by Brenna ShumbamhiniJune 9, 2022
Corne Dames
Condition monitoring is the frequent amassing, measuring, recording, and evaluation of the relevant information relating to an asset’s operation. If we interpret the info appropriately, it can provide us great perception into the asset’s situation.
Frequent monitoring of the asset may end up in much less maintenance required or extra prolonged intervals without any upkeep required.
It is crucial to identify the key parameters which would possibly be wanted to offer us an entire picture of the actual status of the transformer and the motion we want to take to make sure the continued reliability of the asset to achieve the maximum lifetime.
What is the information telling us?
Has the situation of the unit changed for the explanation that final maintenance period?
Is it secure to operate the unit?
Are there indicators of deterioration?
Is it protected to load the unit above the nameplate ranking for a selected 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 consider replacement?
Are the recognized problems of a recurring nature?
Effective condition monitoring define
It is vitally necessary to identify clear objectives as a part of your strategy. What do you wish to achieve by implementing this condition monitoring plan? Is it in-service failure prevention? Or maybe life extension? Maintenance deferral? By stipulating the result and what you wish to accomplish, it will be much simpler to establish the required parameters.
Health indexing of belongings 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 whole worth of the Health Index Value of the transformer. These parameter weight values have been calculated based mostly on the worldwide requirements for mineral oils, indicating the important values stipulated within the numerous standards.
The scope of oil evaluation, interpretation of the info, and significant values
At the beginning of this section, it is essential to state that we deal with completely different measurement transformers within the trade. Transformers are divided into lessons according to the kV rankings of the equipment. It is as much as the reliability or asset supervisor to use the rules for bigger tools, thereby implementing shorter increment oil analysis and electrical exams.
The upkeep engineer or manager wants to discover out what type of testing would profit him in figuring out downside areas inside his fleet. Some of the analysis has been identified as to routine kind tests. Still, there is an extensive vary of checks that may assist in figuring out specific problem standards inside the system, which could not be clear by way of the standard day-to-day analysis often carried out.
Please see the rating courses in TABLE 1
TABLE 2 explains which oil analyses are recommended and how often or underneath which circumstances.
Oil sampling
The taking of the oil sample is among the most crucial and important influencers within the analysis end result. If a pattern is not taken to the prescribed procedure, then there’s a significant chance that the analysis carried out, and the outcomes acquired, are of no use.
Care should be taken to make sure high quality management procedures are utilized in each step of the sampling course of as set out by worldwide standards. A good high quality pattern taken by making use of the correct procedure is essential. A sample may be contaminated by various elements, all of which might influence the result of the ends in a adverse manner.
All steps involved in pattern taking should adhere to high quality management procedures, including the container that is used, the sampling kit, the taking of the sample, the marking of the sample, the closing of the pattern, the storage and dealing with of the pattern, and then the supply of the pattern to the laboratory.
If the sample identification and pattern data are handwritten, the human issue can lead to incorrect interpretation of the information. The label must 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 must be waterproof, and the pen used to write down on the label must be oil- and water resistant. Otherwise, some information might be misplaced, making it extraordinarily troublesome for the laboratory personnel to finish the report or allocate the sample to a selected transformer.
Now, let’s discuss the types of exams to determine the transformer’s condition, the crucial values, and the beneficial actions in each case:
a) Colour and look
This is a routine inspection utilized to every oil sample.
When an oil pattern arrives at the laboratory, one of many “tests” is a visual inspection of the oil sample in a transparent vessel to find out the color, turbidity, and potential particle identification.
Dark oils would possibly indicate chemical degradation or contamination of the oil.
When there might be plenty of turbidity, it might point out a excessive water content in the oil.
If the drain valve was not wiped clean by the sampler, the filth particles within the drain valve could be integrated into the sample. If particles are recognized as carbon, it might point out a attainable electrical fault within the unit. The DGA evaluation of the oil will affirm if this is the case.
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 verify any problems. The oil evaluation outcomes may also decide the degree and type 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 level application<40 kV for OLTC in delta or line-end software
G <30
This is a routine inspection.
Breakdown voltage will point out the water content material or the presence of foreign particles, or both in the oil being analysed.
As the oil in transformers acts as an insulation medium to avoid flashover in the unit, the breakdown voltage must be high.
