by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant information concerning an asset’s operation. If we interpret the information appropriately, it can give us nice insight into the asset’s condition.
Frequent monitoring of the asset may find yourself in less upkeep required or extra prolonged intervals without any upkeep required.
It is crucial to identify the key parameters that are needed to provide us a complete image 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 information telling us?
Has the situation of the unit modified since the last upkeep period?
Is it secure to function the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate rating for a specific period?
Are we required to implement action to make sure the continued reliability of the unit?
How lengthy can we use the unit earlier than we need to think about replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring outline
It is vitally important to establish clear goals as part of your technique. What do you want to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the end result and what you need to accomplish, it will be much easier to establish the required parameters.
Health indexing of assets is changing into a remarkable device in getting a clearer image 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 requirements for mineral oils, indicating the critical values stipulated within the various 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 different size transformers within the industry. 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 make use of the guidelines for bigger tools, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or manager needs to determine 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 in depth vary of tests that can assist in figuring out specific drawback standards within the system, which could not be clear via the typical day-to-day analysis usually carried out.
Please see the ranking classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way typically or under which circumstances.
Oil sampling
The taking of the oil sample is considered one of the most crucial and significant influencers within the analysis consequence. If a pattern just isn’t taken to the prescribed process, then there is a important risk that the analysis carried out, and the results acquired, are of no use.
Care ought to be taken to ensure high quality management procedures are utilized in every step of the sampling course of as set out by worldwide standards. A good quality pattern taken by applying the right process is crucial. A pattern may be contaminated by numerous elements, all of which may influence the finish result of the results in a negative method.
All steps concerned in pattern taking must adhere to high quality management procedures, together with the container that’s used, the sampling kit, the taking of the sample, the marking of the pattern, the closing of the pattern, the storage and dealing with of the pattern, after which 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 information. The label ought to 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 must be waterproof, and the pen used to write down on the label ought to be oil- and water resistant. Otherwise, some information might be misplaced, making it extremely troublesome for the laboratory personnel to finish the report or allocate the pattern to a selected transformer.
Now, let’s discuss the types of checks to determine the transformer’s situation, the critical values, and the recommended actions in every 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 many “tests” is a visual inspection of the oil sample in a transparent vessel to determine the colour, turbidity, and potential particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there’s lots of turbidity, it would indicate a excessive water content in 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 identified as carbon, it would point out a potential electrical fault in the unit. The DGA evaluation of the oil will affirm if that is the case.
Clear oils with out contamination will indicate an excellent situation, and no motion is beneficial.
When oils are darkish or turbid, further evaluation will verify any issues. The oil analysis results may even decide the diploma and type 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 point application<40 kV for OLTC in delta or line-end utility
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 should be excessive.
If the values are Good, it is strongly recommended to proceed with the present pattern 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 suggested to recondition the oil by way of oil reconditioning processes. If different exams 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 elements are removed from the oil. This course of removes acid and water in addition to some other compounds. Another benefit is that the oil could be re-used, and in most situations, this can be carried out without switching off the unit, which contributes to cost-saving. If unsure – as a substitute swap off the unit during this therapy course of.
If the values are Poor, it is advisable to take motion as quickly as potential and not delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content could cause flashover in the unit, resulting in lack of the asset.
c) Water content material (mg/kg at transformer working 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 take a look at for all classes of electrical gear, besides class G
The results of this take a look at ought to at all times be considered in conjunction with the breakdown energy. If it’s discovered that the water content material is high and the breakdown power is low, additional action needs to 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, the place there is no paper current, the breakdown voltage is the figuring out factor.
It ought to be noted that the bounds indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is exterior this temperature vary, it is best to discuss with Annex A of the standard.
When the worth obtained via analyses is GOOD, the traditional sampling interval could be maintained, requiring no further action.
When the worth returns a FAIR end result, extra frequent sampling is recommended. It is also useful to consider other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be carried out.
A POOR result would require immediate action from the asset manager. This might include taking one other sample to confirm the results from the primary analysis. If it is confirmed that the water content is excessive, the oil can be filtered; this course of ought to take away a large portion of the moisture from the oil if utilized accurately. Follow-up samples must be taken to make sure that the moisture content remains to be throughout the required limits. The reason is that probably the most significant slice of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil underneath conditions that favour this movement. It might be discovered later that the oil within the water has elevated once more with none obvious purpose, but the supply could be the paper within the transformer.
