by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the relevant information relating to an asset’s operation. If we interpret the info correctly, it can give us nice perception into the asset’s situation.
Frequent monitoring of the asset can outcome in much less maintenance required or more extended durations without any maintenance required.
It is crucial to determine the necessary thing parameters that are wanted to offer us a whole image of the particular 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 modified because the last maintenance period?
Is it protected to function the unit?
Are there signs of deterioration?
Is it protected 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 lengthy can we use the unit earlier than we need to contemplate replacement?
Are the identified issues of a recurring nature?
Effective condition monitoring outline
It is vitally necessary to identify clear objectives as a half of your technique. What do you want to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the result and what you need to accomplish, it would be a lot easier to determine the required parameters.
Health indexing of property is becoming a outstanding device in getting a clearer picture 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 have been calculated primarily based on the worldwide requirements for mineral oils, indicating the crucial values stipulated in the numerous standards.
The scope of oil analysis, interpretation of the information, and critical values
At the start of this part, it’s essential to state that we deal with different size transformers in the industry. Transformers are divided into courses in accordance with the kV scores of the equipment. It is up to the reliability or asset manager to make use of the rules for larger equipment, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or manager needs to find out what type of testing would benefit him in identifying downside areas within his fleet. Some of the evaluation has been recognized as to routine sort checks. Still, there might be an intensive vary of checks that may assist in figuring out specific downside criteria inside the system, which might not be clear via the everyday day-to-day evaluation often performed.
Please see the ranking lessons in TABLE 1
TABLE 2 explains which oil analyses are beneficial and the way often or beneath which circumstances.
Oil sampling
The taking of the oil pattern is among the most vital and critical influencers in the analysis consequence. If a sample just isn’t taken to the prescribed process, then there is a vital risk that the evaluation carried out, and the outcomes acquired, are of no use.
Care should be taken to make sure high quality management procedures are applied in each step of the sampling course of as set out by international standards. A good high quality pattern taken by making use of the correct procedure is crucial. A pattern may be contaminated by numerous factors, all of which might influence the end result of the leads to a negative manner.
All steps concerned in pattern taking must adhere to high quality control procedures, together with the container that’s used, the sampling kit, the taking of the pattern, the marking of the sample, the closing of the sample, the storage and dealing with of the pattern, and then the supply of the pattern to the laboratory.
If the pattern identification and sample data are handwritten, the human issue can result in 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 accomplished in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to put in writing on the label must be oil- and waterproof. Otherwise, some knowledge could be misplaced, making it extremely tough for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s talk about the forms of tests to determine the transformer’s condition, the important values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection utilized to every oil pattern.
When an oil pattern arrives on the laboratory, one of the “tests” is a visual inspection of the oil sample in a transparent vessel to discover out the colour, turbidity, and potential particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there’s lots of turbidity, it might point out a high water content material in the oil.
If the drain valve was not cleaned by the sampler, the filth particles in the drain valve may be integrated into the sample. If particles are recognized as carbon, it would point out a potential electrical fault within the unit. The DGA evaluation of the oil will verify if so.
Clear oils without contamination will point out a great situation, and no motion is recommended.
When oils are dark or turbid, additional analysis will confirm any problems. The oil analysis outcomes may even decide 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 point 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 international particles, or both in the oil being analysed.
As the oil in transformers acts as an insulation medium to keep away from flashover in the unit, the breakdown voltage have to be high.
If the values are Good, it is strongly recommended to continue with the present pattern interval motion plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter results like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil through oil reconditioning processes. If various checks indicate extreme growing older, the oil could be replaced with new or reclaimed oil. Another possibility could be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water as nicely as some other compounds. Another advantage is that the oil could be re-used, and in most situations, this might be accomplished without switching off the unit, which contributes to cost-saving. If unsure – as a substitute change off the unit throughout this remedy process.
If the values are Poor, it’s advisable to take action as soon as possible and never delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content could cause flashover in the unit, leading to loss of the asset.
c) Water content (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 take a look at
This is a routine test for all classes of electrical gear, besides class G
The outcomes of this check ought to at all times be considered along side the breakdown strength. If it is discovered that the water content material is high and the breakdown power is low, additional action must be taken. It is beneficial that a second pattern from the same unit is tested to confirm the results.
In the case of switching tools, the place there is no paper present, the breakdown voltage is the figuring out factor.
It ought to be noted that the bounds indicated by IEC 60422 Edition four apply to transformers with working temperatures between forty and 70 °C. If it’s found that the unit’s working temperature is exterior this temperature range, it’s best to check with Annex A of the standard.
When the worth obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no additional action.
When the value returns a FAIR outcome, extra frequent sampling is really helpful. It is also useful to contemplate other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be implemented.
