by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the related knowledge concerning an asset’s operation. If we interpret the information accurately, it can provide us nice insight into the asset’s situation.
Frequent monitoring of the asset can outcome in less maintenance required or more prolonged durations with none upkeep required.
It is crucial to determine the key parameters which might be wanted to offer us a complete picture of the actual 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 info telling us?
Has the condition of the unit modified because the final maintenance period?
Is it secure to function the unit?
Are there indicators of deterioration?
Is it safe to load the unit above the nameplate ranking for a particular period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit earlier than we have to consider replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring outline
It is vitally important to identify clear objectives as a half of your strategy. What do you wish to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the finish result and what you wish to accomplish, it might be a lot easier to determine the required parameters.
Health indexing of property is changing into a outstanding software in getting a clearer image of the condition of your transformer. Test parameters carry a numerical value-adding to the entire worth of the Health Index Value of the transformer. These parameter weight values were calculated based on the international requirements for mineral oils, indicating the important values stipulated within the varied requirements.
The scope of oil evaluation, interpretation of the info, and important values
At the start of this part, it’s important to state that we take care of totally different measurement transformers within the business. Transformers are divided into classes according to the kV scores of the gear. It is up to the reliability or asset supervisor to use the guidelines for bigger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or manager needs to discover out what type of testing would benefit him in figuring out drawback areas inside his fleet. Some of the analysis has been recognized as to routine sort tests. Still, there’s an extensive range of checks that can help in figuring out specific downside standards throughout the system, which could not be clear by way of the everyday day-to-day analysis often carried out.
Please see the rating courses in TABLE 1
TABLE 2 explains which oil analyses are recommended and how usually or under which circumstances.
Oil sampling
The taking of the oil pattern is considered one of the most crucial and critical influencers in the evaluation consequence. If a sample just isn’t taken to the prescribed process, then there’s a important possibility that the analysis carried out, and the outcomes acquired, are of no use.
Care ought to be taken to ensure high quality management procedures are utilized in every step of the sampling process as set out by international requirements. A good quality sample taken by making use of the proper procedure is essential. A pattern can be contaminated by numerous components, all of which can affect the outcome of the results in a negative manner.
All steps involved in sample taking must adhere to quality management procedures, including the container that’s used, the sampling package, the taking of the sample, the marking of the pattern, the closing of the pattern, the storage and dealing with of the sample, after which the supply of the sample to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human issue 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 should be waterproof, and the pen used to write on the label should be oil- and waterproof. Otherwise, some information could be lost, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s discuss 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 utilized to every oil sample.
When an oil sample arrives on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a transparent vessel to determine the color, turbidity, and possible particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there’s lots of turbidity, it might point out a excessive water content in the oil.
If the drain valve was not cleaned by the sampler, the dust particles within the drain valve may be incorporated into the sample. If particles are recognized as carbon, it would indicate a possible electrical fault in the unit. The DGA evaluation of the oil will verify if that is so.
Clear oils without contamination will indicate a good condition, and no motion is recommended.
When oils are darkish or turbid, further evaluation will confirm any problems. The oil analysis outcomes 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 point out the water content material or the presence of overseas 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 strongly recommended to continue with the present sample interval motion plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If different tests point out extreme aging, the oil can be changed with new or reclaimed oil. Another choice would be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar parts are removed from the oil. This course of removes acid and water in addition to some other compounds. Another benefit is that the oil may be re-used, and in most conditions, this can be done without switching off the unit, which contributes to cost-saving. If in doubt – as a substitute swap off the unit during this therapy process.
If the values are Poor, it is advisable to take motion as soon as attainable and not delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely excessive water content may cause flashover within the unit, leading to loss 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 test
This is a routine check for all courses of electrical tools, besides class G
The results of this check ought to always be thought-about at the aspect of the breakdown energy. 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 same unit is examined to verify the results.
In the case of switching tools, the place there is no paper present, the breakdown voltage is the figuring out factor.
It must be famous that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s operating temperature is outdoors this temperature range, it’s best to refer to Annex A of the usual.
When the value obtained by way of analyses is GOOD, the traditional sampling interval could be maintained, requiring no further action.
When the value returns a FAIR result, more frequent sampling is recommended. It can be helpful to consider other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be implemented.
A POOR outcome will require immediate action from the asset supervisor. This might include taking another pattern to confirm the outcomes from the first analysis. If it’s confirmed that the water content is excessive, the oil could be filtered; this course of should take away a big portion of the moisture from the oil if utilized appropriately. Follow-up samples have to be taken to make sure that the moisture content material is still throughout 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 transfer from the paper into the oil under situations that favour this movement. It could be discovered later that the oil within the water has increased once more without any apparent cause, but the supply could be the paper in the transformer.
