by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent accumulating, measuring, recording, and analysis of the relevant knowledge relating to an asset’s operation. If we interpret the info appropriately, it can give us nice insight into the asset’s situation.
Frequent monitoring of the asset may find yourself in much less maintenance required or more prolonged durations without any maintenance required.
It is crucial to identify the vital thing parameters that are wanted to give us an entire picture of the actual status of the transformer and the motion we have to take to make sure the continued reliability of the asset to realize the maximum lifetime.
What is the information telling us?
Has the situation of the unit changed since the final maintenance period?
Is it safe to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate ranking for a specific period?
Are we required to implement action to ensure the continued reliability of the unit?
How lengthy can we use the unit earlier than we want to consider replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring outline
It is vitally necessary to determine clear objectives as part of your strategy. What do you need to obtain by implementing this situation monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it will be much easier to identify the required parameters.
Health indexing of assets 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 entire value 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 critical values stipulated in the numerous standards.
The scope of oil analysis, interpretation of the data, and critical values
At the beginning of this part, it is important to state that we deal with totally different dimension transformers in the industry. Transformers are divided into classes in accordance with the kV scores of the equipment. It is as much as the reliability or asset supervisor to make use of the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical checks.
The maintenance engineer or manager wants to discover out what type of testing would profit him in identifying drawback areas within his fleet. Some of the evaluation has been recognized as to routine kind checks. Still, there’s an in depth range of exams that can help in identifying particular drawback standards inside the system, which could not be clear via the everyday day-to-day analysis normally performed.
Please see the score classes in TABLE 1
TABLE 2 explains which oil analyses are really helpful and how typically or underneath which circumstances.
Oil sampling
The taking of the oil pattern is doubtless considered one of the most important and significant influencers in the evaluation consequence. If a sample is not taken to the prescribed procedure, then there’s a significant chance that the evaluation performed, and the results acquired, are of no use.
Care ought to be taken to make sure high quality control procedures are utilized in every step of the sampling process as set out by worldwide standards. A good quality sample taken by applying the correct procedure is important. A pattern may be contaminated by numerous components, all of which may affect the outcome of the results in a negative manner.
All steps concerned in pattern taking should adhere to quality control procedures, together with the container that is used, the sampling kit, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and dealing with of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and pattern data are handwritten, the human factor can result in incorrect interpretation of the information. The label ought to be stuck onto the container to make sure no mix-ups in pattern identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label ought to be waterproof, and the pen used to write down on the label should be oil- and water resistant. Otherwise, some data could be misplaced, making it extraordinarily tough for the laboratory personnel to finish the report or allocate the sample to a particular transformer.
Now, let’s discuss the kinds of tests to find out the transformer’s situation, the important values, and the beneficial actions in each case:
a) Colour and look
This is a routine inspection applied to every oil pattern.
When an oil sample arrives at 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 lots 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 filth particles within the drain valve could be incorporated into the pattern. If particles are identified as carbon, it would indicate a possible electrical fault within the unit. The DGA evaluation of the oil will verify if that is the case.
Clear oils without contamination will indicate a great situation, and no motion is recommended.
When oils are darkish or turbid, further analysis will confirm any issues. The oil evaluation outcomes may even 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 point 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 international particles, or each in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage have to be high.
If the values are Good, it is strongly recommended to proceed with the current pattern interval motion plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with other parameter results just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil by way of oil reconditioning processes. If alternative exams point out extreme getting older, the oil can be changed with new or reclaimed oil. ตัววัดแรงดันน้ำ could be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar elements are removed from the oil. This course of removes acid and water in addition to another compounds. Another benefit is that the oil may be re-used, and in most conditions, this could be done with out switching off the unit, which contributes to cost-saving. If doubtful – instead change off the unit throughout this therapy course of.
If the values are Poor, it’s advisable to take action as quickly as attainable and never delay the upkeep process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content can cause flashover in the unit, leading to loss 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 results of this check should at all times be considered along side the breakdown power. If it is discovered that the water content material is excessive and the breakdown strength is low, additional action must be taken. เกจวัดแรงดันต่ำ is recommended that a second sample from the identical unit is examined to substantiate the outcomes.
In the case of switching tools, where there is no paper present, the breakdown voltage is the figuring out issue.
It must be noted that the limits 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 operating temperature is outdoors this temperature range, it is best to refer to Annex A of the standard.
When the worth obtained via analyses is GOOD, the normal sampling interval may be maintained, requiring no further action.
When the worth returns a FAIR outcome, more frequent sampling is really helpful. It is also helpful to contemplate different parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to determine on the motion to be carried out.
