by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant knowledge concerning an asset’s operation. If we interpret the data appropriately, it can give us nice perception into the asset’s situation.
Frequent monitoring of the asset can result in less upkeep required or more extended intervals without any upkeep required.
It is essential to identify the key parameters that are needed to provide us a complete picture of the particular status of the transformer and the motion we need to take to make sure the continued reliability of the asset to attain the utmost lifetime.
What is the data telling us?
Has the condition of the unit changed since the final upkeep period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it safe to load the unit above the nameplate score for a specific 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 want to contemplate replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring outline
It is vitally essential to determine clear objectives as a half of your strategy. What do you need to obtain by implementing this condition monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it will be a lot simpler to establish the required parameters.
Health indexing of belongings is changing into a remarkable tool in getting a clearer image of the situation of your transformer. Test parameters carry a numerical value-adding to the total value of the Health Index Value of the transformer. These parameter weight values have been calculated based mostly on the international standards for mineral oils, indicating the critical values stipulated in the various requirements.
The scope of oil analysis, interpretation of the info, and critical values
At the beginning of this section, it is essential to state that we take care of completely different measurement transformers in the business. Transformers are divided into classes based on the kV scores of the gear. It is as much as the reliability or asset supervisor to use the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical tests.
The upkeep engineer or manager wants to determine what type of testing would benefit him in figuring out problem areas inside his fleet. Some of the evaluation has been identified as to routine type tests. Still, there may be an intensive vary of tests that can assist in identifying particular problem standards within the system, which could not be clear by way of the everyday day-to-day analysis often performed.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are beneficial and how typically or beneath which circumstances.
Oil sampling
The taking of the oil pattern is probably one of the most crucial and significant influencers in the analysis outcome. If a pattern just isn’t taken to the prescribed process, then there’s a important risk that the analysis carried out, and the results acquired, are of no use.
Care must be taken to ensure high quality management procedures are applied in every step of the sampling process as set out by worldwide requirements. A good quality pattern taken by applying the right process is essential. A sample could be contaminated by numerous components, all of which can influence the finish result of the ends in a negative method.
All steps concerned 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 pattern, the closing of the pattern, the storage and handling of the sample, after which the delivery of the sample to the laboratory.
If the sample identification and sample data are handwritten, the human factor can result in incorrect interpretation of the data. The label should be caught onto the container to make sure 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 down on the label ought to be oil- and waterproof. Otherwise, some information could be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the sample to a selected transformer.
Now, let’s focus on the types of checks to find out the transformer’s situation, the critical values, and the beneficial actions in every case:
a) Colour and appearance
This is a routine inspection applied to each oil sample.
When an oil pattern arrives on the laboratory, one of the “tests” is a visible inspection of the oil pattern in a transparent vessel to determine 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 would indicate a high water content material in the oil.
If the drain valve was not wiped clean by the sampler, the dust particles in the drain valve may be incorporated into the pattern. If particles are identified as carbon, it might point out a potential electrical fault within the unit. The DGA analysis of the oil will confirm if that is so.
เครื่องมือที่ใช้วัดความดันโลหิต without contamination will indicate a great condition, and no action is recommended.
When oils are darkish or turbid, further evaluation will confirm any problems. The oil evaluation outcomes may also decide the diploma 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 point 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 overseas particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to continue with the current sample interval action plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter outcomes like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil via oil reconditioning processes. If various tests indicate severe getting older, the oil may be replaced with new or reclaimed oil. Another option would be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar parts are faraway from the oil. This course of removes acid and water in addition to another compounds. Another advantage is that the oil may be re-used, and in most conditions, this might be done without switching off the unit, which contributes to cost-saving. If unsure – instead switch off the unit throughout this therapy process.
If the values are Poor, it’s advisable to take action as quickly as potential and not delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely excessive water content material can cause flashover in the unit, leading to 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 take a look at
This is a routine take a look at for all classes of electrical equipment, besides class G
The outcomes of this check ought to all the time be thought-about along side the breakdown power. If it’s discovered that the water content material is excessive and the breakdown power is low, further motion must be taken. It is recommended that a second pattern from the same unit is tested to verify the outcomes.
In the case of switching gear, where there is not a paper current, the breakdown voltage is the figuring out factor.
It ought to be noted that the boundaries 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 operating temperature is exterior this temperature vary, it’s best to discuss with Annex A of the usual.
When the value obtained by way of analyses is GOOD, the normal sampling interval could be maintained, requiring no additional motion.
When the value returns a FAIR outcome, more frequent sampling is really helpful. It can additionally be helpful to assume about other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be applied.
