by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant data regarding an asset’s operation. If we interpret the information correctly, it may give us great perception into the asset’s condition.
Frequent monitoring of the asset can lead to much less maintenance required or more prolonged intervals without any maintenance required.
It is crucial to establish the key parameters which may be wanted to offer us a whole 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 attain the utmost lifetime.
What is the information telling us?
Has the situation of the unit modified since the final maintenance period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate ranking 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 have to think about replacement?
Are the recognized issues of a recurring nature?
Effective situation monitoring define
It is vitally essential to establish clear targets as part of your technique. What do you need to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you want to accomplish, it will be a lot easier to establish the required parameters.
Health indexing of assets is turning into a outstanding device in getting a clearer image of the condition 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 based mostly on the worldwide standards for mineral oils, indicating the crucial values stipulated in the various standards.
The scope of oil evaluation, interpretation of the info, and critical values
At the start of this section, it’s important to state that we cope with totally different measurement transformers within the industry. Transformers are divided into classes in accordance with the kV rankings of the equipment. It is up to the reliability or asset manager to make use of the rules for bigger equipment, thereby implementing shorter increment oil evaluation and electrical tests.
The maintenance engineer or supervisor needs to find out what sort of testing would benefit him in identifying drawback areas inside his fleet. Some of the evaluation has been recognized as to routine type checks. Still, there’s an intensive range of checks that can assist in identifying particular problem standards within the system, which could not be clear by way of the standard day-to-day evaluation normally carried out.
Please see the score lessons in TABLE 1
TABLE 2 explains which oil analyses are beneficial and the way typically or beneath which circumstances.
Oil sampling
The taking of the oil sample is amongst the most vital and important influencers within the evaluation consequence. If a sample isn’t taken to the prescribed process, then there’s a vital possibility that the analysis carried out, and the results acquired, are of no use.
Care should be taken to ensure quality management procedures are applied in each step of the sampling process as set out by international standards. A good quality sample taken by making use of the correct process is essential. A sample may be contaminated by various elements, all of which can influence the result of the results in a adverse manner.
All steps involved in sample taking must adhere to high quality management procedures, including the container that’s used, the sampling equipment, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the pattern, after which the delivery of the pattern to the laboratory.
If the sample identification and pattern information are handwritten, the human issue can lead to incorrect interpretation of the info. The label must be caught 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 down on the label must be oil- and waterproof. Otherwise, some data may be misplaced, making it extremely difficult for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s discuss the kinds of checks to discover out the transformer’s condition, the critical values, and the really helpful actions in every case:
a) Colour and look
This is a routine inspection applied to each oil pattern.
When an oil pattern arrives on the laboratory, one of many “tests” is a visual inspection of the oil pattern 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 lots of turbidity, it would point out a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the dust particles in the drain valve might be included into the pattern. If particles are identified as carbon, it would point out a attainable electrical fault within the unit. The DGA analysis of the oil will confirm if that is so.
Clear oils with out contamination will point out a good condition, and no motion is recommended.
When oils are dark or turbid, further analysis will verify any problems. The oil analysis results will also determine the degree and sort of motion.
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 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 avoid flashover within the unit, the breakdown voltage should be excessive.
If the values are Good, it is suggested to continue with the present pattern interval motion plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with different 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 tests indicate extreme aging, the oil can be changed with new or reclaimed oil. Another choice could be to perform on-site oil reclamation using 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. ส่วนประกอบpressuregauge of removes acid and water in addition to another compounds. Another advantage is that the oil can be re-used, and in most conditions, this may be accomplished without switching off the unit, which contributes to cost-saving. If doubtful – instead change off the unit during this therapy course of.
If the values are Poor, it’s advisable to take motion as quickly as attainable and never delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content material may cause flashover in the unit, leading to lack 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 results of this check should at all times be considered along side the breakdown strength. If it’s discovered that the water content material is high and the breakdown power is low, additional motion needs to be taken. It is recommended that a second pattern from the identical unit is examined to substantiate the results.
In the case of switching tools, the place there isn’t a paper present, the breakdown voltage is the figuring out factor.
It ought to be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it’s found that the unit’s working temperature is outside this temperature vary, it is best to refer to Annex A of the usual.
When the value obtained through analyses is GOOD, the conventional sampling interval can be maintained, requiring no further motion.
When the worth returns a FAIR outcome, more frequent sampling is really helpful. It can also be useful to consider different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the motion to be applied.
