by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent collecting, measuring, recording, and analysis of the related data relating to an asset’s operation. If we interpret the information correctly, it may give us nice insight into the asset’s condition.
Frequent monitoring of the asset may end up in less upkeep required or extra extended periods without any maintenance required.
It is crucial to establish the key parameters that are wanted to offer us an entire picture of the actual 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 modified for the explanation that last upkeep 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 selected period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit before we want to consider replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring define
It is vitally essential to identify clear objectives as part of your technique. What do you want to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it will be much easier to establish the required parameters.
Health indexing of assets is turning into a remarkable tool 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 worldwide requirements for mineral oils, indicating the crucial values stipulated in the varied standards.
The scope of oil analysis, interpretation of the data, and important values
At the start of this section, it’s important to state that we deal with different measurement transformers in the trade. Transformers are divided into lessons in accordance with the kV rankings of the tools. It is up to the reliability or asset manager to make use of the rules for bigger tools, thereby implementing shorter increment oil analysis and electrical exams.
The maintenance engineer or supervisor wants to discover out what kind of testing would profit him in figuring out drawback areas inside his fleet. Some of the analysis has been identified as to routine type tests. Still, there might be an in depth vary of checks that may assist in figuring out specific downside standards inside the system, which might not be clear by way of the typical day-to-day analysis often carried out.
Please see the ranking lessons 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 among the most crucial and significant influencers in the evaluation consequence. If a sample just isn’t taken to the prescribed procedure, then there is a vital possibility that the evaluation carried out, and the results acquired, are of no use.
Care should be taken to make sure high quality control procedures are applied in each step of the sampling process as set out by international standards. A good high quality pattern taken by applying the correct process is crucial. A pattern can be contaminated by varied factors, all of which can affect the finish result of the leads to a adverse method.
All steps concerned in sample taking should adhere to quality management procedures, including the container that’s used, the sampling equipment, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and handling of the pattern, after which the delivery of the pattern to the laboratory.
If the pattern identification and pattern knowledge are handwritten, the human factor can lead to incorrect interpretation of the info. The label must 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 water resistant. Otherwise, some information might be lost, 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 kinds of tests to determine the transformer’s condition, the important 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 at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a transparent vessel to determine the color, turbidity, and possible particle identification.
Dark oils would possibly point out chemical degradation or contamination of the oil.
When there is plenty of turbidity, it might point out a high water content within the oil.
If the drain valve was not wiped clean by the sampler, the filth particles within the drain valve could be integrated into the sample. If particles are recognized as carbon, it’d indicate a potential electrical fault in the unit. The DGA analysis of the oil will affirm if so.
Clear oils without contamination will point out a great condition, and no motion is really helpful.
When oils are darkish or turbid, further analysis will affirm any issues. The oil analysis outcomes will also determine the diploma and sort 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 application
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 avoid flashover in the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to proceed with the present pattern interval action 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 recommended to recondition the oil via oil reconditioning processes. If different tests point out severe getting older, the oil may be replaced with new or reclaimed oil. Another possibility would be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar components are faraway from the oil. This course of removes acid and water as well as another compounds. Another advantage is that the oil can be re-used, and in most conditions, this can be accomplished without switching off the unit, which contributes to cost-saving. If unsure – instead switch off the unit during this therapy course of.
If the values are Poor, it’s advisable to take action as soon as attainable and never delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content could cause flashover within 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 check
This is a routine test for all lessons of electrical gear, except class G
The outcomes of this test should all the time be considered along side the breakdown power. If it’s discovered that the water content is excessive and the breakdown energy is low, further motion must be taken. It is beneficial that a second sample from the same unit is examined to confirm the results.
In the case of switching equipment, where there is not a paper current, the breakdown voltage is the determining issue.
It should be famous that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it’s found that the unit’s operating temperature is outside this temperature range, it’s best to check with Annex A of the standard.
When the value obtained via analyses is GOOD, the traditional sampling interval could be maintained, requiring no further motion.
When the worth returns a FAIR outcome, more frequent sampling is recommended. It can be helpful to suppose about other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR outcome would require quick motion from the asset manager. This might embrace taking one other sample to substantiate the outcomes from the first analysis. If it’s confirmed that the water content material is excessive, the oil can be filtered; this course of ought to take away a large portion of the moisture from the oil if applied correctly. Follow-up samples have to be taken to guarantee that the moisture content continues to be within the required limits. The reason is that the most good portion of the water is caught up within the paper system in the transformer. This moisture will move from the paper into the oil beneath situations that favour this motion. It might be discovered later that the oil in the water has increased again with none apparent cause, but the supply could be the paper within the transformer.
