PETROL CHEMICALS
TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Share
Introduction
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and analysis of the related information relating to an asset’s operation. If we interpret the data accurately, it can give us nice perception into the asset’s situation.
Frequent monitoring of the asset can lead to much less upkeep required or more prolonged intervals with none maintenance required.
It is crucial to identify the key parameters which might be needed to give us an entire picture of the actual standing of the transformer and the action we have to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the info telling us?
Has the condition of the unit changed for the explanation that last maintenance period?
Is it protected to operate the unit?
Are there signs of deterioration?
Is it safe to load the unit above the nameplate rating for a specific 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 have to contemplate replacement?
Are the recognized problems of a recurring nature?
Effective situation monitoring outline
It is vitally important to determine clear targets as part of your technique. What do you wish 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 easier to determine the required parameters.
Health indexing of assets is changing into a exceptional software in getting a clearer image 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 had been calculated based on the worldwide standards for mineral oils, indicating the important values stipulated in the various requirements.
The scope of oil evaluation, interpretation of the information, and critical values
At the beginning of this part, it is important to state that we take care of totally different dimension transformers in the trade. Transformers are divided into courses according to the kV ratings of the tools. It is as much as the reliability or asset supervisor to use the rules for larger gear, thereby implementing shorter increment oil analysis and electrical tests.
The upkeep engineer or supervisor needs to discover out what sort of testing would profit him in identifying downside areas inside his fleet. Some of the evaluation has been identified as to routine type exams. Still, there may be an in depth vary of checks that can help in identifying specific problem criteria inside the system, which might not be clear by way of the typical day-to-day analysis usually carried out.
Please see the rating classes in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are beneficial and how usually or under which circumstances.
TABLE 2 FIG
Oil sampling
The taking of the oil pattern is certainly one of the most important and critical influencers within the evaluation outcome. If a sample isn’t taken to the prescribed procedure, then there’s a important possibility that the evaluation carried out, and the results acquired, are of no use.
Care should be taken to make sure quality control procedures are utilized in every step of the sampling process as set out by worldwide requirements. A good high quality sample taken by making use of the correct process is important. A sample may be contaminated by varied elements, all of which may affect the outcome of the leads to a negative method.
All steps concerned in sample taking should adhere to high quality control procedures, including the container that’s used, the sampling kit, the taking of the sample, the marking of the pattern, the closing of the sample, the storage and dealing with of the pattern, and then the delivery 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 make sure no mix-ups in pattern 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 should be oil- and waterproof. Otherwise, some knowledge could be lost, making it extremely tough for the laboratory personnel to complete the report or allocate the pattern to a specific transformer.
Now, let’s talk about the types of checks to determine the transformer’s condition, the important values, and the recommended actions in every case:
a) Colour and look
This is a routine inspection utilized to each oil sample.
When an oil pattern arrives on the laboratory, one of many “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 point out chemical degradation or contamination of the oil.
When there might be a lot of turbidity, it would indicate a high water content in the oil.
If the drain valve was not wiped clean by the sampler, the dust particles in the drain valve could be incorporated into the sample. If particles are identified as carbon, it’d point out a attainable electrical fault within the unit. The DGA analysis of the oil will verify if so.
Clear oils without contamination will indicate a good condition, and no motion is really helpful.
When oils are dark or turbid, additional analysis will verify any issues. The oil evaluation 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 level application<40 kV for OLTC in delta or line-end application
G <30
This is a routine inspection.
Breakdown voltage will point out the water content 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 within the unit, the breakdown voltage have to be excessive.
If the values are Good, it is suggested to proceed with the present pattern interval action plan.
If the values are Fair, more frequent sampling is recommended in collaboration with other parameter results like the water content, DDF (dielectric dissipation factor), and acidity.
If values are Poor – it is strongly recommended to recondition the oil by way of oil reconditioning processes. If alternative checks indicate severe getting older, the oil could be replaced with new or reclaimed oil. Another option can be to perform on-site oil reclamation using a reclamation plant.
