by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant data regarding an asset’s operation. If we interpret the info accurately, it may give us nice insight into the asset’s situation.
Frequent monitoring of the asset can end result in much less upkeep required or extra extended periods without any maintenance required.
It is essential to establish the key parameters which are wanted to provide us a complete picture of the actual standing of the transformer and the action we need to take to make sure the continued reliability of the asset to realize the maximum lifetime.
What is the data telling us?
Has the condition of the unit changed since the last maintenance period?
Is it protected to function the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate ranking for a particular period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit earlier than we want to think about replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring outline
It is vitally important to determine clear targets as a part of your technique. What do you wish to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it will be much easier to determine the required parameters.
Health indexing of belongings is becoming a remarkable tool in getting a clearer picture of the condition of your transformer. Test parameters carry a numerical value-adding to the whole worth of the Health Index Value of the transformer. These parameter weight values were calculated based mostly on the worldwide requirements for mineral oils, indicating the crucial values stipulated in the varied requirements.
The scope of oil evaluation, interpretation of the info, and important values
At the start of this section, it’s important to state that we deal with totally different measurement transformers in the trade. Transformers are divided into lessons according to the kV scores of the tools. It is up to the reliability or asset manager to make use of the rules for bigger equipment, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager wants to discover out what kind of testing would benefit him in figuring out drawback areas within his fleet. Some of the analysis has been identified as to routine sort checks. Still, there is an in depth range of exams that may help in identifying specific drawback criteria within the system, which might not be clear by way of the standard day-to-day evaluation normally performed.
Please see the ranking classes in TABLE 1
TABLE 2 explains which oil analyses are really helpful and how typically or under which circumstances.
Oil sampling
The taking of the oil pattern is amongst the most important and significant influencers within the evaluation outcome. If a sample isn’t taken to the prescribed process, then there is a significant risk that the analysis performed, and the outcomes 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 worldwide requirements. A good high quality sample taken by applying the proper procedure is crucial. A pattern can be contaminated by varied factors, all of which might influence the end result of the leads to a adverse manner.
All steps concerned in pattern taking must adhere to quality management procedures, together with 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 dealing with of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and pattern information are handwritten, the human factor can result in incorrect interpretation of the info. The label should be stuck 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 ought to be waterproof, and the pen used to put in writing on the label must be oil- and water resistant. Otherwise, เกจวัดแรงดันดิจิตอล might be lost, making it extremely troublesome for the laboratory personnel to finish the report or allocate the sample to a particular transformer.
Now, let’s focus on the kinds of checks to determine the transformer’s condition, the important values, and the beneficial actions in each case:
a) Colour and appearance
This is a routine inspection utilized to every oil pattern.
When an oil pattern arrives at the laboratory, one of many “tests” is a visible inspection of the oil sample in a transparent vessel to determine the colour, turbidity, and attainable particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there’s plenty of turbidity, it might indicate a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the dust particles within the drain valve might be integrated into the sample. If particles are identified as carbon, it’d indicate a attainable electrical fault within the unit. The DGA evaluation of the oil will affirm if that is the case.
Clear oils with out contamination will point out an excellent condition, and no action is recommended.
When oils are darkish or turbid, additional evaluation will affirm any problems. The oil evaluation results may even determine 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 utility
G <30
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of foreign particles, or both in 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 have to be high.
If the values are Good, it is suggested to continue with the current sample interval motion plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with different parameter outcomes 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 various exams point out severe getting older, the oil can be changed 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 components are faraway from the oil. This course of removes acid and water as properly as some other compounds. Another benefit is that the oil could be re-used, and in most conditions, this might be carried out without switching off the unit, which contributes to cost-saving. If doubtful – as an alternative change off the unit during this therapy process.
If the values are Poor, it is advisable to take action as soon as attainable and not delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extremely excessive water content can cause flashover within the unit, resulting in 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 take a look at for all lessons of electrical equipment, besides class G
The results of this check ought to at all times be considered at the aspect of the breakdown energy. If it is discovered that the water content material is high and the breakdown power is low, additional action needs to be taken. It is really helpful that a second pattern from the identical unit is tested to confirm the results.
In the case of switching tools, where there is no paper present, the breakdown voltage is the figuring out issue.
It must be famous that the boundaries indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it is found that the unit’s working temperature is outside this temperature range, it is best to discuss with Annex A of the usual.
When the worth obtained by way of analyses is GOOD, the normal sampling interval can be maintained, requiring no additional action.
When the worth returns a FAIR end result, more frequent sampling is really helpful. It is also helpful to suppose about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be applied.
