by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the related knowledge regarding an asset’s operation. If we interpret the information appropriately, it may give us great perception into the asset’s condition.
Frequent monitoring of the asset can lead to less maintenance required or extra extended intervals with none maintenance required.
It is essential to determine the key parameters which are needed to give us an entire picture of the actual standing of the transformer and the action we need to take to ensure the continued reliability of the asset to attain the maximum lifetime.
What is the data telling us?
Has the situation of the unit changed since the last upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate score for a selected period?
Are we required to implement action to make sure the continued reliability of the unit?
How lengthy can we use the unit before we have to contemplate replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring outline
It is vitally important to identify clear objectives as a half of your strategy. What do you want to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the outcome and what you want to accomplish, it will be a lot simpler to establish the required parameters.
Health indexing of property is becoming a exceptional software in getting a clearer image of the situation 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 had been calculated based on the worldwide requirements for mineral oils, indicating the crucial values stipulated within the numerous standards.
The scope of oil evaluation, interpretation of the info, and important values
At the start of this part, it’s important to state that we deal with completely different size transformers within the business. Transformers are divided into courses in accordance with the kV scores of the gear. It is up to the reliability or asset supervisor to use the guidelines for bigger gear, thereby implementing shorter increment oil analysis and electrical exams.
The maintenance engineer or manager wants to find out what kind of testing would profit him in identifying problem areas inside his fleet. Some of the evaluation has been identified as to routine type exams. Still, there is an intensive vary of tests that may help in figuring out specific problem criteria throughout the system, which might not be clear through the everyday day-to-day analysis normally carried out.
Please see the rating lessons in TABLE 1
TABLE 2 explains which oil analyses are recommended and the way typically or beneath which circumstances.
Oil sampling
The taking of the oil pattern is considered one of the most important and significant influencers within the evaluation outcome. If a sample is not taken to the prescribed procedure, then there’s a important risk that the evaluation performed, and the outcomes acquired, are of no use.
Care must be taken to make sure quality management procedures are utilized in each step of the sampling process as set out by worldwide standards. A good high quality sample taken by making use of the correct process is essential. A sample may be contaminated by numerous factors, all of which might affect the outcome of the ends in a unfavorable method.
All steps concerned in pattern taking must adhere to high quality management procedures, including the container that’s used, the sampling package, the taking of the pattern, the marking of the pattern, the closing of the pattern, the storage and dealing with of the sample, and then the delivery of the sample to the laboratory.
If the sample identification and pattern knowledge are handwritten, the human issue can lead to incorrect interpretation of the info. The label ought to be caught onto the container to make sure no mix-ups in sample 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 information could be lost, making it extremely tough for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s discuss the kinds of exams to discover out the transformer’s situation, the important values, and the beneficial actions in every case:
a) Colour and look
This is a routine inspection applied to every oil sample.
When an oil pattern arrives on the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to discover out the colour, turbidity, and possible particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there may be a lot of turbidity, it might indicate a high water content material within the oil.
If the drain valve was not cleaned by the sampler, the dust particles within the drain valve could be incorporated into the pattern. If particles are identified as carbon, it’d point out a potential electrical fault within the unit. The DGA evaluation of the oil will affirm if this is the case.
Clear oils with out contamination will point out an excellent condition, and no motion is beneficial.
When oils are dark or turbid, additional analysis will confirm any issues. The oil evaluation outcomes will also decide the degree and type 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 level application<40 kV for OLTC in delta or line-end application
G <30
This is a routine inspection.
Breakdown voltage will point out the water content or the presence of overseas particles, or both in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage should be excessive.
If the values are Good, it is strongly recommended to continue with the present pattern interval action plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with other parameter results just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If various exams point out extreme aging, the oil can be changed with new or reclaimed oil. Another choice 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 removed from the oil. This process removes acid and water in addition to some other compounds. Another benefit is that the oil may be re-used, and in most conditions, this can be carried out with out switching off the unit, which contributes to cost-saving. If in doubt – as a substitute swap off the unit during this remedy course of.
If the values are Poor, it is advisable to take motion as quickly as potential and not delay the upkeep process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extremely high water content material can cause flashover within the unit, resulting in 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 test
This is a routine take a look at for all courses of electrical equipment, besides class G
The outcomes of this take a look at should all the time be thought-about in conjunction with the breakdown strength. If it’s discovered that the water content material is excessive and the breakdown power is low, additional action must be taken. เครื่องมือที่ใช้ในการวัดความดัน is recommended that a second sample from the identical unit is examined to substantiate the outcomes.
In the case of switching equipment, the place there isn’t a paper current, the breakdown voltage is the figuring out issue.
It ought to be noted that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between forty and 70 °C. If it’s discovered that the unit’s working temperature is outdoors this temperature vary, it is best to check with Annex A of the usual.
When the worth obtained through analyses is GOOD, the normal sampling interval could be maintained, requiring no further action.
When the value returns a FAIR result, extra frequent sampling is recommended. It is also helpful to think about other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be implemented.
