TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related knowledge regarding an asset’s operation. If we interpret the information appropriately, it can give us great insight into the asset’s situation.
Frequent monitoring of the asset may find yourself in less maintenance required or more prolonged intervals without any upkeep required.
It is crucial to establish the key parameters that are wanted to give us a complete 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 information telling us?
Has the situation of the unit changed since the last maintenance period?
Is it safe to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate score for a selected 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 want to consider replacement?
Are the identified issues of a recurring nature?
Effective situation monitoring outline
It is vitally important to determine clear targets as a part of your strategy. What do you wish to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps Best selling ? Maintenance deferral? By stipulating the outcome and what you need to accomplish, it would be much simpler to establish the required parameters.
Health indexing of assets is becoming a exceptional software in getting a clearer image of the situation 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 international standards for mineral oils, indicating the crucial values stipulated in the varied standards.
The scope of oil analysis, interpretation of the data, and important values
At the beginning of this section, it’s essential to state that we cope with totally different size transformers within the business. Transformers are divided into courses according to the kV ratings of the equipment. It is as a lot as the reliability or asset supervisor to make use of the rules for bigger gear, thereby implementing shorter increment oil analysis and electrical checks.
The maintenance engineer or manager wants to determine what kind of testing would benefit him in figuring out downside areas inside his fleet. No obligation of the evaluation has been identified as to routine sort checks. Still, there might be an in depth range of checks that can assist in figuring out specific downside criteria throughout the system, which could not be clear via the standard day-to-day evaluation often performed.
Please see the ranking courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and how usually or underneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is likely certainly one of the most crucial and significant influencers in the evaluation end result. If a sample isn’t taken to the prescribed procedure, then there’s a significant possibility that the analysis performed, and the outcomes acquired, are of no use.
Care must be taken to make sure high quality control procedures are utilized in every step of the sampling course of as set out by international requirements. A good high quality pattern taken by applying the right procedure is important. A sample may be contaminated by varied factors, all of which might affect the result of the ends in a negative manner.
All steps concerned in sample taking should adhere to high quality management procedures, together with the container that’s used, the sampling equipment, the taking of the sample, the marking of the pattern, the closing of the pattern, the storage and dealing with of the pattern, and then the supply of the sample to the laboratory.
If the sample identification and sample data are handwritten, the human issue can lead to incorrect interpretation of the info. The label ought to be caught 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 ought to be waterproof, and the pen used to put in writing on the label should be oil- and water-resistant. Otherwise, some data might be lost, making it extremely difficult for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s talk about the types of exams to find out the transformer’s condition, the important values, and the beneficial actions in each case:
a) Colour and appearance
This is a routine inspection applied to every oil sample.
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 color, turbidity, and potential particle identification.
Dark oils might indicate chemical degradation or contamination of the oil.
When there may be plenty of turbidity, it would point out a excessive water content material within the oil.
If the drain valve was not cleaned by the sampler, the dirt particles in the drain valve may be incorporated into the sample. If particles are recognized as carbon, it might point out a possible electrical fault within the unit. The DGA evaluation of the oil will affirm if this is the case.
Clear oils without contamination will point out a good situation, and no action is recommended.
When oils are dark or turbid, additional evaluation will confirm any problems. The oil analysis outcomes will also determine 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 point out the water content material 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 within the unit, the breakdown voltage must be high.
If the values are Good, it is suggested to continue with the present sample interval action plan.
If the values are Fair, extra frequent sampling is beneficial in collaboration with different parameter outcomes just like the water content material, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is suggested to recondition the oil via oil reconditioning processes. If alternative tests indicate severe getting older, the oil can 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 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 well as some other compounds. Another benefit is that the oil could be re-used, and in most situations, this can be done without switching off the unit, which contributes to cost-saving. If in doubt – as an alternative swap off the unit during this treatment process.
If the values are Poor, it is advisable to take action as soon as potential and not delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime significantly; extraordinarily excessive water content material could cause flashover within the unit, resulting in lack of the asset.
c) Water content (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 test
This is a routine take a look at for all courses of electrical gear, except class G
The outcomes of this test should at all times be considered in conjunction with the breakdown energy. If it is discovered that the water content material is high and the breakdown energy is low, additional motion needs to be taken. It is beneficial that a second pattern from the same unit is examined to confirm the outcomes.
In the case of switching tools, the place there is not a paper current, the breakdown voltage is the figuring out factor.
It ought to be famous that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is outside this temperature vary, it’s best to check with Annex A of the standard.
When the worth obtained by way of analyses is GOOD, the traditional sampling interval may be maintained, requiring no additional motion.
When the worth returns a FAIR end result, extra frequent sampling is really helpful. It can be helpful to consider other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the motion to be applied.
A POOR result would require quick motion from the asset manager. This may embrace taking one other pattern to confirm the outcomes 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 big portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to ensure that the moisture content is still inside the required limits. The cause is that the most significant slice of the water is caught up in the paper system within the transformer. This moisture will move from the paper into the oil underneath situations that favour this motion. It may be found later that the oil in the water has elevated once more with none apparent cause, however the source can be the paper in the transformer.
