by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the related information relating to an asset’s operation. If we interpret the information appropriately, it may give us great perception into the asset’s situation.
Frequent monitoring of the asset can result in less upkeep required or extra prolonged periods without any upkeep required.
It is essential to determine the key parameters which are wanted to provide us an entire picture of the particular standing of the transformer and the motion we want to take to make sure the continued reliability of the asset to realize the utmost lifetime.
What is the information telling us?
Has the situation of the unit modified for the rationale that final upkeep period?
Is it safe to operate the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate score for a particular 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 have to think about replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring outline
It is vitally important to identify clear goals as part of your technique. What do you need to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or possibly life extension? Maintenance deferral? By stipulating the result and what you want to accomplish, it might be much easier to determine the required parameters.
Health indexing of belongings is changing into a outstanding device 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 had been calculated primarily based on the worldwide standards for mineral oils, indicating the important values stipulated within the varied standards.
The scope of oil evaluation, interpretation of the info, and critical values
At the start of this section, it is essential to state that we deal with totally different size transformers within the industry. Transformers are divided into courses based on the kV scores of the gear. It is up to the reliability or asset manager to use the guidelines for bigger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager needs to find out what sort of testing would profit him in figuring out downside areas within his fleet. Some of the analysis has been identified as to routine kind exams. Still, there’s an extensive range of exams that can help in figuring out particular downside criteria inside the system, which might not be clear through the everyday day-to-day analysis often performed.
Please see the ranking lessons in TABLE 1
TABLE 2 explains which oil analyses are recommended and how usually or under which circumstances.
Oil sampling
The taking of the oil pattern is among the most crucial and significant influencers in the evaluation end result. If a pattern isn’t taken to the prescribed process, then there’s a important risk that the analysis performed, and the results acquired, are of no use.
Care must be taken to make sure high quality management procedures are applied in each step of the sampling course of as set out by worldwide standards. A good high quality pattern taken by making use of the proper process is crucial. A sample may be contaminated by numerous factors, all of which might influence the result of the results in a adverse method.
Surge involved in sample taking must adhere to high quality management procedures, including the container that’s used, the sampling equipment, the taking of the sample, the marking of the sample, 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 sample identification and sample knowledge are handwritten, the human factor can result in incorrect interpretation of the information. The label should 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 ought to be waterproof, and the pen used to put in writing on the label ought to be oil- and water resistant. Otherwise, some data might be misplaced, making it extraordinarily tough for the laboratory personnel to finish the report or allocate the pattern to a selected transformer.
Now, let’s discuss the types of exams to find out the transformer’s condition, the critical values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection utilized 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 transparent vessel to discover out the colour, turbidity, and possible particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there might be plenty of turbidity, it’d indicate a high water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dirt particles within the drain valve might be incorporated into the sample. If particles are recognized as carbon, it would indicate a possible electrical fault within the unit. The DGA evaluation of the oil will verify if that’s so.
Clear oils without contamination will indicate a good situation, and no action is really helpful.
When oils are dark or turbid, additional evaluation will verify any issues. The oil evaluation results may also 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 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 material or the presence of foreign particles, or each in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage have to be excessive.
If the values are Good, it is suggested to proceed with the present pattern interval motion plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with different parameter results 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 checks point out severe getting older, the oil could 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 components are removed from the oil. This course of removes acid and water as nicely as some other compounds. Another benefit is that the oil may be re-used, and in most situations, this may 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 motion as quickly as attainable and never delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content may cause flashover in the unit, leading to 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 take a look at
This is a routine take a look at for all classes of electrical equipment, besides class G
The outcomes of this take a look at should at all times be thought of at the facet of the breakdown strength. If it’s found that the water content material is high and the breakdown energy is low, further action needs to be taken. It is recommended that a second pattern from the identical unit is tested to confirm the outcomes.
In the case of switching tools, where there isn’t any paper present, the breakdown voltage is the figuring out issue.
It must be noted that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s operating temperature is outdoors this temperature vary, it is best to check with Annex A of the usual.
When the value obtained by way of analyses is GOOD, the conventional sampling interval may be maintained, requiring no additional motion.
When the value returns a FAIR outcome, more frequent sampling is beneficial. It is also useful to think about other parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be implemented.
A POOR outcome would require instant motion from the asset manager. This might embrace taking one other pattern to confirm the results from the primary analysis. If it’s confirmed that the water content material is high, the oil may be filtered; this course of ought to remove a large portion of the moisture from the oil if utilized accurately. Follow-up samples must be taken to ensure that the moisture content material is still within the required limits. The cause is that essentially the most significant portion of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil under circumstances that favour this movement. It might be discovered later that the oil in the water has increased once more without any apparent cause, but the supply could be the paper within the transformer.
