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 info appropriately, it can provide us nice insight into the asset’s condition.
Frequent monitoring of the asset may end up in less maintenance required or extra extended intervals without any maintenance required.
It is essential to identify the necessary thing parameters which might be needed to give us a complete image of the particular status of the transformer and the motion 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 last upkeep period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate ranking for a specific period?
Are we required to implement motion to make sure the continued reliability of the unit?
How long can we use the unit earlier than we want to think about replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring outline
It is vitally essential to establish clear targets as part of your strategy. What do you want to obtain 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 want to accomplish, it will be a lot easier to determine the required parameters.
Health indexing of belongings is changing into a outstanding tool in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the whole value of the Health Index Value of the transformer. weksler ea14 had been calculated primarily based on the worldwide standards for mineral oils, indicating the important values stipulated in the varied standards.
The scope of oil evaluation, interpretation of the data, and critical values
At the beginning of this section, it’s important to state that we cope with completely different dimension transformers within the trade. Transformers are divided into courses based on the kV scores of the gear. It is as much as the reliability or asset manager to use the guidelines for bigger tools, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or manager wants to discover out what sort of testing would profit him in figuring out downside areas within his fleet. Some of the evaluation has been identified as to routine kind checks. Still, there’s an in depth range of exams that can help in figuring out particular downside criteria inside the system, which could not be clear via the everyday day-to-day analysis normally performed.
Please see the ranking classes in TABLE 1
TABLE 2 explains which oil analyses are recommended and how usually or underneath which circumstances.
Oil sampling
The taking of the oil sample is considered one of the most important and significant influencers in the analysis end result. If a sample is not taken to the prescribed procedure, then there is a vital chance that the evaluation carried out, and the outcomes acquired, are of no use.
Care ought to be taken to make sure quality control procedures are utilized in every step of the sampling course of as set out by international standards. A good high quality sample taken by applying the correct process is crucial. A sample can be contaminated by varied elements, all of which can affect the end result of the results in a negative manner.
All steps involved in pattern taking should adhere to high 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 sample, the storage and handling of the pattern, after which the delivery of the sample to the laboratory.
If the pattern identification and pattern information are handwritten, the human issue can lead to incorrect interpretation of the information. The label must be caught onto the container to ensure 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 down on the label ought to be oil- and waterproof. Otherwise, some knowledge could be lost, making it extremely difficult for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s focus on the forms of exams to determine the transformer’s situation, the critical values, and the beneficial actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil sample.
When an oil sample arrives at the laboratory, one of the “tests” is a visible inspection of the oil sample in a clear vessel to discover out the color, turbidity, and attainable particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there is plenty of turbidity, it’d point out 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 might be incorporated into the sample. If particles are recognized as carbon, it’d indicate a potential electrical fault within the unit. The DGA evaluation of the oil will confirm if so.
Clear oils without contamination will indicate a good situation, and no action is really helpful.
When oils are dark or turbid, additional evaluation will confirm any issues. The oil evaluation outcomes may even decide the degree 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 level application<40 kV for OLTC in delta or line-end software
G <30
This is a routine inspection.
Breakdown voltage will point out the water content 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 recommended to proceed with the present pattern interval motion 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 strongly recommended to recondition the oil through oil reconditioning processes. If different exams point out extreme getting older, the oil could be changed with new or reclaimed oil. Another possibility could be to carry out 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 parts are faraway from the oil. This process removes acid and water as well as 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 in doubt – as an alternative swap off the unit during this treatment process.
If the values are Poor, it is advisable to take action as quickly as potential and never delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content material may cause flashover within the unit, resulting in loss of the asset.
c) Water content (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 test
This is a routine test for all lessons of electrical equipment, except class G
The outcomes of this take a look at should at all times be thought of at the side of the breakdown power. If it is found that the water content material is excessive and the breakdown strength is low, further action must be taken. It is recommended that a second pattern from the identical unit is examined to confirm the outcomes.
In the case of switching gear, where there is not any paper current, the breakdown voltage is the determining factor.
It ought to be noted that the bounds indicated by IEC 60422 Edition 4 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 refer to Annex A of the standard.
When the value obtained via analyses is GOOD, the conventional sampling interval may be maintained, requiring no additional motion.
When the worth returns a FAIR outcome, more frequent sampling is recommended. It can be useful to assume about other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the motion to be applied.
