by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related knowledge concerning an asset’s operation. If we interpret the info accurately, it can give us nice perception into the asset’s condition.
Frequent monitoring of the asset can end result in less upkeep required or more prolonged periods without any maintenance required.
It is crucial to identify the vital thing parameters which would possibly be wanted to provide us a whole 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 realize the utmost lifetime.
What is the info telling us?
Has the condition of the unit changed for the reason that final upkeep period?
Is it safe to operate the unit?
Are there indicators of deterioration?
Is it secure to load the unit above the nameplate ranking for a selected period?
Are we required to implement action to make sure the continued reliability of the unit?
How long can we use the unit earlier than we need to think about replacement?
Are the recognized issues of a recurring nature?
Effective situation monitoring define
It is vitally important to establish clear objectives as part of your strategy. What do you need to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the end result and what you wish to accomplish, it will be a lot easier to determine the required parameters.
Health indexing of property is becoming a exceptional 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 mostly on the international standards for mineral oils, indicating the important values stipulated within the varied standards.
The scope of oil evaluation, interpretation of the information, and critical values
At the start of this part, it’s essential to state that we cope with completely different measurement transformers within the industry. Transformers are divided into lessons according to the kV ratings of the equipment. It is as much as the reliability or asset supervisor to use the rules for larger tools, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager needs to determine what kind of testing would profit him in identifying drawback areas inside his fleet. Some of the analysis has been recognized as to routine kind tests. Still, there’s an extensive range of exams that may assist in identifying specific problem criteria inside the system, which could not be clear by way of the everyday day-to-day analysis usually performed.
Please see the score lessons in TABLE 1
TABLE 2 explains which oil analyses are really helpful and the way often or under which circumstances.
Oil sampling
The taking of the oil sample is one of the most important and important influencers in the evaluation outcome. If a pattern isn’t taken to the prescribed procedure, then there is a important risk that the evaluation performed, and the results acquired, are of no use.
Care ought to be taken to ensure quality management procedures are applied in every step of the sampling course of as set out by international requirements. A good quality sample taken by applying the proper process is crucial. A pattern may be contaminated by varied elements, all of which can influence the result of the ends in a adverse method.
All steps involved in sample taking should adhere to quality management procedures, including the container that is used, the sampling equipment, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and handling of the sample, after which the supply of the pattern to the laboratory.
If the sample identification and pattern information are handwritten, the human issue can result in incorrect interpretation of the info. The label should be stuck 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 put in writing on the label should be oil- and water resistant. Otherwise, some data might be misplaced, making it extremely tough for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s discuss the forms of tests to determine the transformer’s condition, the crucial values, and the beneficial actions in every case:
a) Colour and appearance
This is a routine inspection utilized to each oil pattern.
When an oil sample arrives on the laboratory, one of many “tests” is a visual inspection of the oil pattern in a clear vessel to discover out the colour, turbidity, and attainable particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there might be lots of turbidity, it might point out a excessive water content material within the oil.
If the drain valve was not cleaned by the sampler, the dirt particles within the drain valve could be included into the sample. If particles are identified as carbon, it might point out a potential electrical fault within the unit. The DGA evaluation of the oil will confirm if so.
Clear oils without contamination will point out a good situation, and no action is recommended.
When oils are darkish or turbid, further evaluation will confirm any issues. The oil evaluation outcomes will also determine the degree and kind 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 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 overseas particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to keep away from flashover within the unit, the breakdown voltage should be excessive.
If the values are Good, it is strongly recommended to continue with the current pattern interval action plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with different parameter outcomes just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil via oil reconditioning processes. If different exams indicate severe getting older, the oil may be changed with new or reclaimed oil. Another option would 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 components are faraway from the oil. This course of removes acid and water in addition to some other compounds. Another benefit is that the oil can be re-used, and in most situations, this can be carried out with out switching off the unit, which contributes to cost-saving. If doubtful – instead swap off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as quickly as possible and not delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content could cause flashover in the unit, leading to lack 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 take a look at
This is a routine check for all courses of electrical gear, except class G
The results of this take a look at ought to all the time be thought of in conjunction with the breakdown power. If it is discovered that the water content material is high and the breakdown strength is low, further motion must be taken. It is really helpful that a second pattern from the same unit is examined to verify the results.
In the case of switching gear, the place there isn’t any paper current, the breakdown voltage is the figuring out factor.
It must be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it’s found that the unit’s working temperature is outside this temperature range, it is best to check with Annex A of the usual.
When the worth obtained through analyses is GOOD, the conventional sampling interval can be maintained, requiring no further action.
When the value returns a FAIR end result, extra frequent sampling is beneficial. It is also helpful to contemplate different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be applied.
A POOR outcome would require immediate motion from the asset supervisor. This might embody taking another pattern to verify the outcomes from the primary evaluation. If it is confirmed that the water content material is excessive, the oil can be filtered; this course of should take away a big portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to guarantee that the moisture content material remains to be throughout the required limits. The cause is that essentially the most good portion 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 may be found later that the oil within the water has increased once more without any obvious purpose, but the supply can be the paper within the transformer.