A visible inspection is also recommended to discover out if any water would possibly move into the transformer or electrical equipment through leaks. This drawback could be extra severe if the transformer or electrical tools is outside and not in a covered 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<0.100.10 to 0.25>0.20
C<0.100.15 to zero.30>0.30
F, G Not a routine test
This is a routine check for all classes except F and G
The acids in oils are shaped as a end result of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help within the formation of the acids. Because acids are polar compounds, it’ll adversely have an result on the insulation properties of the oil and can improve paper degradation. If left untreated in transformers, this will lead to sludge formation, usually across the lower parts of the transformer core. The sludge will eventually type a semi-solid substance that is extraordinarily tough to take away.
If the result is GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to suit the situation. Future evaluation should include a visual inspection of the oil for sediment and sludge.
If the result is 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 possibility may swimsuit their necessities 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 zero.20>0.20
B, C<0.one 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 test
This is a routine check for all classes of electrical equipment, except F and G
The dielectric dissipation factor or tan delta of this take a look at supplies info relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating material.
When oil ages, we now have the formation of polar compounds, leading to phase displacement and dielectric losses. Other impurities that might influence the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking additional 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 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 take a look at
DC resistivity of the oil is doubtless considered one of the key parameters to assess the transformer insulation situation; this is based on the truth that DC resistance is delicate to oil degradation.
When the result is FAIR, extra frequent sampling and checking further 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 authentic worth
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 relating to additives.
The two commonest oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The function of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the growing older course of in the oil and the stable insulation.
If the result’s FAIR, it is suggested to top up the inhibitor stage to the prescribed stage per provider directions. It is suggested to use a area skilled educated in the procedure to perform this task.
If the outcome obtained is POOR, the advice for this situation would suggest that the top person continues to make use of the oil “uninhibited,” but this may result in more fast degradation of each the liquid and stable insulation.
It must be famous that some transformers have already got a built-in oil preservation system; that is designed to keep dissolved oxygen at ranges under 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 is not required, although it might add further safety in opposition to 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 lower of >10mg/kg/year<50 and reducing at >10mg/kg/year
Passivators, also identified as metal deactivators, react with reactive steel surfaces and dissolved metals corresponding 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 fundamental varieties, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the author is aware, 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 speed up when the oil is un-inhibited.
With GOOD outcomes, regular sample intervals can be maintained.
With FAIR results, keep common monitoring.
When POOR, it’s suggested to remove the oil or remove the supply of corrosivity from the oil via particular oil therapy.
I) Sediment and sludge
This just isn’t a routine test.
It is advised that this take a look at is carried out when the oil outcomes indicate a high acid value and the dissipation factor is close to the unacceptable restrict.
The outcomes must be less than 0.02% by mass to be negligible. If the outcomes return a value of greater than 0.02% by mass, it is 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 28 20 to 25<22 <20
ENot a routine take a look at
F, GNot Applicable
The interfacial rigidity between transformer oil and water reduces through the getting older course of. What this implies in sensible phrases is there is extra polar compound current in the oil, decreasing the ability of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial pressure becomes a high quality criterion: the oil must be modified under a predefined limit.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If outcomes are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine take a look at.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its impact on the transformer system may be important. The extent of the corrosion harm brought on by the sulfur may be so extreme that it’d cause failure of the equipment if not checked. The addition of a copper passivator can reduce the impact of this compound on the transformer system.
In a examine by Doble, it has been found that rubber products utilized in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions primarily based on this institute’s risk evaluation study. 
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 
m) Flashpoint ° C
Not a routine take a look at
If there is a most lower in flashpoints by 10%, the equipment may require further inspection. This value would possibly differ in several nations.
It is suggested to perform this check when an uncommon odour is seen, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to decide the situation of the transformer; it is a well being and security impact check. PCB is hazardous to each people and the surroundings; it’s critical to test for PCBs after the retro fill of a transformer. It is also required every time any maintenance has been accomplished on the unit, and the possibility of contamination is present. If PCB content material exceeds the recommended limits, the appropriate action must be taken.
Units with a PCB content of greater than 50ppm require a hearth 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 protected disposal issued to the tools owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we are going to discuss this phenomenon partly II of the article. The limits for the totally different gases and the interpretation of this data according to worldwide requirements will be mentioned intimately, forming a part of the overall health ranking willpower of the transformer.
Transformer situation monitoring is an interlaced, highly thrilling area of examine. In this article, we focused on the kinds of tests to determine the condition of the transformer, the crucial values, and the recommended actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a specific date and time. This makes it possible to make sure greatest apply utility and optimised maintenance. It also make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition four.zero 2013-01 International Standard (Mineral insulating oils in electrical equipment – supervision and upkeep guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
three. Passivators, what they’re and how 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 energy.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer marketing consultant. She has 20+ years’ expertise within the trade, having beforehand worked as laboratory supervisor for a significant industrial laboratory group, specializing in transformer health. She has been intrigued by transformer chemistry proper from the start of her profession, significantly within the analysis of check data. Corné has vast practical and theoretical data of reliability maintenance programmes.