A POOR result would require instant action from the asset supervisor. This may include taking one other pattern to confirm the outcomes from the first evaluation. If it is confirmed that the water content material is excessive, the oil could be filtered; this course of should take away a big portion of the moisture from the oil if applied accurately. Follow-up samples need to be taken to ensure that the moisture content is still throughout the required limits. The purpose is that the most significant slice of the water is caught up in the paper system within the transformer. This moisture will transfer from the paper into the oil under situations that favour this motion. It could be found later that the oil within the water has increased once more with none obvious cause, however the supply would be the paper in the transformer.
A visual inspection can be really helpful to determine if any water may transfer into the transformer or electrical tools through leaks. This problem might be more severe if the transformer or electrical gear is outside and not in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D< hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to zero.25>0.20
C< hundred.15 to 0.30>0.30
F, G Not a routine test
This is a routine check for all lessons except F and G
The acids in oils are fashioned due to chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist in the formation of the acids. Because acids are polar compounds, it’s going to adversely have an result on the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this can result in sludge formation, often across the decrease elements of the transformer core. The sludge will ultimately type a semi-solid substance that’s extremely difficult to remove.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval ought to be decreased to fit the situation. Future evaluation should embody a visual 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 could decide to reclaim the oil or exchange it with new or reclaimed oil, whichever possibility might swimsuit their necessities the best.
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< hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E< 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, besides F and G
The dielectric dissipation issue or tan delta of this take a look at provides data regarding the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that might affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result is 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 broken, in effect 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 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 amongst the key parameters to assess the transformer insulation situation; that is based on the truth that DC resistance is delicate to oil degradation.
When the result’s FAIR, more frequent sampling and checking extra parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original value
This check is restricted to oils with this additive.
It would be advisable to contact the oil provider to verify the small print regarding additives.
The two commonest 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 objective of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the getting older course of in the oil and the stable insulation.
If the result’s FAIR, it is suggested to prime up the inhibitor level to the prescribed degree per provider instructions. It is advised to use a field skilled educated in the process to perform this activity.
If the outcome obtained is POOR, the recommendation for this scenario would recommend that the end user continues to make use of the oil “uninhibited,” however this will likely lead to more speedy degradation of both the liquid and strong insulation.
It ought to be noted that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels beneath 1000ppm. This would be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances isn’t required, although it would add extra protection towards oil oxidation if the preservation system ever fails. [2]
h) Passivator content
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 lowering at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and cut back their fee of reaction with compounds in the oil. This contains oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental varieties, 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 would possibly deplete more rapidly; this depletion may accelerate when the oil is un-inhibited.
With GOOD results, regular sample intervals could be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it is advised to remove the oil or remove the supply of corrosivity from the oil via particular oil therapy.
I) Sediment and sludge
This is not a routine take a look at.
It is advised that this check is carried out when the oil outcomes point out a excessive acid value and the dissipation issue is close to the unacceptable limit.
The results have to be lower than zero.02% by mass to be negligible. If the outcomes return a value of more than 0.02% by mass, it is instructed that or not it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial pressure
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 take a look at
F, GNot Applicable
The interfacial pressure between transformer oil and water reduces through the growing older course of. What this means in practical terms is there could be extra polar compound current within the oil, reducing 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 tension becomes a quality criterion: the oil have to be changed below a predefined restrict.
If outcomes are GOOD, continue the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This just 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 could be significant. The extent of the corrosion injury attributable to the sulfur could be so severe that it would cause failure of the gear if not checked. The addition of a copper passivator can scale back the influence of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber merchandise used in transformers might 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 primarily based on this institute’s threat assessment research. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (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 is a most decrease in flashpoints by 10%, the equipment would possibly require additional inspection. This value would possibly differ in different countries.
It is suggested to carry out this check when an unusual odour is seen, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not determine the condition of the transformer; this can be a well being and security impression check. PCB is hazardous to both humans and the environment; it is important to check 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 risk of contamination is current. If PCB content exceeds the recommended limits, the suitable motion must be taken.
Units with a PCB content material of more than 50ppm require a hearth security plan, environmental protection plan, and further precautionary measures when upkeep is finished. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the equipment proprietor.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of information and interpretation, we’ll focus on this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data according to international requirements might be discussed in detail, forming part of the general well being ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling field of study. In this text, we centered on the forms of tests to determine the condition of the transformer, the important values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a particular date and time. This makes it potential to make sure finest practice software and optimised maintenance. It also make it simpler to draw up a maintenance plan and motion plan.
1. IEC 60422 Edition 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
three. 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 published by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise in the industry, having previously labored as laboratory supervisor for a major industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, particularly within the evaluation of test information. Corné has huge practical and theoretical knowledge of reliability upkeep programmes.

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