TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant knowledge concerning an asset’s operation. If we interpret the information accurately, it can give us great insight into the asset’s condition.
Frequent monitoring of the asset may find yourself in less maintenance required or more extended durations without any upkeep required.
It is essential to establish the important thing parameters that are needed to provide us a whole image of the particular status of the transformer and the action we want to take to make sure the continued reliability of the asset to realize the utmost lifetime.
What is the data telling us?
Has the situation of the unit modified because the last upkeep period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate rating for a particular period?
Are we required to implement action to make sure the continued reliability of the unit?
How long can we use the unit before we need to consider replacement?
Are the identified problems of a recurring nature?
Effective condition monitoring define
It is vitally important to determine clear objectives as a half 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 wish to accomplish, it would be a lot simpler to identify the required parameters.
Health indexing of property is changing into a remarkable tool in getting a clearer image of the condition of your transformer. Test parameters carry a numerical value-adding to the whole value of the Health Index Value of the transformer. These parameter weight values were calculated based on the international requirements for mineral oils, indicating the critical values stipulated within the numerous standards.
The scope of oil evaluation, interpretation of the information, and critical values
At the beginning of this part, it is essential to state that we deal with totally different measurement transformers within the business. Transformers are divided into courses according to the kV ratings of the gear. It is up to the reliability or asset supervisor to make use of the rules for bigger equipment, thereby implementing shorter increment oil evaluation and electrical checks.
The maintenance engineer or supervisor wants to find out what kind of testing would benefit him in figuring out downside areas within his fleet. Some of the analysis has been identified as to routine sort checks. Still, there may be an extensive range of checks that can assist in identifying specific problem standards throughout the system, which could not be clear by way of the standard day-to-day analysis usually performed.
Please see the score classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and how often or under which circumstances.
TABLE 2 FIG
The taking of the oil pattern is considered one of the most important and critical influencers within the analysis outcome. If a pattern is not taken to the prescribed procedure, then there’s a significant chance that the analysis performed, and the results acquired, are of no use.
Care ought to be taken to ensure high quality control procedures are utilized in each step of the sampling course of as set out by international standards. A good high quality pattern taken by making use of the right process is essential. A pattern can be contaminated by numerous components, all of which might affect the outcome of the results in a adverse method.
All steps involved in pattern taking should adhere to high quality control procedures, together with 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 handling of the pattern, and then the supply of the sample to the laboratory.
If the sample identification and sample information are handwritten, the human issue can result in incorrect interpretation of the info. The label should be caught 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 must be oil- and waterproof. Otherwise, some information could be lost, making it extraordinarily troublesome for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s discuss the forms of checks to find out the transformer’s condition, the important values, and the really helpful actions in each case:
a) Colour and look
This is a routine inspection utilized to every oil pattern.
When an oil sample arrives at the laboratory, one of many “tests” is a visible inspection of the oil pattern in a transparent vessel to determine the color, turbidity, and potential particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there might be plenty of turbidity, it might point out a excessive water content in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve could be included into the sample. If particles are identified as carbon, it would point out a potential electrical fault within the unit. The DGA analysis of the oil will confirm if that’s so.
Clear oils without contamination will indicate a good situation, and no motion is really helpful.
When oils are darkish or turbid, further analysis will verify any problems. The oil analysis outcomes may also determine 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
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of international particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover within the unit, the breakdown voltage must be high.
If the values are Good, it is strongly recommended to proceed with the current pattern interval action plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with different parameter outcomes just like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil by way of oil reconditioning processes. If alternative exams point out severe growing older, the oil can be changed with new or reclaimed oil. Another option would be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the color of the oil is restored, and the polar elements are removed from the oil. This course of removes acid and water as nicely as some other compounds. Another advantage is that the oil may be re-used, and in most situations, this can be accomplished without switching off the unit, which contributes to cost-saving. If in doubt – as an alternative switch off the unit during this therapy process.
If the values are Poor, it is advisable to take action as quickly as attainable and never delay the upkeep course of. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily high water content material may cause flashover in the unit, leading to loss 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 test
This is a routine check for all classes of electrical gear, except class G
The outcomes of this check should all the time be thought of in conjunction with the breakdown energy. If it is discovered that the water content material is high and the breakdown strength is low, additional action needs to be taken. It is beneficial that a second pattern from the identical unit is examined to verify the results.
In the case of switching tools, where there isn’t any paper present, the breakdown voltage is the determining issue.
It ought to be famous that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between 40 and 70 °C. If it’s discovered that the unit’s operating temperature is exterior this temperature range, it is best to check with Annex A of the standard.
When the value obtained by way of analyses is GOOD, the traditional sampling interval can be maintained, requiring no additional motion.
When the value returns a FAIR end result, extra frequent sampling is beneficial. It can be helpful to suppose about other parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the action to be carried out.
A POOR end result would require quick action from the asset manager. This would possibly embody taking another sample to verify the results from the primary evaluation. If it is confirmed that the water content is excessive, the oil could be filtered; this course of ought to take away a large portion of the moisture from the oil if applied appropriately. Follow-up samples have to be taken to ensure that the moisture content material remains to be within the required limits. The purpose is that probably the most significant slice of the water is caught up within the paper system within the transformer. This moisture will move from the paper into the oil underneath circumstances that favour this movement. It may be found later that the oil in the water has increased once more with none obvious reason, however the source could be the paper within the transformer.