A visual inspection is also really helpful to determine if any water would possibly transfer into the transformer or electrical equipment through leaks. This drawback could be extra extreme if the transformer or electrical tools is outdoors and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to zero.25>0.20
C<0.100.15 to 0.30>0.30
F, G Not a routine check
This is a routine test for all lessons besides F and G
The acids in oils are fashioned as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will help in the formation of the acids. Because acids are polar compounds, it will adversely affect the insulation properties of the oil and will increase paper degradation. If left เกจวัดแรงดัน300psi in transformers, this can lead to sludge formation, normally around the decrease components of the transformer core. The sludge will ultimately type a semi-solid substance that is extraordinarily tough to remove.
If the result’s GOOD, the common sampling interval can continue.
In case of a FAIR result, the sampling interval ought to be decreased to fit the scenario. Future evaluation ought to embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset manager may resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever choice might go nicely with their necessities the most effective.
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<0.a 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 check
This is a routine test for all courses of electrical equipment, besides F and G
The dielectric dissipation factor or tan delta of this check provides info 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 phase displacement and dielectric losses. Other impurities that may influence the dissipation factor embody water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended. The structure of the oil is damaged, in impact 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 90 °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 certainly one of the key parameters to evaluate the transformer insulation situation; that is based on the truth that DC resistance is delicate to grease degradation.
When the result’s FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of unique worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to verify the details 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 significantly slows the aging course of within the oil and the strong insulation.
If the result is FAIR, it’s suggested to top up the inhibitor level to the prescribed level per provider directions. It is advised to use a subject professional trained within the process to carry out this activity.
If the result obtained is POOR, the recommendation for this situation would counsel that the tip user continues to use the oil “uninhibited,” however this may result in extra fast degradation of each the liquid and solid insulation.
It should be noted that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at levels below 1000ppm. This can be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances just isn’t required, though it might add extra safety against oil oxidation if the preservation system ever fails. [2]
h) Passivator content material
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and stable, (rate of decrease < 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 metallic deactivators, react with reactive metallic surfaces and dissolved metals similar to copper and silver and reduce their price of response with compounds in the oil. This contains oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental types, sulfur-based and nitrogen-based. The first instructed use of passivators in transformer oil, of which the author 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 may deplete extra quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals may be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it is advised to take away the oil or take away the supply of corrosivity from the oil by way of special oil remedy.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this test is carried out when the oil results indicate a high acid worth and the dissipation factor is near the unacceptable restrict.
The results need to be less than zero.02% by mass to be negligible. If the results return a value of greater than 0.02% by mass, it is suggested that or not it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial rigidity
This just isn’t a routine check
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 rigidity between transformer oil and water reduces through the growing older course of. What this means in practical terms is there’s more polar compound current within the oil, decreasing the ability of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial pressure becomes a quality criterion: the oil must be changed beneath a predefined limit.
If outcomes are GOOD, continue the common sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine test.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be vital. The extent of the corrosion injury attributable to the sulfur can be so severe that it would cause failure of the equipment if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber products utilized 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 need of corrective actions based mostly on this institute’s danger evaluation examine. [4]
l) Particle counting and sizing
Table three: 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 maximum decrease in flashpoints by 10%, the gear would possibly require further inspection. This worth would possibly differ in different nations.
It is advised to carry out 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 take a look at is not to decide the condition of the transformer; this can be a well being and safety influence take a look at. PCB is hazardous to each humans and the setting; it’s critical to test for PCBs after the retro fill of a transformer. It can be required every time any maintenance has been accomplished on the unit, and the potential for contamination is present. If PCB content exceeds the really helpful limits, the suitable action needs to be taken.
Units with a PCB content of more than 50ppm require a hearth safety plan, environmental safety plan, and additional precautionary measures when upkeep is finished. 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 boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we’ll talk about this phenomenon partially II of the article. The limits for the different gases and the interpretation of this information based on international standards might be discussed in detail, forming part of the general health score dedication of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling subject of examine. In this article, we focused on the kinds of tests to determine the condition of the transformer, the critical values, and the beneficial 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 greatest practice application and optimised upkeep. It additionally make it simpler to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 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 are and how 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
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience within the industry, having previously worked 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 career, significantly in the analysis of check knowledge. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.

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