TALKING TRANSFORMER TESTING

PETROL CHEMICALS
TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
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Introduction
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the relevant knowledge concerning an asset’s operation. If we interpret the info appropriately, it can provide us nice perception into the asset’s situation.
Frequent monitoring of the asset can result in much less upkeep required or extra prolonged durations with none maintenance required.
It is essential to identify the important thing parameters which are needed to give us an entire image of the particular standing of the transformer and the motion we need to take to ensure the continued reliability of the asset to achieve the maximum lifetime.
What is the info telling us?
Has the situation of the unit modified since the last maintenance period?
Is it safe to operate the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate score for a selected period?
Are we required to implement action to ensure the continued reliability of the unit?
How long can we use the unit before we have to contemplate replacement?
Are the identified problems of a recurring nature?
Effective situation monitoring outline
It is vitally important to establish clear targets as part of your technique. What do you need to achieve by implementing this situation monitoring plan? Is it in-service failure prevention? Or perhaps life extension? Maintenance deferral? By stipulating the result and what you need to accomplish, it will be a lot simpler to establish the required parameters.
Health indexing of property is becoming a exceptional device in getting a clearer picture of the situation of your transformer. Test parameters carry a numerical value-adding to the total worth of the Health Index Value of the transformer. These parameter weight values were calculated based on the international requirements for mineral oils, indicating the important values stipulated within the numerous standards.
The scope of oil analysis, interpretation of the info, and critical values
At the beginning of this part, it’s essential to state that we deal with totally different measurement transformers in the trade. Transformers are divided into lessons in accordance with the kV scores of the tools. It is as much as the reliability or asset supervisor to use the rules for larger tools, thereby implementing shorter increment oil analysis and electrical checks.
The upkeep engineer or manager needs to discover out what kind of testing would benefit him in figuring out problem areas inside his fleet. Some of the analysis has been recognized as to routine type checks. Still, there is an in depth range of tests that may assist in identifying specific drawback standards throughout the system, which might not be clear by way of the typical day-to-day evaluation normally carried out.
Please see the rating courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are really helpful and how often or under which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is among the most crucial and significant influencers in the analysis outcome. If a pattern is not taken to the prescribed procedure, then there is a vital possibility that the analysis performed, and the outcomes acquired, are of no use.
Care must be taken to make sure quality control procedures are applied in each step of the sampling process as set out by international standards. A good high quality pattern taken by applying the right process is important. A sample can be contaminated by varied elements, all of which might influence the outcome of the results in a unfavorable manner.
All steps involved in pattern taking should adhere to high quality control procedures, together with the container that is used, the sampling kit, the taking of the pattern, the marking of the sample, the closing of the pattern, the storage and handling of the sample, after which the supply of the pattern to the laboratory.
If the pattern identification and pattern data are handwritten, the human issue can result in incorrect interpretation of the information. The label must be caught onto the container to make sure no mix-ups in sample identification. Ensure that the label is accomplished in neat, easy-to-read handwriting. The label must be waterproof, and the pen used to write down on the label must be oil- and waterproof. Otherwise, some information could be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the pattern to a particular transformer.
Now, let’s talk about the forms of exams to discover out the transformer’s situation, the important values, and the really helpful actions in every case:
a) Colour and appearance
This is a routine inspection utilized to every oil pattern.
When an oil sample arrives on the laboratory, one of the “tests” is a visible inspection of the oil sample in a transparent vessel to determine the colour, turbidity, and attainable particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there’s lots of turbidity, it might indicate a excessive water content material within the oil.
If the drain valve was not wiped clean by the sampler, the dust particles within the drain valve may be included into the sample. If particles are recognized as carbon, it might point out a potential electrical fault within the unit. The DGA analysis of the oil will confirm if so.
Clear oils with out contamination will point out a great condition, and no action is recommended.
When oils are dark or turbid, further evaluation will confirm any problems. The oil analysis results may even determine the diploma and sort of action.
b) Breakdown Voltage
PropertyCategoryGoodFairPoor
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
G <30
This is a routine inspection.
Breakdown voltage will point out 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 keep away from flashover within the unit, the breakdown voltage must be high.
If the values are Good, it is recommended to continue with the present sample interval motion plan.