A visible inspection can be really helpful to discover out if any water might move into the transformer or electrical equipment via leaks. This problem might be extra extreme if the transformer or electrical equipment is outdoors and never in a covered 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 zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine take a look at for all courses besides F and G
The acids in oils are shaped as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will help within the formation of the acids. Because acids are polar compounds, it’s going to adversely affect the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this will result in sludge formation, normally around the decrease components of the transformer core. The sludge will finally type a semi-solid substance that is extraordinarily difficult to remove.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval ought to be decreased to fit the scenario. Future evaluation ought to embody a visible inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition, the asset supervisor might decide to reclaim the oil or replace it with new or reclaimed oil, whichever option would possibly suit 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.a hundred.10 to zero.20>0.20
B, C<0.100.10 to zero.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 check
This is a routine take a look at for all lessons of electrical equipment, except F and G
The dielectric dissipation issue or tan delta of this check offers information concerning 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 section displacement and dielectric losses. Other impurities which may affect the dissipation factor include water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking extra parameters is recommended.
When the result’s POOR, reclamation or an oil change is recommended. The construction of the oil is broken, in effect 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 amongst the key parameters to assess the transformer insulation condition; that is based mostly on the fact that DC resistance is delicate to oil degradation.
When the result is FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result’s 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 unique worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the details regarding components.
The two most typical oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the aging process within the oil and the solid insulation.
If the result’s FAIR, it is suggested to top up the inhibitor stage to the prescribed level per provider instructions. It is suggested to use a field skilled trained in the process to perform this activity.
If the result obtained is POOR, the recommendation for this scenario would recommend that the end person continues to use the oil “uninhibited,” but this will result in more speedy degradation of each the liquid and stable insulation.
It ought to be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels under 1000ppm. This can 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 underneath these circumstances just isn’t required, although it would add further 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 recognized as metallic deactivators, react with reactive metallic surfaces and dissolved metals similar to copper and silver and reduce their fee of reaction with compounds within the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental types, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the creator is conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator might deplete extra rapidly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, regular pattern intervals may be maintained.
With FAIR outcomes, preserve common monitoring.
When POOR, it’s advised to remove the oil or take away the source of corrosivity from the oil via special oil therapy.
I) Sediment and sludge
This is not a routine check.
It is suggested that this take a look at is carried out when the oil results point out a excessive acid value and the dissipation factor is near the unacceptable restrict.
The outcomes must be less than 0.02% by mass to be negligible. If the outcomes return a value of greater than 0.02% by mass, it’s advised that it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial rigidity
This isn’t a routine take a look at
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to 28 20 to 25<22 <20
ENot a routine test
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the aging course of. What this implies in practical phrases is there’s more polar compound present within the oil, decreasing the power of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial rigidity turns into a top quality criterion: the oil must be changed under a predefined restrict.
If results are GOOD, continue the regular sampling interval.
If results are FAIR, decrease the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t 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 may be important. The extent of the corrosion injury attributable to the sulfur could be so severe that it would trigger 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 discovered that rubber merchandise used in transformers may 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 mostly 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 utilizing IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there’s a maximum lower in flashpoints by 10%, the gear might require additional inspection. This worth may differ in several international locations.
It is suggested to carry out this test when an uncommon odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB ( ส่วนประกอบpressuregauge )
This test is not to decide the situation of the transformer; this could be a well being and security impression test. PCB is hazardous to both people and the setting; it’s critical to check for PCBs after the retro fill of a transformer. It can also be required each time any maintenance has been done on the unit, and the potential for contamination is present. If PCB content material exceeds the really helpful limits, the appropriate motion must be taken.
Units with a PCB content of greater than 50ppm require a fire safety plan, environmental safety plan, and extra precautionary measures when maintenance is finished. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the equipment owner.
Local regulatory bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we are going to discuss this phenomenon in part II of the article. The limits for the different gases and the interpretation of this information according to international standards shall be discussed in detail, forming part of the general well being score determination of the transformer.
Transformer situation monitoring is an interlaced, extremely thrilling area of research. In this article, we focused on the types of checks to determine the condition of the transformer, the critical values, and the really helpful actions.
The Health Index indication makes it possible to see the supposed reliability of a specific unit at a particular date and time. This makes it attainable to make sure greatest follow software and optimised maintenance. 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 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.
four. CIGRE technical brochure 378, 2009 “Copper sulfide in Transformer Insulation.”
5. CIGRE Technical Brochure 157, 2000 “Effect of particles on transformer dielectric energy.”
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’ experience within the industry, having previously labored as laboratory manager for a major industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her career, particularly in the evaluation of test information. Corné has huge sensible and theoretical data of reliability maintenance programmes.

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