A visible inspection can be recommended to discover out if any water would possibly transfer into the transformer or electrical tools by way of leaks. This drawback may be more extreme if the transformer or electrical equipment is outside and never 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<0.100.10 to 0.25>0.20
C<0.100.15 to 0.30>0.30
F, G Not a routine check
This is a routine check for all classes besides 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 will adversely affect the insulation properties of the oil and will enhance paper degradation. If left untreated in transformers, this could result in sludge formation, usually around the decrease components of the transformer core. The sludge will eventually type a semi-solid substance that’s extraordinarily tough to remove.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR result, the sampling interval ought to be decreased to suit the situation. Future evaluation should embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR in accordance with the prescribed values in IEC 60422 Edition four.0, the asset supervisor could decide to reclaim the oil or replace it with new or reclaimed oil, whichever choice might suit their requirements the best.
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 0.20>0.20
B, C< hundred.10 to 0.50>0.50
D<0.010.01 to 0.03>0.03
E<0.100.01 to zero.30>0.03
F, GNot a routine check
This is a routine test for all classes of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this check supplies data relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that may influence the dissipation factor include water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking additional parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful. The construction of the oil is damaged, in impact that the chemical bonds between the molecules have broken down, and even with filtration, the really helpful 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 among the key parameters to assess the transformer insulation condition; this is based on the reality that DC resistance is delicate 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 beneficial.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original worth
This take a look at is restricted to oils with this additive.
It would be advisable to contact the oil supplier to confirm the details concerning additives.
The two most typical 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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the growing older process within the oil and the solid insulation.
If the result is FAIR, it is advised to top up the inhibitor degree to the prescribed degree per supplier instructions. It is suggested to use a subject skilled educated within the process to perform this task.
If the end result obtained is POOR, the advice for this state of affairs would counsel that the top user continues to use the oil “uninhibited,” however this will likely result in extra rapid degradation of each the liquid and stable 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 under 1000ppm. This would be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, though it would add further 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 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 reducing at >10mg/kg/year
Passivators, also referred to as steel deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and scale back their rate 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 sorts, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the writer 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 more rapidly; this depletion might accelerate when the oil is un-inhibited.
With GOOD results, common sample intervals may be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it is suggested to remove the oil or take away the source of corrosivity from the oil by way of special oil therapy.
I) Sediment and sludge
This isn’t a routine test.
It is advised that this check is carried out when the oil results indicate a high acid value and the dissipation issue is near the unacceptable limit.
The outcomes need to be less than zero.02% by mass to be negligible. If the outcomes return a worth of greater than 0.02% by mass, it is instructed that it be reclaimed; in any other case, an oil change is recommended.
j) Interfacial pressure
This is not 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 pressure between transformer oil and water reduces in the course of the growing older course of. What this implies in practical terms is there might be extra polar compound present within the oil, reducing the ability of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure becomes a quality criterion: the oil have to be modified below a predefined restrict.
If outcomes are GOOD, continue the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system could be important. The extent of the corrosion damage caused by the sulfur could be so extreme that it’d cause failure of the equipment 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 discovered that rubber products utilized in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based on this institute’s threat assessment 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 decrease in flashpoints by 10%, the tools might require additional inspection. This worth would possibly differ in different international locations.
It is suggested to carry out this check when an uncommon odour is noticed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not decide the condition of the transformer; it is a well being and security impact test. PCB is hazardous to both people and the setting; it is vital to test for PCBs after the retro fill of a transformer. It can also be required each time any upkeep has been carried out on the unit, and the potential for contamination is current. If PCB content exceeds the really helpful limits, the appropriate motion must be taken.
Units with a PCB content material of more than 50ppm require a fireplace security plan, environmental safety plan, and further precautionary measures when upkeep 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 tools proprietor.
Local regulatory our bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of data and interpretation, we are going to focus on this phenomenon partly II of the article. The limits for the different gases and the interpretation of this data in accordance with worldwide requirements shall be mentioned intimately, forming part of the overall well being rating dedication of the transformer.
Transformer condition monitoring is an interlaced, extremely thrilling subject of examine. In this article, we targeted on the kinds of checks to determine the condition of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a specific date and time. This makes it attainable to ensure finest practice application and optimised maintenance. It also make it simpler to attract up a upkeep plan and action plan.
1. เกจวัดแรงลม -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’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 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 in the business, having previously labored as laboratory manager for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the beginning of her career, notably within the evaluation of check information. Corné has huge practical and theoretical information of reliability upkeep programmes.