If the values are Fair, more frequent sampling is beneficial in collaboration with different parameter results like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is recommended to recondition the oil through oil reconditioning processes. If various tests indicate extreme aging, the oil can be replaced with new or reclaimed oil. Another choice would be to perform 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 removed from the oil. This course of removes acid and water as nicely as some other compounds. Another advantage is that the oil can 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 a substitute switch off the unit during this therapy course of.
If the values are Poor, it is advisable to take motion as quickly as attainable and not delay the maintenance course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content material can cause flashover in the unit, leading to loss of the asset.
c) Water content material (mg/kg at transformer working temperature)
PropertyCategoryGoodFairPoor
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 lessons of electrical tools, besides class G
The results of this check ought to at all times be considered along side the breakdown power. If it is discovered that the water content is excessive and the breakdown energy is low, additional action needs to be taken. It is really helpful that a second pattern from the same unit is tested to confirm the outcomes.
In the case of switching tools, the place there isn’t a paper present, the breakdown voltage is the determining issue.
It should be noted that the limits indicated by IEC 60422 Edition 4 apply to transformers with operating temperatures between forty and 70 °C. If it’s 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 worth obtained through analyses is GOOD, the traditional sampling interval may be maintained, requiring no further motion.
When the value returns a FAIR result, extra frequent sampling is really helpful. It can be useful to contemplate different parameters like the breakdown voltage, particle content and DDF/resistivity, and acidity to decide on the motion to be applied.
A POOR outcome will require quick action from the asset manager. This may embody taking another sample to substantiate the outcomes from the first analysis. If it is confirmed that the water content material is excessive, the oil can be filtered; this course of should take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to ensure that the moisture content remains to be throughout the required limits. The purpose is that probably 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 underneath conditions that favour this movement. It could be discovered later that the oil within the water has increased once more with none obvious purpose, but the supply would be the paper within the transformer.
A visible inspection can additionally be really helpful to determine if any water may move into the transformer or electrical equipment through leaks. This drawback may be extra severe if the transformer or electrical equipment is exterior and never in a lined space.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.100.10 to 0.15>0.15
B, E<0.a hundred.10 to zero.25>0.20
C<0.one hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine check for all courses besides F and G
The acids in oils are formed because of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will help in 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 increase paper degradation. If left untreated in transformers, this could result in sludge formation, often around the decrease elements of the transformer core. The sludge will ultimately kind 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 result, the sampling interval should be decreased to suit the situation. Future evaluation ought to include 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 manager might determine to reclaim the oil or replace it with new or reclaimed oil, whichever option would possibly go well with their necessities the most effective.
e) Dielectric Dissipation issue at 40Hz to 60Hz at 90° C
PropertyCategoryGoodFairPoor
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to zero.20>0.20
B, C<0.100.10 to 0.50>0.50
D<0.010.01 to 0.03>0.03
E<0.one hundred.01 to 0.30>0.03
F, GNot a routine take a look at
This is a routine take a look at for all classes of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this check supplies data concerning 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, resulting in phase displacement and dielectric losses. Other impurities that may affect the dissipation issue embrace water, dissolved insulating resin, and paper.
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 really helpful. The construction of the oil is broken, in effect that the chemical bonds between the molecules have damaged down, and even with filtration, the recommended dielectric values can’t be achieved.
f) Resistivity (GΩm) at 20 °C or 90 °C
PropertyCategoryGoodFairPoor
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 probably 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, more frequent sampling and checking extra parameters is beneficial.
When the result’s POOR, reclamation or an oil change is recommended.
g) Inhibitor content material %
PropertyCategoryGoodFairPoor
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.
เกจ์แรงดัน can be advisable to contact the oil supplier to confirm the primary points regarding additives.
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 objective of the inhibitor is to prevent oxygen from reacting with the oil. This significantly slows the growing older process within the oil and the solid insulation.
If the result is FAIR, it is advised to prime up the inhibitor stage to the prescribed stage per provider directions. It is suggested to use a area professional trained in the procedure to carry out this task.
If the end result obtained is POOR, the advice for this situation would counsel that the end user continues to make use of the oil “uninhibited,” but this may lead to extra rapid degradation of each the liquid and stable insulation.