A POOR end result would require quick action from the asset supervisor. This may include taking another sample to confirm the results from the first analysis. If it is confirmed that the water content 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 need to be taken to make sure that the moisture content is still throughout the required limits. The reason is that probably the most significant slice of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under conditions that favour this motion. It could be discovered later that the oil in the water has increased once more with none obvious purpose, however the supply would be the paper in the transformer.
A visible inspection can additionally be recommended to find out if any water would possibly transfer into the transformer or electrical tools via leaks. This downside might be extra severe if the transformer or electrical equipment is outside and never in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to zero.15>0.15
B, E<0.100.10 to zero.25>0.20
C< hundred.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine take a look at 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 will increase 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 could result in sludge formation, often around the lower elements of the transformer core. The sludge will ultimately type a semi-solid substance that is extremely difficult to take away.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval ought to be decreased to fit the situation. Future evaluation ought to embrace a visual inspection of the oil for sediment and sludge.
If the result’s POOR according to the prescribed values in IEC 60422 Edition, the asset manager may determine to reclaim the oil or replace it with new or reclaimed oil, whichever possibility would possibly 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<0.100.10 to 0.20>0.20
B, C<0.a hundred.10 to zero.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 classes of electrical equipment, except F and G
The dielectric dissipation issue or tan delta of this test offers info relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities that might influence the dissipation issue embody water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is beneficial.
When the result is 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 beneficial 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 assess the transformer insulation condition; that is based on the truth that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is recommended.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of authentic value<40% of original value
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil supplier to confirm the details relating to components.
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 objective of the inhibitor is to forestall oxygen from reacting with the oil. This significantly slows the growing older process in the oil and the stable insulation.
If the result is FAIR, it’s suggested to prime up the inhibitor degree to the prescribed degree per supplier instructions. It is advised to make use of a field professional trained within the procedure to carry out this process.
If the result obtained is POOR, the recommendation for this state of affairs would suggest that the top person continues to use the oil “uninhibited,” but this will likely lead to extra speedy degradation of each the liquid and strong insulation.
It should be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels below 1000ppm. This would be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank geared up with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances is not required, though it would add extra safety against 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 secure, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of lower of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and scale back their price of response with compounds within the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic sorts, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the author 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 may deplete extra rapidly; this depletion may speed up when the oil is un-inhibited.
With GOOD results, regular pattern intervals may be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it’s suggested to remove the oil or take away the source of corrosivity from the oil through particular oil treatment.
I) Sediment and sludge
This isn’t a routine take a look at.
It is advised that this take a look at is performed when the oil outcomes indicate a excessive acid value and the dissipation issue is close to the unacceptable limit.
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’s instructed that or not it’s reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
This just 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 check
F, GNot Applicable
The interfacial tension between transformer oil and water reduces in the course of the getting older process. What this implies in practical terms is there may be more polar compound present in the oil, decreasing the power of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, the interfacial tension turns into a quality criterion: the oil should be modified beneath a predefined restrict.
If results are GOOD, continue the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, check 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 impact on the transformer system may be important. The extent of the corrosion harm brought on by the sulfur may be so extreme that it might trigger failure of the equipment if not checked. The addition of a copper passivator can scale back the impression of this compound on the transformer system.
In a research 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 accredited for oil filtration might 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 examine. [4]
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination levels (particles) encountered on power 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 may require additional inspection. This worth would possibly differ in different nations.
It is advised to perform this check when an uncommon odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is to not determine the condition of the transformer; this may be a well being and safety impression take a look at. PCB is hazardous to both humans and the surroundings; it’s critical to check for PCBs after the retro fill of a transformer. It can additionally be required every time any maintenance has been done on the unit, and the chance of contamination is present. If PCB content material exceeds the beneficial limits, the appropriate action needs to be taken.
Units with a PCB content of greater than 50ppm require a fireplace security plan, environmental safety plan, and extra precautionary measures when upkeep is finished. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the equipment proprietor.
Local regulatory our bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of information and interpretation, we will focus on this phenomenon partially II of the article. The limits for the different gases and the interpretation of this information in accordance with worldwide requirements shall be mentioned intimately, forming a part of the overall health score determination of the transformer.
Transformer situation monitoring is an interlaced, extremely exciting field of examine. In this text, we targeted on the types of tests to determine the situation of the transformer, the critical values, and the really helpful 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 possible to make sure best practice application and optimised maintenance. It also make it easier to draw up a maintenance plan and motion 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 how 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 sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise in the business, having beforehand labored as laboratory manager for a major industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the start of her career, notably within the analysis of check knowledge. Corné has vast sensible and theoretical information of reliability maintenance programmes.