A visible inspection can also be really helpful to discover out if any water may transfer into the transformer or electrical equipment via leaks. This drawback may be more severe if the transformer or electrical tools is outside and never in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to zero.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 take a look at
This is a routine take a look at for all courses except F and G
The acids in oils are formed as a end result 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’s going to adversely affect the insulation properties of the oil and can improve paper degradation. If left untreated in transformers, this could lead to sludge formation, often around the lower elements of the transformer core. The sludge will finally form a semi-solid substance that’s extraordinarily tough to remove.
If the result is GOOD, the regular sampling interval can continue.
In case of a FAIR result, the sampling interval ought to be decreased to suit the scenario. Future analysis ought to embrace a visible inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition, the asset manager could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever choice may swimsuit their requirements the most effective.
e) Dielectric Dissipation factor 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< hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E<0.a hundred.01 to 0.30>0.03
F, GNot a routine test
This is a routine take a look at for all courses of electrical gear, besides F and G
The dielectric dissipation issue or tan delta of this check provides data regarding 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, leading to section displacement and dielectric losses. Other impurities that might influence the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result is FAIR, extra frequent sampling and checking further parameters is beneficial.
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 broken 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 check
DC resistivity of the oil is among the key parameters to evaluate the transformer insulation situation; that is based mostly on the fact 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 is POOR, reclamation or an oil change is recommended.
g) Inhibitor content material percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original worth
This check is restricted to oils with this additive.
It could be advisable to contact the oil supplier to verify the small print relating to components.
The two commonest oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The objective of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the getting older course of in the oil and the solid insulation.
If the result is FAIR, it’s suggested to top up the inhibitor level to the prescribed degree per supplier instructions. It is suggested to make use of a subject professional educated within the procedure to perform this process.
If the outcome obtained is POOR, the recommendation for this scenario would suggest that the tip user continues to use the oil “uninhibited,” but this may result in extra rapid degradation of both the liquid and stable insulation.
It should be famous that some transformers have already got a built-in oil preservation system; that is designed to keep dissolved oxygen at levels beneath 1000ppm. This would be in the type 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 is not required, though it’d add additional protection 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 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 decreasing at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and cut back their price 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 primary varieties, sulfur-based and nitrogen-based. The first instructed 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 would possibly deplete more quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD results, regular sample intervals can be maintained.
With pressure gauge , keep common monitoring.
When POOR, it’s suggested to remove the oil or remove the source of corrosivity from the oil by way of special oil therapy.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is advised that this test is carried out when the oil results point out a excessive acid worth and the dissipation issue is near the unacceptable restrict.
The results have to be less than 0.02% by mass to be negligible. If the outcomes return a value of more than 0.02% by mass, it is advised that or not it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
This 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 check
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces during the aging course of. What this implies in sensible phrases is there might be more polar compound current within the oil, reducing the power 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 turns into a excessive quality criterion: the oil have to be changed under a predefined limit.
If outcomes are GOOD, proceed the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system can be important. The extent of the corrosion injury attributable to the sulfur could be so extreme that it’d trigger failure of the equipment if not checked. The addition of a copper passivator can reduce the influence of this compound on the transformer system.
In a examine by Doble, it has been discovered 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 need of corrective actions 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 power transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine test
If there is a most lower in flashpoints by 10%, the equipment may require additional inspection. This worth may differ in different countries.
It is suggested to carry out this take a look at when an unusual odour is observed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not decide the condition of the transformer; this may be a health and security impact test. PCB is hazardous to both humans and the setting; it is important to check for PCBs after the retro fill of a transformer. It can be required whenever any maintenance has been accomplished on the unit, and the potential of contamination is current. If PCB content exceeds the really helpful limits, the appropriate action needs to be taken.
Units with a PCB content material of greater than 50ppm require a fire security plan, environmental protection plan, and further precautionary measures when maintenance is finished. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the equipment owner.
Local regulatory bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we’ll discuss this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this data according to worldwide requirements shall be discussed in detail, forming part of the general health rating willpower of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling field of study. In this article, we targeted on the types of checks to determine the condition of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a particular date and time. This makes it potential to make sure best practice utility and optimised upkeep. It additionally make it simpler to draw up a upkeep plan and action plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical tools – supervision and maintenance 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.
four. 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 advisor. She has 20+ years’ expertise within the business, having previously worked as laboratory supervisor for a significant industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her profession, notably in the analysis of test knowledge. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.

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