It ought to be famous that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at levels beneath 1000ppm. This can be within the form of a nitrogen system, a nitrogen tank or generator, or a conservator tank equipped with a rubber diaphragm (bladder). Using inhibited oils under these circumstances isn’t required, though it’d add additional safety against oil oxidation if the preservation system ever fails. [2]
h) Passivator content material
PropertyCategoryGoodFairPoor
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 lower of >10mg/kg/year<50 and decreasing at >10mg/kg/year
Passivators, also referred to as steel deactivators, react with reactive metal surfaces and dissolved metals corresponding to copper and silver and reduce their price of reaction with compounds in the oil. This contains oxidation reactions with natural compounds and reactions with corrosive sulfur. Passivators are composed of two basic varieties, sulfur-based and nitrogen-based. The first instructed 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.[3]
As the oil ages, the passivator would possibly deplete extra quickly; this depletion may speed up when the oil is un-inhibited.
With GOOD outcomes, common sample intervals could be maintained.
With FAIR outcomes, keep common monitoring.
When POOR, it is suggested to take away the oil or take away the supply of corrosivity from the oil through special oil therapy.
I) Sediment and sludge
This isn’t a routine test.
It is suggested that this check is carried out when the oil outcomes indicate a high acid worth and the dissipation factor is close to the unacceptable limit.
The outcomes need to be less than zero.02% by mass to be negligible. If the results return a value of more than 0.02% by mass, it’s suggested that or not it’s reclaimed; in any other case, an oil change is beneficial.
j) Interfacial tension
This just isn’t a routine check
PropertyCategoryGoodFairPoor
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 tension between transformer oil and water reduces during the aging course of. What this implies in sensible terms is there’s extra polar compound current within the oil, reducing the flexibility of an oil to serve as an insulator within the transformer system. There is a direct correlation between interfacial rigidity and neutralisation quantity. Therefore, the interfacial pressure turns into a quality criterion: the oil must be changed beneath a predefined limit.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If results are POOR, check the oil for sediment and/or sludge.
k) Corrosive sulfur
This isn’t a routine test.
Oil is both 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 injury brought on by the sulfur can be so severe that it would cause failure of the equipment if not checked. The addition of a copper passivator can scale back the impact of this compound on the transformer system.
In a research by Doble, it has been found that rubber merchandise utilized in transformers would possibly add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not accredited for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based mostly on this institute’s threat assessment research. [4]
l) Particle counting and sizing
Table three: Particles
Table B.1 – Typical contamination levels (particles) encountered on energy transformer insulating oil as measured using IEC 60970 [5]
m) Flashpoint ° C
Not a routine take a look at
If there is a maximum lower in flashpoints by 10%, the equipment may require further inspection. This worth would possibly differ in different countries.
It is advised to carry out this take a look at when an unusual odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not determine the condition of the transformer; this may be a well being and security impression take a look at. PCB is hazardous to both humans and the environment; it is important to check for PCBs after the retro fill of a transformer. It is also required whenever any upkeep has been done on the unit, and the potential for contamination is present. If PCB content exceeds the beneficial limits, the appropriate motion needs to be taken.
Units with a PCB content of more than 50ppm require a hearth security plan, environmental safety plan, and additional precautionary measures when upkeep is finished. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the equipment proprietor.
Local regulatory bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we are going to discuss this phenomenon in part II of the article. The limits for the completely different gases and the interpretation of this information according to international requirements shall be discussed in detail, forming a part of the general well being rating willpower of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, highly thrilling area of research. In this article, we focused on the kinds of checks to discover out the condition of the transformer, the important values, and the recommended actions.
The Health Index indication makes it potential to see the supposed reliability of a selected unit at a selected date and time. This makes it attainable to make sure best practice utility and optimised maintenance. It also make it easier to draw up a maintenance plan and action plan.
References:
1. IEC 60422 Edition four.zero 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’re and the way they work, by Lance Lewand, Doble Engineering Company.
four. 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 published by Transformer Technology Magazine Issue sixteen – December 2021 https://www.transformer-technology.com
Author bio:
Corné Dames is a WearCheck transformer consultant. She has 20+ years’ experience within the trade, having beforehand worked as laboratory supervisor for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the start of her profession, significantly in the evaluation of test information. Corné has vast practical and theoretical knowledge of reliability upkeep programmes.
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