TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant knowledge regarding an asset’s operation. If we interpret the data correctly, it can provide us nice insight into the asset’s situation.
Frequent monitoring of the asset may find yourself in less upkeep required or more extended intervals with none maintenance required.
It is crucial to establish the vital thing parameters that are needed to provide us an entire image of the actual standing of the transformer and the motion we need to take to ensure the continued reliability of the asset to attain the utmost lifetime.
What is the info telling us?
Has the situation of the unit changed because the last upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it protected to load the unit above the nameplate ranking 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 think about replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring outline
It is vitally necessary to establish clear goals as part of your technique. What do you need to obtain by implementing this condition 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 might be a lot easier to establish the required parameters.
Health indexing of assets is becoming a exceptional 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 had been calculated based on the international requirements for mineral oils, indicating the crucial values stipulated within the varied requirements.
The scope of oil analysis, interpretation of the information, and important values
At the start of this part, it’s important to state that we take care of different measurement transformers within the industry. Transformers are divided into lessons according to the kV scores 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 upkeep engineer or supervisor wants to determine what type of testing would profit him in identifying drawback areas within his fleet. Some of the analysis has been recognized as to routine kind checks. Still, there’s an intensive range of exams that can assist in identifying specific downside criteria inside the system, which could not be clear via the standard day-to-day analysis normally carried out.
Please see the score courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and the way usually or under which circumstances.
TABLE 2 FIG
The taking of the oil pattern is likely considered one of the most crucial and important influencers in the evaluation outcome. If a pattern is not taken to the prescribed procedure, then there is a significant chance that the analysis performed, and the outcomes acquired, are of no use.
Care ought to be taken to make sure quality control procedures are applied in each step of the sampling course of as set out by worldwide requirements. A good high quality sample taken by applying the correct procedure is essential. A pattern can be contaminated by numerous components, all of which might affect the end result of the ends in a unfavorable manner.
All steps concerned in sample taking should adhere to quality management procedures, including the container that is 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 pattern information are handwritten, the human issue can lead to incorrect interpretation of the info. The label ought to be stuck onto the container to ensure no mix-ups in sample identification. Ensure that the label is completed in neat, easy-to-read handwriting. The label should be waterproof, and the pen used to write down on the label ought to be oil- and waterproof. Otherwise, some data could be misplaced, making it extremely tough for the laboratory personnel to complete the report or allocate the pattern to a selected transformer.
Now, let’s focus on the forms of tests to determine the transformer’s situation, the crucial values, and the beneficial actions in each case:
a) Colour and appearance
This is a routine inspection utilized to every oil sample.
When an oil sample arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern in a clear vessel to determine the colour, turbidity, and possible particle identification.
Dark oils may point out chemical degradation or contamination of the oil.
When there’s a lot of turbidity, it’d indicate a excessive water content in the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve may be included into the sample. If particles are identified as carbon, it might point out a attainable electrical fault in the unit. The DGA analysis of the oil will affirm if that is the case.
Clear oils with out contamination will point out an excellent situation, and no motion is really helpful.
When oils are darkish or turbid, further evaluation will affirm any issues. The oil evaluation outcomes may even decide the diploma 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 software
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of overseas particles, or each within the oil being analysed.
As the oil in transformers acts as an insulation medium to keep away from flashover within the unit, the breakdown voltage have to be high.
If เกจวัดความดันน้ำ are Good, it is strongly recommended to proceed with the present pattern interval motion plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with other parameter results like the water content material, 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 indicate severe getting older, the oil could be changed with new or reclaimed oil. Another possibility can be to carry out on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the benefit that the colour of the oil is restored, and the polar components are removed from the oil. This course of removes acid and water as properly as another 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 unsure – as an alternative change off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as quickly as possible and never delay the maintenance process. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content material may 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 check
This is a routine take a look at for all classes of electrical tools, except class G
The results of this check should always be thought-about at the side of the breakdown strength. If it is discovered that the water content is excessive and the breakdown strength is low, further motion 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 equipment, the place there isn’t a paper current, the breakdown voltage is the determining issue.
It must be famous that the bounds indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it is found that the unit’s operating temperature is outdoors this temperature range, it is best to refer to Annex A of the usual.
When the value obtained by way of analyses is GOOD, the normal sampling interval could be maintained, requiring no further motion.
When the value returns a FAIR end result, extra frequent sampling is beneficial. It can be useful to consider different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to determine on the motion to be carried out.
A POOR end result will require instant motion from the asset manager. This would possibly include taking another pattern to confirm the results from the primary analysis. If it’s confirmed that the water content is high, the oil can be filtered; this process should 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 continues to be throughout the required limits. The purpose is that essentially the most significant slice of the water is caught up in the paper system in the transformer. This moisture will move from the paper into the oil beneath circumstances that favour this motion. It might be discovered later that the oil within the water has increased again without any obvious reason, however the supply could be the paper within the transformer.