A POOR result would require quick action from the asset supervisor. This would possibly include taking one other sample to substantiate the results from the first evaluation. If it’s confirmed that the water content is excessive, the oil can be filtered; this process ought to take away a large portion of the moisture from the oil if applied appropriately. Follow-up samples have to be taken to guarantee that the moisture content remains to be inside the required limits. The reason is that the most significant slice of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil underneath circumstances that favour this movement. It could be found later that the oil in the water has elevated once more with none apparent purpose, but the source could be the paper in the transformer.
A visible inspection can also be really helpful to find out if any water would possibly transfer into the transformer or electrical equipment via leaks. This downside could be extra extreme if the transformer or electrical equipment is exterior and not in a lined space.
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 zero.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine check
This is a routine test for all courses besides 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’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, usually around the decrease elements of the transformer core. The sludge will ultimately kind a semi-solid substance that’s extremely troublesome to remove.
If the result’s GOOD, the regular sampling interval can continue.
In case of a FAIR end result, the sampling interval should be decreased to suit the scenario. Future evaluation ought to embrace a visual inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition, the asset supervisor might resolve to reclaim the oil or replace it with new or reclaimed oil, whichever possibility might suit their requirements one of the best.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A< hundred.10 to 0.20>0.20
B, C<0.a hundred.10 to zero.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 take a look at for all courses of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this take a look at supplies information regarding the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we have the formation of polar compounds, resulting in part displacement and dielectric losses. Other impurities which may influence the dissipation issue embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking additional parameters is beneficial.
When the result is POOR, reclamation or an oil change is recommended. The structure of the oil is broken, 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 test
DC resistivity of the oil is likely certainly one of the key parameters to evaluate the transformer insulation situation; this is primarily based on the truth that DC resistance is sensitive to oil degradation.
When the result is FAIR, extra frequent sampling and checking additional parameters is recommended.
When the result’s POOR, reclamation or an oil change is really helpful.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of authentic value
This test is restricted to oils with this additive.
It could be advisable to contact the oil provider to verify the details 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 purpose of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the growing older process within the oil and the strong insulation.
If the result is FAIR, it is suggested to prime up the inhibitor stage to the prescribed degree per supplier directions. It is advised to use a area professional trained in the procedure to carry out this task.
If the end result obtained is POOR, the recommendation for this situation would suggest that the end user continues to make use of the oil “uninhibited,” however this will lead to extra rapid degradation of each the liquid and strong insulation.
It should be famous that some transformers already have a built-in oil preservation system; that is designed to keep 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 outfitted with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances just isn’t required, though it would add extra safety towards 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 reducing at >10mg/kg/year
Passivators, also called steel deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and scale back their fee of response with compounds in the oil. This consists of oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary 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 would possibly deplete extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD results, regular sample intervals could be maintained.
With FAIR results, maintain common monitoring.
When POOR, it’s suggested to take away the oil or remove the source 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 test is performed when the oil results indicate a high acid worth and the dissipation factor is near the unacceptable restrict.
The outcomes have to be less than 0.02% by mass to be negligible. If the results return a worth of greater than zero.02% by mass, it is instructed that it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial pressure
This isn’t a routine take a look at
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 pressure between transformer oil and water reduces in the course of the growing older course of. What this implies in sensible terms is there is extra polar compound present in the oil, reducing the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation number. Therefore, the interfacial rigidity becomes a excessive quality criterion: the oil must be changed beneath a predefined limit.
If results are GOOD, proceed the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine test.
Oil is either 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 damage brought on by the sulfur could be so extreme that it would trigger failure of the equipment 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 approved for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s risk evaluation study. [4]
l) Particle counting and sizing
Table 3: 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 check
If there is a most lower in flashpoints by 10%, the gear may require further inspection. This value might differ in numerous countries.
Controversial is advised to carry out this check when an unusual odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the situation of the transformer; this may be a well being and security influence check. PCB is hazardous to both people and the environment; it’s critical to test for PCBs after the retro fill of a transformer. It can be required each time any maintenance has been accomplished on the unit, and the risk of contamination is current. If PCB content exceeds the really helpful limits, the suitable action needs to be taken.
Units with a PCB content of more than 50ppm require a fireplace security plan, environmental safety plan, and extra precautionary measures when upkeep is completed. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the equipment owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of data and interpretation, we are going to talk about this phenomenon partially II of the article. The limits for the completely different gases and the interpretation of this knowledge according to worldwide requirements shall be discussed intimately, forming a part of the overall well being ranking willpower of the transformer.
Transformer condition monitoring is an interlaced, highly thrilling field of research. In this article, we focused on the kinds of exams to determine the situation of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it attainable to see the supposed reliability of a particular unit at a selected date and time. This makes it potential to ensure greatest practice application and optimised maintenance. It also make it simpler to draw up a maintenance 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
three. 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 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 in the industry, having previously worked as laboratory supervisor for a significant industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her career, particularly within the analysis of test knowledge. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.

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