A visible inspection can be really helpful to find out if any water would possibly move into the transformer or electrical gear via leaks. This problem might be more severe if the transformer or electrical gear is exterior and never in a covered space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to zero.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C<0.one hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine take a look at for all lessons besides F and G
The acids in oils are fashioned 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’s going to adversely have an effect on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this will result in sludge formation, often around the decrease components of the transformer core. The sludge will ultimately form a semi-solid substance that is extremely troublesome to take away.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR end result, the sampling interval must be decreased to suit the scenario. Future analysis should embrace a visible inspection of the oil for sediment and sludge.
If the result is POOR in accordance with the prescribed values in IEC 60422 Edition four.zero, the asset supervisor might determine to reclaim the oil or substitute it with new or reclaimed oil, whichever possibility might swimsuit their necessities 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 zero.20>0.20
B, C<0.one hundred.10 to 0.50>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 take a look at
This is a routine test for all lessons of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this test offers data concerning the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, leading to phase displacement and dielectric losses. Other impurities which may influence the dissipation factor embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is beneficial. The structure of the oil is damaged, 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 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 test
DC resistivity of the oil is considered one of the key parameters to evaluate the transformer insulation situation; that is based mostly on the reality that DC resistance is sensitive to grease degradation.
When the result is FAIR, extra frequent sampling and checking extra 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 original value<40% of authentic worth
This take a look at is restricted to oils with this additive.
It could be advisable to contact the oil provider to confirm the main points concerning additives.
เพรสเชอร์เกจวัดแรงดันน้ำ for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and a pair of,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the growing older process in the oil and the solid insulation.
If the result’s FAIR, it is advised to top up the inhibitor level to the prescribed level per provider instructions. It is suggested to make use of a subject skilled trained within the procedure to perform this activity.
If the result obtained is POOR, the advice for this situation would recommend that the tip user continues to use the oil “uninhibited,” but this will likely lead to extra rapid degradation of both the liquid and stable insulation.
It ought to be noted that some transformers already have a built-in oil preservation system; that is designed to keep dissolved oxygen at ranges below 1000ppm. This would be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils underneath these circumstances isn’t required, although it might add additional safety towards oil oxidation if the preservation system ever fails. 
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 decrease of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive metallic surfaces and dissolved metals corresponding to copper and silver and cut back their rate of reaction with compounds within the oil. This consists of oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic types, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the writer 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 more quickly; this depletion would possibly accelerate when the oil is un-inhibited.
With GOOD results, 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 source of corrosivity from the oil by way of special oil remedy.
I) Sediment and sludge
This just isn’t a routine check.
It is suggested that this take a look at is carried out when the oil outcomes indicate a excessive acid worth and the dissipation issue is close to the unacceptable limit.
The results need to be less than 0.02% by mass to be negligible. If the results return a value of more than zero.02% by mass, it’s suggested that it’s reclaimed; otherwise, an oil change is really helpful.
j) Interfacial tension
This isn’t a routine test
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 through the getting older process. What this implies in sensible phrases is there’s more polar compound current in the oil, decreasing the power of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, the interfacial pressure turns into a quality criterion: the oil have to be changed beneath a predefined restrict.
If results are GOOD, continue the common sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system could be important. The extent of the corrosion harm caused by the sulfur could be so severe that it might trigger failure of the gear if not checked. The addition of a copper passivator can scale back the impact of this compound on the transformer system.
In a study by Doble, it has been discovered that rubber merchandise used in transformers might 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 necessity of corrective actions primarily based on this institute’s threat evaluation research. 
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured utilizing IEC 60970 
m) Flashpoint ° C
Not a routine take a look at
If there’s a maximum decrease in flashpoints by 10%, the equipment would possibly require additional inspection. This worth may differ in numerous international locations.
It is suggested 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 not to determine the situation of the transformer; this can be a health and safety impression test. PCB is hazardous to both humans and the surroundings; it’s critical to test for PCBs after the retro fill of a transformer. It can be required whenever any maintenance has been carried out on the unit, and the risk of contamination is current. If PCB content material exceeds the recommended limits, the appropriate motion needs to be taken.
Units with a PCB content of greater than 50ppm require a fireplace security plan, environmental protection plan, and additional precautionary measures when upkeep is completed. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the tools proprietor.
Local regulatory bodies outline the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of information and interpretation, we’ll talk about this phenomenon partially II of the article. The limits for the different gases and the interpretation of this knowledge in accordance with international standards might be mentioned in detail, forming part of the overall health ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, extremely exciting field of research. In this article, we centered on the forms of exams to determine the condition of the transformer, the critical values, and the beneficial actions.
The Health Index indication makes it potential to see the supposed reliability of a particular unit at a specific date and time. This makes it possible to ensure finest practice software and optimised maintenance. It also make it simpler to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 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’re 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 printed by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise in the trade, having previously labored as laboratory supervisor for a serious industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry proper from the start of her profession, significantly in the analysis of take a look at data. Corné has huge practical and theoretical information of reliability upkeep programmes.