A POOR result will require quick action from the asset supervisor. This might include taking another pattern to confirm the outcomes from the primary evaluation. If it is confirmed that the water content material is excessive, the oil may be filtered; this process ought to take away a big portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to ensure that the moisture content remains to be throughout the required limits. The purpose 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 conditions that favour this movement. It could be discovered later that the oil in the water has elevated again with none obvious purpose, but the supply would be the paper within the transformer.
A visual inspection is also recommended to discover out if any water might move into the transformer or electrical equipment via leaks. This downside may be more extreme if the transformer or electrical tools is outdoors and never in a lined area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E< hundred.10 to zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine test for all classes except F and G
The acids in oils are fashioned as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist within the formation of the acids. Because acids are polar compounds, it will adversely have an result on the insulation properties of the oil and will increase paper degradation. If left untreated in transformers, this can lead to sludge formation, often around the lower elements of the transformer core. The sludge will ultimately form a semi-solid substance that’s extremely tough to take away.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to suit the state of affairs. Future analysis should embrace a visible inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition four.0, the asset supervisor could determine to reclaim the oil or substitute it with new or reclaimed oil, whichever option may suit their requirements the most effective.
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 take a look at
This is a routine check for all courses of electrical tools, except F and G
The dielectric dissipation issue or tan delta of this take a look at supplies information relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we now have the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities which may influence the dissipation factor include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result is POOR, reclamation or an oil change is recommended. The structure of the oil is damaged, 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 certainly one of the key parameters to evaluate the transformer insulation situation; that is based on the truth that DC resistance is delicate to grease degradation.
When the result is FAIR, more frequent sampling and checking extra parameters is beneficial.
When the result’s POOR, reclamation or an oil change is beneficial.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original 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 verify the small print 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 function of the inhibitor is to stop oxygen from reacting with the oil. This considerably slows the growing older process within the oil and the solid insulation.
If the result’s FAIR, it’s advised to prime up the inhibitor level to the prescribed degree per provider instructions. It is suggested to use a area professional skilled within the process to carry out this activity.
If the end result obtained is POOR, the advice for this situation would counsel that the top person continues to use the oil “uninhibited,” but this may lead to more speedy degradation of each the liquid and solid insulation.
It should be famous that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels below 1000ppm. This can be within the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, although it might add additional protection 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 lower < 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 metallic deactivators, react with reactive steel 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 natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic 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 quickly; this depletion may accelerate when the oil is un-inhibited.
With GOOD outcomes, common sample intervals could be maintained.
With FAIR outcomes, maintain common monitoring.
When POOR, it is advised to take away the oil or remove the source of corrosivity from the oil via particular oil remedy.
I) Sediment and sludge
This is not a routine check.
It is suggested that this take a look at is performed when the oil results indicate a high acid value and the dissipation factor is near the unacceptable restrict.
The outcomes need to be lower than 0.02% by mass to be negligible. If the results return a value of more than 0.02% by mass, it’s advised that or not it’s reclaimed; in any other case, an oil change is really helpful.
j) Interfacial pressure
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 rigidity between transformer oil and water reduces through the getting older course of. What this means in sensible phrases is there is extra polar compound present within the oil, reducing the flexibility 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 tension 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, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine test.
Oil is both corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system may be significant. The extent of the corrosion damage attributable to the sulfur can be so extreme that it’d cause failure of the equipment if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber merchandise utilized 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 based on this institute’s threat evaluation study. [4]
l) Particle counting and sizing
Table three: 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’s a maximum decrease in flashpoints by 10%, the equipment might require additional inspection. This worth might differ in different countries.
It is advised to perform this check when an uncommon odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the condition of the transformer; it is a well being and security impression test. PCB is hazardous to both humans and the environment; it’s critical to check for PCBs after the retro fill of a transformer. It is also required each time any upkeep has been accomplished on the unit, and the potential for contamination is present. If PCB content exceeds the recommended limits, the suitable motion must be taken.
Units with a PCB content of greater than 50ppm require a fire security plan, environmental protection plan, and additional precautionary measures when upkeep is done. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the gear proprietor.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of information and interpretation, we are going to discuss this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this information according to worldwide standards will be discussed intimately, forming a half of the general health score willpower of the transformer.
Transformer condition monitoring is an interlaced, highly exciting subject of research. In this text, we targeted on the kinds of tests to determine the condition of the transformer, the important values, and the really helpful actions.
The Health Index indication makes it potential to see the supposed reliability of a specific unit at a selected date and time. This makes it attainable to ensure best apply application and optimised maintenance. It also make it simpler to attract up a upkeep plan and motion plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical tools – supervision and maintenance 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 energy.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ expertise in the trade, having beforehand worked as laboratory manager for a serious industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the start of her career, particularly in the analysis of take a look at knowledge. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.

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