A visible inspection is also beneficial to discover out if any water may move into the transformer or electrical gear via leaks. This problem might be more severe if the transformer or electrical gear is outside and not in a covered area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to 0.15>0.15
B, E<0.a hundred.10 to 0.25>0.20
C<0.100.15 to 0.30>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 because of chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist within 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 lead to sludge formation, often around the decrease components of the transformer core. The sludge will finally form a semi-solid substance that is extraordinarily troublesome to remove.
If the result is GOOD, the common sampling interval can proceed.
In case of a FAIR outcome, the sampling interval ought to be decreased to suit the scenario. Future analysis should embody 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 4.zero, the asset supervisor could resolve to reclaim the oil or replace it with new or reclaimed oil, whichever choice would possibly 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.one hundred.10 to zero.20>0.20
B, C<0.one hundred.10 to zero.50>0.50
D<0.010.01 to 0.03>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 classes of electrical tools, except F and G
The dielectric dissipation issue or tan delta of this test provides data concerning the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, resulting in phase displacement and dielectric losses. Other impurities which may affect the dissipation factor 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 really helpful. The construction 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 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 certainly one of the key parameters to evaluate the transformer insulation condition; that is based on the fact that DC resistance is delicate to grease degradation.
When the result is FAIR, more frequent sampling and checking additional parameters is really helpful.
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 authentic 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 primary points relating to components.
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 purpose of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the growing older process in the oil and the strong insulation.
If the result is FAIR, it is suggested to high up the inhibitor stage to the prescribed degree per supplier directions. It is suggested to make use of a area professional skilled within the process to perform this activity.
If the end result obtained is POOR, the advice for this state of affairs would suggest that the end consumer continues to make use of the oil “uninhibited,” but this may lead to extra fast degradation of each the liquid and solid insulation.
It should be noted that some transformers already have a built-in oil preservation system; that is designed to maintain dissolved oxygen at ranges below 1000ppm. This could be in the type of a nitrogen system, a nitrogen tank or generator, or a conservator tank outfitted with a rubber diaphragm (bladder). Using inhibited oils under these circumstances just isn’t required, though it might add additional safety towards 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 steady, (rate of lower < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also known as metallic deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and scale back their rate of reaction with compounds within the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary types, sulfur-based and nitrogen-based. The first advised 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.
As the oil ages, the passivator might deplete extra quickly; this depletion may accelerate when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR results, keep common monitoring.
When POOR, it is advised to remove the oil or take away the source of corrosivity from the oil through special oil therapy.
I) Sediment and sludge
This is not a routine take a look at.
It is suggested that this test is carried out when the oil results point out a high acid worth and the dissipation factor is near the unacceptable limit.
The outcomes need to be less than 0.02% by mass to be negligible. If the results return a price of more than 0.02% by mass, it’s instructed that it be reclaimed; in any other case, an oil change is recommended.
j) Interfacial pressure
This 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 pressure between transformer oil and water reduces in the course of the getting older process. What this implies in practical phrases is there might be more polar compound present in the oil, reducing the flexibility of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial tension becomes a excessive quality criterion: the oil have to be modified beneath a predefined limit.
If outcomes 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 vital. The extent of the corrosion harm attributable to the sulfur can be so severe that it’d cause failure of the gear if not checked. The addition of a copper passivator can cut 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 would possibly 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 need of corrective actions based on this institute’s risk evaluation research. 
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 
m) Flashpoint ° C
Not a routine check
If there’s a maximum decrease in flashpoints by 10%, the tools may require additional inspection. This value might differ in numerous international locations.
It is suggested to perform this test when an uncommon odour is observed, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is not to determine the condition of the transformer; this could be a health and security impression test. PCB is hazardous to both people and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It can also be required whenever any maintenance has been carried out on the unit, and the potential for contamination is current. If PCB content exceeds the beneficial limits, the appropriate action needs to be taken.
Units with a PCB content of greater than 50ppm require a fireplace safety plan, environmental safety plan, and further precautionary measures when upkeep is completed. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the equipment owner.
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 data in accordance with international standards might be discussed intimately, forming a half of the general well being ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, highly exciting field of research. In this text, we focused on the kinds of exams to discover out 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 selected unit at a particular date and time. This makes it possible to ensure finest practice utility and optimised upkeep. It additionally make it easier to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition four.zero 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.
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 printed by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience in the trade, having beforehand labored as laboratory supervisor for a significant industrial laboratory group, specializing in transformer well being. She has been intrigued by transformer chemistry right from the beginning of her profession, significantly in the evaluation of take a look at knowledge. Corné has vast practical and theoretical information of reliability upkeep programmes.