TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent collecting, measuring, recording, and evaluation of the relevant data relating to an asset’s operation. If we interpret the data appropriately, it may give us great insight into the asset’s situation.
Frequent monitoring of the asset can lead to less maintenance required or extra prolonged periods with none maintenance required.
It is crucial to establish the key parameters which are needed to offer us a complete image of the actual status of the transformer and the motion we need 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 modified for the rationale that last upkeep period?
Is it protected to function the unit?
Are there signs of deterioration?
Is it safe 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 lengthy can we use the unit before we have to contemplate replacement?
Are the recognized issues of a recurring nature?
Effective condition monitoring define
It is vitally essential to determine clear objectives as a half of your technique. What do you want to achieve 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 simpler to establish the required parameters.
Health indexing of belongings is turning into a outstanding device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the whole worth of the Health Index Value of the transformer. These parameter weight values have been calculated primarily based on the worldwide requirements for mineral oils, indicating the critical values stipulated in the numerous requirements.
The scope of oil analysis, interpretation of the information, and important values
At the start of this section, it’s important to state that we deal with different dimension transformers in the trade. Transformers are divided into courses based on the kV scores of the equipment. It is up to the reliability or asset supervisor to make use of the rules for larger gear, thereby implementing shorter increment oil evaluation and electrical checks.
The upkeep engineer or supervisor wants to discover out what kind of testing would profit him in figuring out problem areas inside his fleet. เกจวัดถังแก๊ส of the evaluation has been identified as to routine sort tests. Still, there might be an in depth range of exams that may assist in identifying specific problem standards inside the system, which could not be clear by way of the typical day-to-day analysis normally performed.
Please see the rating lessons in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and how usually or beneath which circumstances.
TABLE 2 FIG
The taking of the oil sample is among the most vital and critical influencers in the analysis end result. If a pattern is not taken to the prescribed procedure, then there is a important possibility 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 every step of the sampling process as set out by worldwide requirements. A good high quality pattern taken by making use of the proper procedure is important. A sample may be contaminated by numerous elements, all of which may affect the finish result of the results in a negative manner.
All steps involved in sample taking should adhere to quality control procedures, together with the container that’s used, the sampling package, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and handling of the sample, and then the delivery 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 data. The label ought to 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 ought to be waterproof, and the pen used to write on the label must be oil- and water resistant. Otherwise, some data may be lost, making it extraordinarily tough for the laboratory personnel to finish the report or allocate the sample to a specific transformer.
Now, let’s discuss the forms of tests to determine the transformer’s situation, the critical values, and the recommended actions in every case:
a) Colour and appearance
This is a routine inspection utilized to every oil pattern.
When an oil pattern arrives on the laboratory, one of many “tests” is a visible inspection of the oil pattern in a clear vessel to discover out the color, turbidity, and potential particle identification.
Dark oils may indicate chemical degradation or contamination of the oil.
When there is plenty of turbidity, it might indicate a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the dust particles in the drain valve could be incorporated into the pattern. If particles are identified as carbon, it’d indicate a possible electrical fault within the unit. The DGA analysis of the oil will confirm if that is the case.
Clear oils without contamination will indicate an excellent situation, and no action is beneficial.
When oils are dark or turbid, further analysis will affirm any problems. The oil evaluation outcomes will also decide the degree and sort 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 utility
This is a routine inspection.
Breakdown voltage will indicate the water content material or the presence of foreign particles, or both within the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage should be high.
If the values are Good, it is recommended to proceed with the present pattern interval action plan.
If the values are Fair, extra frequent sampling is really helpful in collaboration with other parameter outcomes like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil via oil reconditioning processes. If alternative exams point out severe aging, the oil could 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 colour of the oil is restored, and the polar components are removed from the oil. This process removes acid and water in addition to some other compounds. Another benefit is that the oil can be re-used, and in most situations, this can be accomplished without switching off the unit, which contributes to cost-saving. If doubtful – instead change off the unit during this therapy course of.
If the values are Poor, it is advisable to take motion as quickly as attainable and never delay the upkeep process. Excess water within the transformer system decreases the projected transformer lifetime significantly; extremely high water content material could cause flashover in the unit, resulting in 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 test
This is a routine check for all courses of electrical tools, besides class G
The results of this test ought to all the time be thought-about in conjunction with the breakdown energy. If it’s discovered that the water content material is excessive and the breakdown energy is low, further action needs to be taken. It is really helpful that a second sample from the identical unit is tested to confirm the outcomes.
In the case of switching gear, where there isn’t a paper current, the breakdown voltage is the determining factor.
It should be noted that the bounds indicated by IEC 60422 Edition 4 apply to transformers with working temperatures between forty and 70 °C. If it is found 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 can be maintained, requiring no further motion.
When the worth returns a FAIR end result, more frequent sampling is recommended. It can additionally be useful to contemplate different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the motion to be implemented.
A POOR outcome would require instant action from the asset manager. This might embody taking one other sample to substantiate the results from the primary evaluation. If it is confirmed that the water content is high, the oil may be filtered; this course of ought to remove a large portion of the moisture from the oil if applied correctly. Follow-up samples have to be taken to guarantee that the moisture content is still throughout the required limits. The purpose is that probably 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 beneath conditions that favour this motion. It may be found later that the oil within the water has elevated again without any apparent cause, however the source can be the paper within the transformer.
