TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the relevant knowledge relating to an asset’s operation. If we interpret the info appropriately, it may give us great perception into the asset’s situation.
Frequent monitoring of the asset may find yourself in much less upkeep required or extra prolonged durations without any maintenance required.
It is essential to identify the important thing parameters which may be wanted to provide us a complete 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 attain the maximum lifetime.
What is the data telling us?
Has the condition of the unit changed because the final upkeep period?
Is it secure to operate the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate ranking for a particular period?
Are we required to implement motion to ensure the continued reliability of the unit?
How long can we use the unit earlier than we need to think about 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 achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the outcome and what you wish to accomplish, it might be a lot easier to identify the required parameters.
Health indexing of assets is becoming a remarkable tool in getting a clearer image 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 primarily based on the international standards for mineral oils, indicating the crucial values stipulated in the numerous standards.
The scope of oil evaluation, interpretation of the information, and important values
At the start of this part, it is important to state that we take care of totally different size transformers within the business. Transformers are divided into courses in accordance with the kV scores of the tools. It is up to the reliability or asset manager to use the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The maintenance engineer or supervisor wants to determine what type of testing would benefit him in identifying downside areas inside his fleet. Some of the evaluation has been identified as to routine sort tests. Still, there’s an in depth vary of tests that can assist in figuring out particular problem criteria inside the system, which might not be clear through the typical day-to-day analysis usually carried out.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and the way typically or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is doubtless certainly one of the most crucial and important influencers within the analysis end result. If a pattern is not taken to the prescribed process, then there’s a significant chance that the analysis performed, and the outcomes acquired, are of no use.
Care ought to be taken to make sure high quality management procedures are utilized in every step of the sampling course of as set out by international requirements. A good quality pattern taken by applying the right process is essential. A sample could be contaminated by numerous components, all of which might affect the outcome of the ends in a unfavorable method.
All steps involved in pattern taking should adhere to quality control procedures, including the container that is used, the sampling equipment, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and dealing with of the pattern, and then the supply of the sample to the laboratory.
If the pattern identification and sample data are handwritten, the human issue can lead to incorrect interpretation of the info. The label should be stuck onto the container to ensure 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 write on the label ought to be oil- and waterproof. Otherwise, some knowledge might be misplaced, making it extraordinarily tough for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s talk about the kinds of exams to find out the transformer’s condition, the crucial values, and the beneficial actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visual inspection of the oil sample in a clear vessel to determine the colour, turbidity, and potential particle identification.
Dark oils might point out chemical degradation or contamination of the oil.
When there might be a lot of turbidity, it might indicate a high water content 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 point out a attainable electrical fault within the unit. The DGA analysis of the oil will confirm if that’s so.
Clear oils with out contamination will indicate a great condition, and no action is really helpful.
When oils are dark or turbid, further evaluation will verify any problems. The oil analysis results may even determine the degree and type 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 point application<40 kV for OLTC in delta or line-end application
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 keep away from flashover in the unit, the breakdown voltage have to be high.
If the values are Good, it is recommended to continue with the present pattern interval action plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with other parameter results like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil through oil reconditioning processes. If alternative checks indicate extreme getting older, the oil may be changed with new or reclaimed oil. Another possibility would be to carry out on-site oil reclamation using a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water in addition to some other compounds. Another benefit is that the oil could be re-used, and in most situations, this can be carried out with out switching off the unit, which contributes to cost-saving. If doubtful – as a substitute switch off the unit throughout this treatment course of.
If the values are Poor, it’s advisable to take motion as quickly as potential and never delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content may cause flashover in the unit, leading to lack 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 take a look at
This is a routine test for all courses of electrical equipment, besides class G
The outcomes of this take a look at ought to all the time be thought of at the facet of the breakdown strength. If it’s found that the water content is excessive and the breakdown power is low, further action must be taken. It is recommended that a second sample from the same unit is examined to confirm the results.
In the case of switching tools, where there isn’t a paper current, the breakdown voltage is the figuring out factor.
It should be noted that the limits indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between 40 and 70 °C. If it is discovered that the unit’s operating temperature is outside this temperature range, it’s best to check with Annex A of the standard.
When the worth obtained via analyses is GOOD, the normal sampling interval could be maintained, requiring no additional motion.
When the worth returns a FAIR result, extra frequent sampling is beneficial. It can also be helpful to consider different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be carried out.
A POOR outcome will require instant action from the asset manager. This may include taking one other sample to confirm the results from the primary analysis. If it’s confirmed that the water content is excessive, the oil can be filtered; this course of ought to take away a big portion of the moisture from the oil if applied correctly. Follow-up samples have to be taken to ensure that the moisture content material remains to be throughout the required limits. The cause is that probably the most significant portion of the water is caught up in the paper system in the transformer. This moisture will transfer from the paper into the oil beneath conditions that favour this motion. It may be found later that the oil within the water has increased again with none apparent cause, but the supply could be the paper within the transformer.