A POOR end result will require quick action from the asset manager. This would possibly embody taking another sample to substantiate the outcomes from the primary evaluation. If it is confirmed that the water content material is high, the oil may be filtered; this process should take away a big portion of the moisture from the oil if utilized accurately. Follow-up samples have to be taken to make sure that the moisture content continues to be inside the required limits. The cause is that the most good portion of the water is caught up in the paper system in the transformer. This moisture will move from the paper into the oil underneath situations that favour this motion. It may be found later that the oil in the water has increased again with none obvious cause, however the source could be the paper within the transformer.
A visible inspection can be beneficial to find out if any water may transfer into the transformer or electrical equipment by way of leaks. This drawback might be more extreme if the transformer or electrical tools is outside and not in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.15>0.15
B, E<0.a hundred.10 to zero.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine test for all classes except F and G
The acids in oils are formed because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help within the formation of the acids. Because acids are polar compounds, it’ll adversely have an result on the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this can result in sludge formation, normally around the decrease components of the transformer core. The sludge will finally type a semi-solid substance that’s extraordinarily tough to take away.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to fit the situation. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset supervisor might decide to reclaim the oil or substitute it with new or reclaimed oil, whichever possibility would possibly swimsuit their requirements the most effective.
e) Dielectric Dissipation issue at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A< hundred.10 to zero.20>0.20
B, C<0.a hundred.10 to zero.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 take a look at
This is a routine test for all courses of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this take a look at provides info relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we now have the formation of polar compounds, leading to section displacement and dielectric losses. Other impurities that might influence the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
When the result is POOR, reclamation or an oil change is beneficial. The structure of the oil is damaged, in impact that the chemical bonds between the molecules have damaged down, and even with filtration, the recommended 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 take a look at
DC resistivity of the oil is certainly one of the key parameters to evaluate the transformer insulation situation; that is based on the reality that DC resistance is sensitive to grease degradation.
When the result is FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of unique worth
This test is restricted to oils with this additive.
It could be advisable to contact the oil supplier to confirm the small print concerning 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 stop oxygen from reacting with the oil. This significantly slows the getting older course of within the oil and the strong insulation.
If the result’s FAIR, it’s advised to top up the inhibitor degree to the prescribed level per provider instructions. It is advised to make use of a field skilled skilled within the process to perform this activity.
If the end result obtained is POOR, the recommendation for this situation would recommend that the end user continues to make use of the oil “uninhibited,” however this will likely result in more speedy degradation of each the liquid and stable insulation.
It should be famous that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels under 1000ppm. This could be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, although it might add additional safety in opposition to 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 decrease of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and reduce their fee of response with compounds within the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary types, sulfur-based and nitrogen-based. The first suggested 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 more quickly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it’s suggested to remove the oil or take away the supply of corrosivity from the oil through special oil remedy.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this check is carried out when the oil outcomes point out a high acid value and the dissipation factor is near the unacceptable limit.
The results need to be less than zero.02% by mass to be negligible. If the results return a price of greater than 0.02% by mass, it’s advised 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 twenty-eight 20 to 25<22 <20
ENot a routine test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces during the getting older process. What this means in sensible phrases is there is more polar compound current within the oil, reducing the ability of an oil to function 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 have to be changed under a predefined restrict.
If results are GOOD, proceed the common 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 test.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be significant. The extent of the corrosion damage brought on by the sulfur could be so extreme that it’d cause failure of the gear if not checked. The addition of a copper passivator can reduce the impression of this compound on the transformer system.
In a research 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 would possibly 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 energy transformer insulating oil as measured utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine test
If there’s a most lower in flashpoints by 10%, the gear might require further inspection. This value may differ in several international locations.
It is advised to perform this check when an uncommon odour is noticed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not decide the condition of the transformer; this can be a health and safety impact test. PCB is hazardous to both people and the environment; 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 chance of contamination is current. If PCB content exceeds the beneficial limits, the appropriate action must be taken.
Units with a PCB content of greater than 50ppm require a fireplace safety plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the gear proprietor.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of information and interpretation, we will talk about this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this information in accordance with international requirements might be discussed in detail, forming part of the overall health score determination of the transformer.
Transformer situation monitoring is an interlaced, highly exciting field of study. In this text, we focused on the kinds of exams to determine the condition of the transformer, the crucial values, and the really helpful actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a selected date and time. This makes it attainable to make sure greatest apply application and optimised maintenance. It additionally make it simpler to draw up a maintenance plan and action plan.
1. IEC 60422 Edition 4.0 2013-01 International Standard (Mineral insulating oils in electrical gear – 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.
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 published by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the business, having beforehand labored as laboratory manager for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, particularly within the evaluation of check knowledge. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.

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