Reclamation of oil has the advantage that the color of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water in addition to some other compounds. Another advantage is that the oil may be re-used, and in most situations, this could be done without switching off the unit, which contributes to cost-saving. If unsure – as an alternative change off the unit during this remedy course of.
If the values are Poor, it’s advisable to take action as quickly as attainable and never delay the maintenance process. Excess water in the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content material may cause flashover in the unit, resulting in loss of the asset.
c) Water content (mg/kg at transformer operating 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 check
This is a routine take a look at for all courses of electrical tools, besides class G
The results of this test should at all times be considered along side the breakdown energy. If it’s found that the water content material is high and the breakdown strength is low, additional motion must be taken. It is really helpful that a second pattern from the same unit is examined to verify the results.
In the case of switching tools, where there is no paper present, the breakdown voltage is the figuring out 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’s found that the unit’s operating temperature is outside this temperature range, it’s 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 worth returns a FAIR result, more frequent sampling is recommended. It can also be helpful to assume about different parameters just like the breakdown voltage, particle content and DDF/resistivity, and acidity to resolve on the action to be carried out.
A POOR result will require instant motion from the asset manager. This might include taking another sample to verify the results from the primary evaluation. If it is confirmed that the water content material is high, the oil can be filtered; this course of ought to take away a big portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to make sure that the moisture content material is still inside the required limits. The cause is that the most significant portion of the water is caught up in the paper system within the transformer. This moisture will transfer from the paper into the oil under situations that favour this movement. It could be discovered later that the oil within the water has increased again with none apparent purpose, however the source would be the paper in the transformer.
A visual inspection can additionally be beneficial to find out if any water might transfer into the transformer or electrical tools through leaks. This problem might be extra severe if the transformer or electrical equipment is exterior and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
PropertyCategoryGoodFairPoor
Acidity mgKOH/g oilO, A, D<0.one hundred.10 to zero.15>0.15
B, E<0.one hundred.10 to 0.25>0.20
C<0.one hundred.15 to zero.30>0.30
F, G Not a routine test
This is a routine take a look at for all lessons except F and G
The acids in oils are shaped as a outcome of chemical reactions between the oil, water, and paper. Higher temperatures or load will increase will assist 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 will increase paper degradation. If left untreated in transformers, this can lead to sludge formation, usually across the decrease elements of the transformer core. เกจวัดแรงดันไอน้ำ will ultimately type a semi-solid substance that’s extremely difficult to take away.
If the result is GOOD, the common sampling interval can continue.
In case of a FAIR outcome, the sampling interval must be decreased to fit the situation. Future analysis should embody 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 4.zero, the asset manager could decide to reclaim the oil or exchange it with new or reclaimed oil, whichever option would possibly suit their necessities one of the best.
e) Dielectric Dissipation factor at 40Hz to 60Hz at 90° C
PropertyCategoryGoodFairPoor
Dielectric Dissipation factorAt 40Hz to 60Hz at 90°CO, A<0.100.10 to 0.20>0.20
B, C<0.one hundred.10 to zero.50>0.50
D<0.010.01 to zero.03>0.03
E<0.a hundred.01 to zero.30>0.03
F, GNot a routine check
This is a routine test for all lessons of electrical gear, besides F and G
The dielectric dissipation factor or tan delta of this test offers info relating to the extent of the dielectric losses in transformer oil. This check measures the inefficiency of insulating materials.
When oil ages, we’ve the formation of polar compounds, leading to phase displacement and dielectric losses. Other impurities that might affect the dissipation issue include water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking additional parameters is really helpful.
When the result is 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 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 take a look at
DC resistivity of the oil is amongst the key parameters to assess the transformer insulation condition; that is primarily based on the reality that DC resistance is sensitive to oil degradation.
When the result’s FAIR, extra frequent sampling and checking additional parameters is really helpful.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content %
PropertyCategoryGoodFairPoor
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of authentic value
This test 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 most typical oxidation inhibitors for transformer oils are 2,6-di-tertiary-butyl para-cresol (DBPC) and 2,6-di-tertiary butyl-phenol (DBP). The purpose of the inhibitor is to prevent oxygen from reacting with the oil. This considerably slows the aging process in the oil and the solid insulation.
