different dimension transformers in the industry. Transformers are divided into lessons according to the kV ratings of the gear. It is as a lot as the reliability or asset manager to make use of the guidelines for larger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager wants to determine what type of testing would profit him in figuring out downside areas within his fleet. Some of the evaluation has been recognized as to routine kind tests. Still, there is an in depth range of tests that may help in identifying particular problem standards inside the system, which could not be clear through the everyday day-to-day evaluation often performed.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and how often or underneath which circumstances.
Oil sampling
The taking of the oil pattern is one of the most crucial and significant influencers in the evaluation outcome. If a sample just isn’t taken to the prescribed procedure, then there is a important possibility that the evaluation performed, and the outcomes acquired, are of no use.
Care should be taken to ensure quality management procedures are applied in each step of the sampling course of as set out by international requirements. A good quality pattern taken by making use of the right process is important. A pattern can be contaminated by varied elements, all of which might influence the outcome of the results 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 kit, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and handling of the sample, after which the delivery of the pattern to the laboratory.
If the sample identification and sample data are handwritten, the human factor can lead to incorrect interpretation of the info. The label should 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 write on the label should be oil- and waterproof. Otherwise, some data could be lost, making it extraordinarily tough for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s talk about the forms of tests to determine the transformer’s condition, the critical values, and the recommended actions in each 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 the “tests” is a visible inspection of the oil sample in a clear vessel to determine the color, turbidity, and possible particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there is a lot of turbidity, it’d indicate a high water content within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles within the drain valve may be integrated into the pattern. If particles are recognized as carbon, it might point out a attainable electrical fault within the unit. The DGA analysis of the oil will affirm if this is the case.
Clear oils with out contamination will point out an excellent condition, and no motion is recommended.
When oils are darkish or turbid, additional evaluation will verify any problems. The oil evaluation results may also determine the degree and sort of action.
b) Breakdown Voltage
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 software
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of foreign particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage have to be high.
If the values are Good, it is suggested to continue with the present sample interval action plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with different parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If different exams point out extreme growing older, the oil may be changed with new or reclaimed oil. Another option can be to perform on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar elements are removed from the oil. This process removes acid and water as properly as some other compounds. Another benefit is that the oil may be re-used, and in most situations, this can be accomplished with out switching off the unit, which contributes to cost-saving. If in doubt – as a substitute change off the unit during this therapy process.
If the values are Poor, it is advisable to take action as quickly as potential and not delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content material could cause flashover within the unit, leading to loss of the asset.
c) Water content material (mg/kg at transformer operating temperature)
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, except class G
The outcomes of this take a look at ought to always be thought-about along side the breakdown power. If it is found that the water content is high and the breakdown energy is low, further motion needs to be taken. It is really helpful that a second pattern from the identical unit is tested to verify the results.
In the case of switching gear, the place there is not any paper current, the breakdown voltage is the determining issue.
It must be famous that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it is found that the unit’s working temperature is outside 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 may be maintained, requiring no further action.
When the value returns a FAIR result, extra frequent sampling is beneficial. It is also helpful to consider different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the motion to be applied.
A POOR outcome will require immediate motion from the asset supervisor. This would possibly include taking another sample to verify the results from the first evaluation. If it is confirmed that the water content material is excessive, the oil may be filtered; this process ought to take away a large portion of the moisture from the oil if utilized appropriately. Follow-up samples have to be taken to guarantee that the moisture content material remains to be inside the required limits. The reason is that essentially the most significant portion of the water is caught up within the paper system in the transformer. This moisture will transfer from the paper into the oil underneath circumstances that favour this motion. It might be found later that the oil in the water has elevated again without any obvious cause, but the source can be the paper within the transformer.
A visual inspection is also recommended to discover out if any water may transfer into the transformer or electrical tools via leaks. This downside might be extra extreme if the transformer or electrical equipment is outside and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E< hundred.10 to 0.25>0.20
C< hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine check for all lessons except F and G
The acids in oils are shaped because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist within the formation of the acids. Because acids are polar compounds, it’s going to adversely have an result on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this can result in sludge formation, usually across the lower elements of the transformer core. The sludge will eventually form a semi-solid substance that’s extraordinarily tough to take away.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to suit the scenario. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result’s POOR according to the prescribed values in IEC 60422 Edition, the asset supervisor could resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever option may go nicely with their necessities one of the best.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to zero.20>0.20
B, C< hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.a hundred.01 to 0.30>0.03
F, GNot a routine take a look at
This is a routine test for all courses of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this test provides info regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we now have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities which may influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended. ร้านซ่อมเครื่องวัดความดันomron of the oil is damaged, in effect 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 ninety °C
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 test
DC resistivity of the oil is considered one of the key parameters to evaluate the transformer insulation situation; this is primarily based on the reality 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 percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of original value
This check is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the small print regarding additives.
The two most typical 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 aging process within the oil and the stable insulation.
If the result’s FAIR, it’s advised to prime up the inhibitor stage to the prescribed stage per supplier directions. It is suggested to make use of a subject skilled skilled in the process to carry out this activity.
If the outcome obtained is POOR, the recommendation for this scenario would suggest that the top person continues to make use of the oil “uninhibited,” but this may result in more rapid degradation of each the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at ranges below 1000ppm. This could be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils under these circumstances isn’t required, though it might add additional safety in opposition to oil oxidation if the preservation system ever fails. [2]
h) Passivator content
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and steady, (rate of decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive metallic surfaces and dissolved metals such as copper and silver and scale back their rate of response with compounds within the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic sorts, sulfur-based and nitrogen-based. The first instructed 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 rapidly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals could be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it’s advised to remove the oil or remove the supply of corrosivity from the oil via particular oil treatment.
I) Sediment and sludge
This isn’t a routine check.
It is suggested that this take a look at is performed when the oil outcomes indicate a excessive acid value and the dissipation issue is close to the unacceptable limit.
The outcomes need to be less than zero.02% by mass to be negligible. If the results return a value of greater than zero.02% by mass, it’s suggested that it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial rigidity
This is not a routine test
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 tension between transformer oil and water reduces in the course of the aging process. What this means in sensible phrases is there may be more polar compound current within the oil, decreasing the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation number. Therefore, the interfacial pressure becomes a quality criterion: the oil should be changed under a predefined restrict.
If outcomes are GOOD, proceed the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not 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 can be significant. The extent of the corrosion harm attributable to the sulfur may be so extreme that it might cause failure of the tools if not checked. The addition of a copper passivator can reduce the impact 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 might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based on this institute’s danger assessment research. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there’s a maximum lower in flashpoints by 10%, the equipment might require further inspection. This worth may differ in numerous nations.
It is advised to perform this take a look at when an uncommon odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not decide the condition of the transformer; this is a well being and safety influence test. PCB is hazardous to both humans and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It can be required whenever any upkeep has been done on the unit, and the potential for contamination is current. If PCB content exceeds the really helpful limits, the appropriate motion needs to be taken.
Units with a PCB content material of more than 50ppm require a fireplace security plan, environmental protection plan, and further precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificate 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 information and interpretation, we’ll talk about this phenomenon partly II of the article. The limits for the different gases and the interpretation of this data in accordance with worldwide standards shall be discussed in detail, forming a part of the general well being score determination of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling subject of research. In this article, we targeted 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 potential to see the supposed reliability of a selected unit at a specific date and time. This makes it potential to ensure greatest apply application and optimised upkeep. It also make it simpler to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition four.0 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
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 published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience within the trade, having beforehand labored as laboratory manager for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her profession, particularly within the evaluation of take a look at knowledge. Corné has vast practical and theoretical information of reliability upkeep programmes.

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