If the values are Good, it is strongly recommended to proceed with the present sample interval action plan.
If the values are Fair, more frequent sampling is really helpful in collaboration with other parameter results just 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 checks indicate extreme getting older, the oil may be replaced with new or reclaimed oil. Another choice could 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 elements are faraway from the oil. pressure gauge nuova fima ราคา removes acid and water as nicely as some other compounds. Another advantage is that the oil may be re-used, and in most conditions, this might be accomplished with out switching off the unit, which contributes to cost-saving. If in doubt – as an alternative swap off the unit throughout this treatment process.
If the values are Poor, it’s advisable to take motion as soon as attainable and not delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily high water content may cause flashover within the unit, resulting in loss of the asset.
c) Water content (mg/kg at transformer operating 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 take a look at
This is a routine test for all lessons of electrical gear, besides class G
The results of this take a look at ought to always be considered at the facet of the breakdown power. If it’s found that the water content 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 tested to confirm the outcomes.
In the case of switching tools, where there isn’t any paper present, the breakdown voltage is the figuring out issue.
It ought to be noted that the bounds indicated by IEC 60422 Edition four apply to transformers with operating temperatures between 40 and 70 °C. If it is found that the unit’s operating temperature is outside this temperature range, it is best to discuss with Annex A of the usual.
When the worth obtained by way of analyses is GOOD, the traditional sampling interval can be maintained, requiring no additional action.
When the worth returns a FAIR outcome, extra frequent sampling is beneficial. It can additionally be useful to contemplate other parameters just like the breakdown voltage, particle content material and DDF/resistivity, and acidity to resolve on the action to be implemented.
A POOR outcome would require quick action from the asset supervisor. This may embody taking another pattern to confirm the results from the first analysis. If it is confirmed that the water content is high, the oil may be filtered; this process should take away a large portion of the moisture from the oil if utilized correctly. Follow-up samples must be taken to make certain that the moisture content material continues to be inside the required limits. The purpose is that essentially the most good portion of the water is caught up within the paper system in 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 without any apparent cause, however the supply can be the paper in the transformer.
A visual inspection can also be really helpful to determine if any water may move into the transformer or electrical gear by way of leaks. This problem might be more extreme if the transformer or electrical tools is exterior and not in a coated space.
d) Acidity (mgKOH/g oil) Neutralization Number
Acidity mgKOH/g oilO, A, D<0.a hundred.10 to 0.15>0.15
B, E<0.100.10 to zero.25>0.20
C<0.a hundred.15 to 0.30>0.30
F, G Not a routine take a look at
This is a routine take a look at for all lessons besides F and G
The acids in oils are shaped because 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’ll adversely have an result on the insulation properties of the oil and can increase paper degradation. If left untreated in transformers, this can result in sludge formation, often around the decrease components of the transformer core. The sludge will finally form a semi-solid substance that’s extraordinarily difficult to remove.
If the result’s GOOD, the regular sampling interval can proceed.
In case of a FAIR result, the sampling interval must be decreased to fit the state of affairs. Future analysis should embody a visual inspection of the oil for sediment and sludge.
If the result’s POOR in accordance with the prescribed values in IEC 60422 Edition, the asset manager might determine to reclaim the oil or replace it with new or reclaimed oil, whichever option would possibly suit their requirements 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< hundred.10 to 0.50>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 courses of electrical gear, except F and G
The dielectric dissipation issue or tan delta of this take a look at supplies data regarding the extent of the dielectric losses in transformer oil. This take a look at measures the inefficiency of insulating materials.
When oil ages, we have the formation of polar compounds, leading to part displacement and dielectric losses. Other impurities which may influence the dissipation issue embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, more frequent sampling and checking extra parameters is recommended.
When the result is POOR, reclamation or an oil change is beneficial. The construction of the oil is damaged, 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 amongst the key parameters to assess the transformer insulation condition; this is primarily based on the truth that DC resistance is sensitive to grease degradation.
When the result’s FAIR, more frequent sampling and checking extra parameters is beneficial.
When the result is POOR, reclamation or an oil change is beneficial.
g) Inhibitor content material %
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of unique value<40% of original worth
This test is restricted to oils with this additive.