A POOR outcome will require instant action from the asset manager. This might embody taking another sample to verify the results from the primary analysis. If it’s confirmed that the water content is excessive, the oil could be filtered; this course of should remove a large portion of the moisture from the oil if applied appropriately. Follow-up samples need to be taken to guarantee that the moisture content material remains to be inside the required limits. The purpose is that essentially 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 movement. It may be found later that the oil in the water has elevated once more without any obvious purpose, however the supply can be the paper in the transformer.
A visual inspection can also be beneficial to discover out if any water would possibly transfer into the transformer or electrical gear through leaks. This problem could be more extreme if the transformer or electrical equipment is outdoors and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D< hundred.10 to zero.15>0.15
B, E< hundred.10 to 0.25>0.20
C< hundred.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine test for all lessons besides F and G
The acids in oils are fashioned 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’ll adversely affect the insulation properties of the oil and can improve paper degradation. If left untreated in transformers, this can lead to sludge formation, normally across the decrease components of the transformer core. The sludge will finally kind a semi-solid substance that is extremely troublesome to remove.
If the result is GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to suit the situation. Future analysis ought to embody a visible inspection of the oil for sediment and sludge.
If หลักการทํางานของpressuregauge according to the prescribed values in IEC 60422 Edition 4.0, the asset manager might resolve to reclaim the oil or exchange it with new or reclaimed oil, whichever option would possibly go properly with their requirements the best.
e) Dielectric Dissipation issue 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< hundred.10 to zero.50>0.50
D<0.010.01 to zero.03>0.03
E<0.100.01 to 0.30>0.03
F, GNot a routine test
This is a routine take a look at for all lessons of electrical tools, except F and G
The dielectric dissipation issue or tan delta of this take a look at provides info concerning 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 that might affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking further parameters is really helpful.
When the result’s 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 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 doubtless one of the key parameters to evaluate the transformer insulation condition; that is based mostly on the fact that DC resistance is sensitive to grease degradation.
When the result’s FAIR, extra frequent sampling and checking further parameters is recommended.
When the result is POOR, reclamation or an oil change is recommended.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of authentic value
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil provider to confirm the details concerning components.
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 aging process in the oil and the stable insulation.
If the result’s FAIR, it is suggested to high up the inhibitor level to the prescribed degree per supplier instructions. It is suggested to make use of a area skilled educated in the procedure to carry out this activity.
If the result obtained is POOR, the recommendation for this state of affairs would recommend that the end person continues to make use of the oil “uninhibited,” however this will likely result in extra fast degradation of each the liquid and solid insulation.
It ought to be famous that some transformers have already got a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges below 1000ppm. This can be in the type 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, though it’d add extra 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 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 reducing at >10mg/kg/year
Passivators, also called metal deactivators, react with reactive steel surfaces and dissolved metals similar to copper and silver and reduce their price of response with compounds within the oil. This includes oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two primary types, 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 might speed up when the oil is un-inhibited.
With GOOD outcomes, common sample intervals could be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it’s advised to remove the oil or take away the supply of corrosivity from the oil by way of special oil remedy.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is advised that this test is performed when the oil outcomes point out a excessive acid worth and the dissipation issue is near the unacceptable restrict.
The results need to be less than 0.02% by mass to be negligible. If the results return a price of more than zero.02% by mass, it’s instructed that or not it’s reclaimed; otherwise, an oil change is beneficial.
j) Interfacial pressure
This is not a routine test
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 during the aging process. What this implies in practical phrases is there’s extra polar compound current within the oil, decreasing the power of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial pressure turns into a quality criterion: the oil must be changed under a predefined limit.
If results are GOOD, continue the common sampling interval.
If results are FAIR, decrease the sampling interval.
If results 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 important. The extent of the corrosion damage caused by the sulfur can be so extreme that it might cause failure of the gear if not checked. The addition of a copper passivator can reduce the influence of this compound on the transformer system.
In a study by Doble, it has been found that rubber merchandise utilized 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 based on this institute’s threat evaluation examine. [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 is a maximum decrease in flashpoints by 10%, the tools may require further inspection. This worth would possibly differ in several international locations.
It is advised to perform this check when an unusual odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not determine the condition of the transformer; this may be a well being and security impact test. PCB is hazardous to each humans and the environment; it’s critical 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 potential for contamination is current. If PCB content exceeds the really helpful limits, the appropriate motion must be taken.
Units with a PCB content of greater than 50ppm require a hearth safety plan, environmental protection plan, and further precautionary measures when maintenance is completed. This oil must be changed, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the tools owner.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of knowledge and interpretation, we’ll talk about this phenomenon partially II of the article. The limits for the different gases and the interpretation of this data in accordance with international standards shall be discussed intimately, forming a part of the overall health ranking willpower of the transformer.
Transformer condition monitoring is an interlaced, extremely thrilling subject of research. In this article, we focused on the kinds of checks to find out the situation of the transformer, the important values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a particular unit at a selected date and time. This makes it possible to ensure greatest follow application and optimised upkeep. It also make it simpler to draw up a upkeep 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
3. 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 power.”
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’ experience in the business, having previously worked as laboratory supervisor for a serious industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry right from the beginning of her career, particularly in the analysis of take a look at information. Corné has vast practical and theoretical knowledge of reliability maintenance programmes.