A visible inspection can be beneficial to find out if any water may move into the transformer or electrical equipment via leaks. This downside could be more extreme if the transformer or electrical equipment is outdoors and never in a coated area.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.one hundred.10 to zero.25>0.20
C<0.one hundred.15 to zero.30>0.30
F, G Not a routine check
This is a routine check for all classes besides F and G
The acids in oils are shaped 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’s going to adversely have an result on the insulation properties of the oil and will improve paper degradation. If left untreated in transformers, this will lead to sludge formation, normally around the decrease elements of the transformer core. The sludge will ultimately type a semi-solid substance that is extremely difficult to remove.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR outcome, the sampling interval must be decreased to suit the state of affairs. Future analysis ought to include a visible inspection of the oil for sediment and sludge.
If the result is POOR based on the prescribed values in IEC 60422 Edition four.0, the asset manager might decide to reclaim the oil or exchange it with new or reclaimed oil, whichever choice might go properly with their requirements the most effective.
e) Dielectric Dissipation issue at 40Hz to 60Hz at 90° C
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.a hundred.10 to 0.20>0.20
B, C<0.one hundred.10 to 0.50>0.50
D<0.010.01 to zero.03>0.03
E<0.one hundred.01 to zero.30>0.03
F, GNot a routine take a look at
This is a routine check for all classes of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this test supplies information relating to the extent of the dielectric losses in transformer oil. This test measures the inefficiency of insulating material.
When oil ages, we’ve the formation of polar compounds, resulting in section displacement and dielectric losses. Other impurities that may affect the dissipation factor embrace 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 construction of the oil is broken, in impact that the chemical bonds between the molecules have damaged down, and even with filtration, the beneficial 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 check
DC resistivity of the oil is among the key parameters to assess the transformer insulation situation; this is based mostly on the reality that DC resistance is delicate to grease degradation.
When the result is FAIR, extra frequent sampling and checking extra parameters is really helpful.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of unique worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil supplier to verify the details concerning additives.
The two most typical 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 forestall oxygen from reacting with the oil. This considerably slows the aging course of within the oil and the strong insulation.
If the result’s FAIR, it is advised to prime up the inhibitor level to the prescribed stage per provider directions. It is advised to make use of a subject professional trained in the procedure to carry out this activity.
If the end result obtained is POOR, the advice for this situation would suggest that the top person continues to use the oil “uninhibited,” however this may result in more speedy degradation of each the liquid and solid insulation.
It must be famous that some transformers already have a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This can 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, though 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 stable, (rate of decrease < 10/mg/kg/year)50-70mg/kg or<70mg/kg, with a significantrate of decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also known as metallic deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and cut back their rate of response with compounds in the oil. This includes oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two basic sorts, sulfur-based and nitrogen-based. The first advised use of passivators in transformer oil, of which the writer is aware, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.
As the oil ages, the passivator would possibly deplete more quickly; this depletion may accelerate when the oil is un-inhibited.
With GOOD results, regular pattern intervals could be maintained.
With FAIR results, maintain common monitoring.
When POOR, it is advised to remove the oil or take away the source of corrosivity from the oil through particular oil therapy.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is advised that this test is carried out when the oil results point out a high acid value and the dissipation issue is close to the unacceptable limit.
The results need to be lower than 0.02% by mass to be negligible. If the outcomes return a price of greater than zero.02% by mass, it is advised that it be reclaimed; in any other case, an oil change is recommended.
j) Interfacial tension
This is not a routine take a look at
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 means in sensible terms is there is more polar compound current within the oil, decreasing the ability of an oil to function an insulator in the transformer system. There is a direct correlation between interfacial tension and neutralisation quantity. Therefore, the interfacial rigidity becomes a quality criterion: the oil should be modified under a predefined limit.
If results are GOOD, continue the common sampling interval.
If results are FAIR, decrease the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine test.
Oil is either 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 damage attributable to the sulfur may be so extreme that it would cause failure of the gear 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 merchandise used in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration would possibly contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions primarily based on this institute’s danger evaluation study. 
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination levels (particles) encountered on power transformer insulating oil as measured using IEC 60970 
m) Flashpoint ° C
Not a routine take a look at
If there is a most lower in flashpoints by 10%, the tools would possibly require additional inspection. This worth would possibly differ in different international locations.
It is advised to carry out this check when an uncommon odour is observed, the unit has been refilled, or an inside fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This check is not to determine the condition of the transformer; it is a well being and safety impact check. PCB is hazardous to each humans and the surroundings; it is vital to test for PCBs after the retro fill of a transformer. It is also required every time any upkeep has been done on the unit, and the possibility of contamination is current. If PCB content exceeds the really helpful limits, the appropriate action needs to be taken.
Units with a PCB content material of greater than 50ppm require a fireplace safety plan, environmental protection plan, and extra precautionary measures when upkeep is done. This oil needs to be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the gear owner.
Local regulatory bodies define the limits.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with a lot of information 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 in accordance with worldwide requirements shall be mentioned in detail, forming a part of the overall health rating dedication of the transformer.
Transformer situation monitoring is an interlaced, highly exciting area of examine. In this text, we centered on the types of checks to discover out the condition of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it possible to see the supposed reliability of a selected unit at a selected date and time. This makes it potential to ensure best follow application and optimised maintenance. It also make it simpler to draw up a maintenance plan and action plan.
1. IEC 60422 Edition 4.zero 2013-01 International Standard (Mineral insulating oils in electrical gear – supervision and upkeep 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 strength.”
6. Article initially published 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 within the industry, having beforehand worked as laboratory manager for a serious industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the start of her profession, significantly within the evaluation of check knowledge. Corné has vast practical and theoretical data of reliability upkeep programmes.