A visual inspection is also really helpful to determine if any water might transfer into the transformer or electrical gear by way of leaks. This downside might be extra severe if the transformer or electrical tools is outside and not in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to zero.25>0.20
C<0.a hundred.15 to zero.30>0.30
F, G Not a routine take a look at
This is a routine test for all lessons except F and G
The acids in oils are shaped due to 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 improve paper degradation. If left untreated in transformers, this can result in sludge formation, usually around the decrease components of the transformer core. The sludge will finally kind a semi-solid substance that is extraordinarily tough to remove.
If the result’s GOOD, the common sampling interval can proceed.
In case of a FAIR result, the sampling interval must be decreased to suit the situation. Future analysis should embrace a visible 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 exchange it with new or reclaimed oil, whichever possibility would possibly suit their requirements one of the best.
e) Dielectric Dissipation issue 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<0.100.10 to zero.50>0.50
D<0.010.01 to 0.03>0.03
E<0.a hundred.01 to zero.30>0.03
F, GNot a routine test
This is a routine test for all lessons of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this test offers data concerning 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 might influence the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is beneficial. The construction of the oil is broken, in effect that the chemical bonds between the molecules have broken 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 considered one of the key parameters to evaluate the transformer insulation condition; this is based on the reality that DC resistance is sensitive to oil degradation.
When the result is FAIR, more frequent sampling and checking further parameters is beneficial.
When the result is POOR, reclamation or an oil change is really helpful.
g) Inhibitor content material percent
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 can be advisable to contact the oil supplier to confirm the small print regarding 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 objective of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the aging course of in the oil and the stable insulation.
If the result is FAIR, it is suggested to top up the inhibitor level to the prescribed degree per supplier directions. It is advised to make use of a subject skilled educated within the procedure to carry out this process.
If the end result obtained is POOR, the advice for this scenario would recommend that the tip person continues to make use of the oil “uninhibited,” but this may result in extra rapid degradation of both the liquid and solid insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This would 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 underneath these circumstances isn’t required, although it might add additional safety in opposition to oil oxidation if the preservation system ever fails. [2]
h) Passivator content
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 lower of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also referred to as metal deactivators, react with reactive steel surfaces and dissolved metals corresponding to copper and silver and reduce their fee of reaction with compounds in the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental sorts, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the author 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 quickly; this depletion may accelerate when the oil is un-inhibited.
With GOOD outcomes, common pattern intervals may be maintained.
With FAIR results, preserve regular monitoring.
When POOR, it is 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 check.
It is advised that this take a look at is carried out when the oil outcomes point out a high acid worth and the dissipation issue is near the unacceptable restrict.
The results need to be lower than 0.02% by mass to be negligible. If the results return a worth of greater than 0.02% by mass, it is suggested that it’s reclaimed; otherwise, an oil change is recommended.
j) Interfacial rigidity
This just isn’t 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 test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces through the getting older course of. What this implies in practical terms is there might be more polar compound present in the oil, decreasing the flexibility of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation number. Therefore, Authoritative becomes a high quality criterion: the oil have to be modified under a predefined restrict.
If results are GOOD, proceed the common sampling interval.
If results are FAIR, lower the sampling interval.
If outcomes are POOR, verify 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 impact on the transformer system can be important. The extent of the corrosion harm 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 scale back the influence of this compound on the transformer system.
In a examine by Doble, it has been found that rubber products used 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 necessity of corrective actions based mostly on this institute’s danger assessment research. [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 test
If there is a maximum lower in flashpoints by 10%, the gear would possibly require further inspection. This value might differ in numerous countries.
It is suggested to perform this test when an unusual odour is noticed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to determine the situation of the transformer; it is a well being and security impact check. PCB is hazardous to both humans and the surroundings; it is important to check for PCBs after the retro fill of a transformer. It is also required every time any maintenance has been done on the unit, and the risk of contamination is current. If PCB content material exceeds the really helpful limits, the appropriate action 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 must be changed, and the oil disposed of as hazardous waste, with a certificate of protected disposal issued to the gear proprietor.
Local regulatory our bodies define the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of knowledge and interpretation, we will focus on this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data based on worldwide standards might be mentioned in detail, forming a half of the overall well being ranking dedication of the transformer.
Transformer condition monitoring is an interlaced, extremely thrilling subject of examine. In this article, we centered on the forms of checks to determine the situation of the transformer, the crucial values, and the really helpful actions.
The Health Index indication makes it attainable to see the supposed reliability of a particular unit at a particular date and time. This makes it attainable to make sure best follow software and optimised upkeep. It additionally 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 equipment – supervision and maintenance guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
3. 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 strength.”
6. Article initially printed by Transformer Technology Magazine Issue sixteen – December 2021
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the trade, having previously labored as laboratory supervisor for a major industrial laboratory group, focusing on transformer well being. She has been intrigued by transformer chemistry proper from the beginning of her profession, significantly in the evaluation of test knowledge. Corné has vast practical and theoretical data of reliability upkeep programmes.

Leave a Comment