If the result’s FAIR, it’s suggested to high up the inhibitor stage to the prescribed level per provider instructions. It is suggested to make use of a field skilled educated within the procedure to perform this activity.
If the end result obtained is POOR, the recommendation for this scenario would recommend that the end person continues to make use of the oil “uninhibited,” but this may result in extra speedy degradation of both the liquid and stable insulation.
It should 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 would 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 beneath these circumstances isn’t required, though it might add extra protection towards 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 secure, (rate of decrease < 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 referred to as metallic 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 organic compounds and reactions with corrosive sulfur. Passivators are composed of two basic types, sulfur-based and nitrogen-based. The first instructed 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.[3]
As the oil ages, the passivator would possibly deplete extra quickly; this depletion might speed up when the oil is un-inhibited.
With GOOD outcomes, regular pattern intervals can be maintained.
With FAIR outcomes, preserve regular monitoring.
When POOR, it’s suggested to take away the oil or remove 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 outcomes point out a excessive acid value and the dissipation issue is near the unacceptable restrict.
The results must be lower than 0.02% by mass to be negligible. If the outcomes return a price of more than zero.02% by mass, it is instructed that it be reclaimed; otherwise, an oil change is recommended.
j) Interfacial tension
This just isn’t a routine take a look at
PropertyCategoryGoodFairPoor
Interfacial Tension (mN/m)O, A, B, C, DInhibited Uninhibited>28 >2522 to twenty-eight 20 to 25<22 <20
ENot a routine test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces in the course of the getting older process. What this implies in sensible phrases is there is extra polar compound current within the oil, reducing the ability of an oil to serve as an insulator in the transformer system. There is a direct correlation between interfacial pressure and neutralisation quantity. Therefore, the interfacial rigidity becomes a high quality criterion: the oil must be modified under a predefined limit.
If results are GOOD, continue the regular sampling interval.
If results are FAIR, lower the sampling interval.
If results are POOR, examine the oil for sediment and/or sludge.
k) Corrosive sulfur
This just isn’t 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 can be significant. The extent of the corrosion injury caused by the sulfur could be so extreme that it might trigger failure of the tools if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a examine by Doble, it has been discovered that rubber products utilized in transformers might add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not approved for oil filtration might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s danger assessment examine. [4]
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 [5]
m) Flashpoint ° C
Not a routine test
If there’s a most decrease in flashpoints by 10%, the gear might require further inspection. This value might differ in numerous countries.
It is advised to carry out this test when an uncommon odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This take a look at is to not determine the situation of the transformer; this is a health and security impression check. PCB is hazardous to each people and the setting; it’s critical to check for PCBs after the retro fill of a transformer. It is also required each time any upkeep has been carried out on the unit, and the possibility of contamination is current. If PCB content material exceeds the beneficial limits, the appropriate motion must be taken.
Units with a PCB content material of greater than 50ppm require a fire security plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil must be replaced, and the oil disposed of as hazardous waste, with a certificates of safe disposal issued to the gear owner.
Local regulatory our bodies outline the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with plenty of knowledge and interpretation, we will focus on this phenomenon partly II of the article. The limits for the completely different gases and the interpretation of this information based on international standards will be mentioned intimately, forming a half of the general health ranking determination of the transformer.
Conclusion
Transformer condition monitoring is an interlaced, extremely thrilling area of research. In this text, we targeted on the kinds of checks to determine the condition of the transformer, the crucial values, and the beneficial actions.
The Health Index indication makes it potential to see the supposed reliability of a specific unit at a specific date and time. This makes it potential to make sure best practice software and optimised maintenance. It additionally make it simpler to draw up a upkeep plan and motion plan.
References:
1. IEC 60422 Edition four.0 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
3. Passivators, what they are 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 printed by Transformer Technology Magazine Issue 16 – 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 previously 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 career, particularly in the analysis of check data. Corné has huge practical and theoretical knowledge of reliability maintenance programmes.
Share