A visual inspection can be recommended to find out if any water may transfer into the transformer or electrical gear by way of leaks. This drawback could be more extreme if the transformer or electrical tools is outside and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to zero.15>0.15
B, E<0.one hundred.10 to zero.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine check
This is a routine take a look at for all classes besides F and G
The acids in oils are fashioned as a end result 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’s going to adversely affect the insulation properties of the oil and will enhance paper degradation. If left Private in transformers, this will result in sludge formation, normally across the decrease components of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily difficult to take away.
If the result is GOOD, the regular sampling interval can continue.
In case of a FAIR outcome, the sampling interval ought to be decreased to suit the situation. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition 4.0, the asset supervisor might decide to reclaim the oil or replace it with new or reclaimed oil, whichever possibility might suit 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<0.100.10 to 0.20>0.20
B, C<0.a hundred.10 to 0.50>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 lessons of electrical equipment, except F and G
The dielectric dissipation issue or tan delta of this take a look at supplies info concerning the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating materials.
When oil ages, we have the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities that might affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result is FAIR, more frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is really helpful. The structure 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 take a look at
DC resistivity of the oil is probably considered one of the key parameters to evaluate the transformer insulation condition; this is primarily based on the reality that DC resistance is sensitive to oil degradation.
When the result’s FAIR, more frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is really helpful.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of unique value
This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the small print concerning components.
The two most common oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the aging course of in the oil and the strong insulation.
If the result’s FAIR, it is advised to high up the inhibitor stage to the prescribed stage per provider directions. It is advised to make use of a subject professional trained within the process to perform this process.
If the outcome obtained is POOR, the advice for this scenario would counsel that the tip person continues to use the oil “uninhibited,” however this will lead to extra fast degradation of both the liquid and stable insulation.
It must be famous that some transformers have already got a built-in oil preservation system; this is designed to keep dissolved oxygen at levels beneath 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 underneath these circumstances just isn’t required, although it’d add further protection in opposition to oil oxidation if the preservation system ever fails. 
h) Passivator content material
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and stable, (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 metal deactivators, react with reactive metallic surfaces and dissolved metals similar to copper and silver and scale back their fee of reaction with compounds within the oil. This contains oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two fundamental varieties, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the creator is conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.
As the oil ages, the passivator may deplete extra rapidly; this depletion would possibly speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals may be maintained.
With FAIR results, keep regular 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 therapy.
I) Sediment and sludge
This isn’t a routine check.
It is suggested that this test is performed when the oil results point out a high acid worth and the dissipation issue is close to the unacceptable restrict.
The results 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’s suggested that it be reclaimed; otherwise, an oil change is really helpful.
j) Interfacial tension
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 tension between transformer oil and water reduces in the course of the growing older process. What this implies in sensible terms is there is more polar compound current within the oil, reducing the ability of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation number. Therefore, the interfacial tension turns into a high quality criterion: the oil should be modified under a predefined limit.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes 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 can be significant. The extent of the corrosion injury brought on by the sulfur can be so severe that it’d trigger failure of the tools 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 found that rubber merchandise used in transformers may 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 on this institute’s danger evaluation research. 
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 
m) Flashpoint ° C
Not a routine take a look at
If there’s a most decrease in flashpoints by 10%, the tools may require additional inspection. This worth may differ in numerous international locations.
It is advised to perform this check when an unusual odour is observed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to determine the situation of the transformer; it is a well being and safety impact test. PCB is hazardous to each humans and the environment; it is vital to check for PCBs after the retro fill of a transformer. It is also required whenever any upkeep has been accomplished on the unit, and the chance of contamination is present. If PCB content exceeds the recommended limits, the suitable motion needs to be taken.
Units with a PCB content of more than 50ppm require a fire security plan, environmental protection plan, and extra precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificate of safe disposal issued to the tools proprietor.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of data and interpretation, we’ll talk about this phenomenon partly II of the article. The limits for the different gases and the interpretation of this knowledge according to international requirements shall be mentioned intimately, forming a half of the overall well being ranking determination of the transformer.
Transformer situation monitoring is an interlaced, extremely thrilling field of research. In this text, we targeted on the types of checks to determine the situation of the transformer, the important values, and the beneficial actions.
The Health Index indication makes it attainable to see the supposed reliability of a specific unit at a specific date and time. This makes it attainable to ensure finest follow application and optimised upkeep. It also make it easier to draw up a maintenance plan and action plan.
1. IEC 60422 Edition four.0 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and maintenance 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 strength.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience in the trade, having previously labored as laboratory manager 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, notably within the analysis of check information. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.