It would be advisable to contact the oil provider to confirm the major points relating to 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 prevent oxygen from reacting with the oil. This significantly slows the aging course of within the oil and the stable insulation.
If the result’s FAIR, it is suggested to high up the inhibitor level to the prescribed level per supplier directions. It is advised to make use of a area skilled trained in the process to perform this process.
If the outcome obtained is POOR, the advice for this situation would recommend that the tip person continues to make use of the oil “uninhibited,” but this will lead to extra speedy degradation of each the liquid and solid insulation.
It must be noted that some transformers have already got a built-in oil preservation system; this is designed to maintain dissolved oxygen at ranges beneath 1000ppm. This would be in the form 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 is not required, though it might add extra safety towards oil oxidation if the preservation system ever fails. [2]
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 decrease of >10mg/kg/year<50 and lowering at >10mg/kg/year
Passivators, also called metallic deactivators, react with reactive steel surfaces and dissolved metals such as copper and silver and scale back their fee 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 varieties, sulfur-based and nitrogen-based. The first suggested use of passivators in transformer oil, of which the creator is conscious, was in 1967 by J.J. Melchiore and I.W. Mills of the Sun Oil Company.[3]
As the oil ages, the passivator might deplete extra rapidly; this depletion may accelerate when the oil is un-inhibited.
With GOOD outcomes, common sample intervals may be maintained.
With FAIR results, keep common monitoring.
When POOR, it is advised to take away the oil or remove the source of corrosivity from the oil via special oil remedy.
I) Sediment and sludge
This just isn’t a routine take a look at.
It is suggested that this take a look at is carried out when the oil results point out a high acid value and the dissipation factor is near the unacceptable restrict.
The outcomes have to be less than zero.02% by mass to be negligible. If the results return a worth of greater than 0.02% by mass, it’s instructed that it be reclaimed; otherwise, an oil change is beneficial.
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 test
F, GNot Applicable
The interfacial tension between transformer oil and water reduces during the growing older course of. What this means in sensible terms is there’s more polar compound current in the oil, decreasing 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 rigidity becomes a quality criterion: the oil should be changed beneath a predefined restrict.
If results are GOOD, continue the regular sampling interval.
If outcomes are FAIR, lower the sampling interval.
If results are POOR, verify 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 effect on the transformer system may be vital. The extent of the corrosion damage caused by the sulfur could be so severe that it might cause failure of the equipment if not checked. The addition of a copper passivator can cut back the impact of this compound on the transformer system.
In a study 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 might contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the need of corrective actions based on this institute’s risk assessment study. [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 take a look at
If there is a maximum decrease in flashpoints by 10%, the gear might require additional inspection. This value would possibly differ in numerous countries.
It is advised to carry out this check when an uncommon odour is observed, the unit has been refilled, or an inner fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is not to determine the situation of the transformer; this can be a health and safety impression test. PCB is hazardous to each humans and the environment; it is important to test for PCBs after the retro fill of a transformer. It can be required each time any maintenance has been done on the unit, and the potential for contamination is present. If PCB content material exceeds the recommended limits, the suitable action must be taken.
Units with a PCB content material of more than 50ppm require a hearth safety plan, environmental safety plan, and additional precautionary measures when maintenance is done. This oil must be changed, and the oil disposed of as hazardous waste, with a certificates of secure disposal issued to the tools proprietor.
Local regulatory our bodies define the bounds.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of knowledge and interpretation, we are going to talk about this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this data according to worldwide requirements might be mentioned intimately, forming a part of the overall health score dedication of the transformer.
Transformer condition monitoring is an interlaced, extremely thrilling field of examine. In this text, we focused on the types of checks to discover out the situation 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 potential to make sure best apply application and optimised maintenance. It additionally make it easier to draw up a upkeep plan and motion plan.
1. IEC 60422 Edition 2013-01 International Standard (Mineral insulating oils in electrical tools – supervision and upkeep guide)
2. Oxidation inhibitor and reinhabiting oil-filled transformers, by Andy Shkolnik
three. Passivators, what they are and how 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
Author bio:
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ expertise within the industry, having previously labored as laboratory supervisor for a major industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry right from the start of her profession, notably within the analysis of test knowledge. Corné has vast practical and theoretical knowledge of reliability maintenance programmes.

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