A POOR result will require immediate action from the asset supervisor. This may embody taking one other pattern to confirm the outcomes from the first analysis. If it is confirmed that the water content is excessive, the oil could be filtered; this course of ought to remove a big portion of the moisture from the oil if utilized accurately. Follow-up samples must be taken to make certain that the moisture content material continues to be inside the required limits. The reason is that the most significant portion of the water is caught up in the paper system in the transformer. This moisture will move from the paper into the oil under circumstances that favour this movement. It may be found later that the oil in the water has elevated again without any obvious purpose, but the supply can be the paper within the transformer.
A visual inspection can additionally be really helpful to discover out if any water would possibly transfer into the transformer or electrical gear via leaks. This drawback may be extra extreme if the transformer or electrical tools is outdoors and not in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.a 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 test for all classes besides F and G
The acids in oils are fashioned as a end result 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’ll adversely have an result on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this could result in sludge formation, usually across the decrease elements of the transformer core. The sludge will eventually type a semi-solid substance that is extremely troublesome to take away.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR end result, the sampling interval must be decreased to fit the scenario. Future analysis should include a visible inspection of the oil for sediment and sludge.
If the result is POOR according to the prescribed values in IEC 60422 Edition 4.0, the asset manager may determine to reclaim the oil or replace it with new or reclaimed oil, whichever choice may suit their necessities the best.
e) Dielectric Dissipation issue at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.a hundred.10 to 0.20>0.20
B, C< hundred.10 to 0.50>0.50
D<0.010.01 to 0.03>0.03
E< hundred.01 to zero.30>0.03
F, GNot a routine test
This is a routine test for all classes of electrical tools, besides F and G
The dielectric dissipation factor or tan delta of this check supplies info relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities which may influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result’s POOR, reclamation or an oil change is really helpful. The construction 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
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 one of the key parameters to evaluate the transformer insulation situation; that is based on the reality that DC resistance is delicate to oil degradation.
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.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of unique value
This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to confirm the small print relating to additives.
The two most typical oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a couple of,6-di-tertiary butyl-phenol (DBP). The function of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the getting older process within the oil and the strong insulation.
If the result is FAIR, it is suggested to top up the inhibitor stage to the prescribed stage per provider instructions. It is suggested to make use of a subject professional trained in the process to carry out this task.
If the outcome obtained is POOR, the recommendation for this state of affairs would counsel that the tip consumer continues to make use of the oil “uninhibited,” but this will likely result in extra rapid degradation of both the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; that is designed to keep dissolved oxygen at ranges below 1000ppm. This would be in the type 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, although it might add further 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 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 known as metal deactivators, react with reactive metallic surfaces and dissolved metals similar to 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 fundamental sorts, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the writer 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 more quickly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD results, common sample intervals could be maintained.
With FAIR outcomes, preserve common monitoring.
When POOR, it’s suggested to take away the oil or take away the supply of corrosivity from the oil via special oil remedy.
I) Sediment and sludge
This is not a routine test.
It is advised that this test is carried out when the oil results point out a excessive acid value and the dissipation issue is near the unacceptable restrict.
The results have to be less than zero.02% by mass to be negligible. If the results return a worth of greater than zero.02% by mass, it’s instructed that it be reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
This just isn’t a routine test
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to 28 20 to 25<22 <20
ENot a routine check
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the getting older course of. What this means in sensible phrases is there is extra polar compound current in the oil, lowering the ability of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial rigidity turns into a high quality criterion: the oil should be modified under a predefined restrict.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes 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 effect on the transformer system can be significant. The extent of the corrosion injury 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 impact 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 based on this institute’s threat 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 take a look at
If there’s a most lower in flashpoints by 10%, the tools may require further inspection. This value might differ in several countries.
It is advised to perform this take a look at when an uncommon odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to determine the situation of the transformer; it is a well being and safety impact test. PCB is hazardous to each people and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It can be required every time any maintenance has been carried out on the unit, and the chance of contamination is present. If PCB content material exceeds the recommended limits, the suitable action needs to be taken.
Units with a PCB content of more than 50ppm require a fire safety plan, environmental safety plan, and further precautionary measures when upkeep is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment proprietor.
Local regulatory our bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we’ll talk about this phenomenon partly II of the article. The limits for the different gases and the interpretation of this information based on worldwide requirements might be mentioned in detail, forming a half of the overall well being score willpower of the transformer.
Transformer condition monitoring is an interlaced, extremely exciting field of research. In this text, we centered on the forms of tests to determine the situation of the transformer, the critical values, and the beneficial actions.
The Health Index indication makes it attainable to see the supposed reliability of a selected unit at a selected date and time. This makes it potential to make sure greatest follow application and optimised maintenance. It also make it simpler to attract up a upkeep plan and action plan.
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 revealed by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ experience within the trade, having previously worked as laboratory manager for a serious industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her career, notably in the analysis of check data. Corné has huge practical and theoretical data of reliability maintenance programmes.

Scroll to Top