TALKING TRANSFORMER TESTING
by Brenna ShumbamhiniJune 9, 2022
Condition monitoring is the frequent accumulating, measuring, recording, and evaluation of the relevant information concerning an asset’s operation. If we interpret the info correctly, it can give us great insight into the asset’s condition.
Frequent monitoring of the asset may end up in less upkeep required or more prolonged periods with none upkeep required.
It is essential to determine the important thing parameters which may be wanted to give us a whole picture of the particular status of the transformer and the motion we have to take to make sure the continued reliability of the asset to realize the utmost lifetime.
What is the info telling us?
Has the condition of the unit changed because the final upkeep period?
Is it secure to function the unit?
Are there signs of deterioration?
Is it secure to load the unit above the nameplate rating for a selected period?
Are we required to implement motion to make sure the continued reliability of the unit?
How long can we use the unit earlier than we have to think about replacement?
Are the recognized issues of a recurring nature?
Effective condition monitoring outline
It is vitally essential to determine clear goals as part of your technique. What do you want to obtain by implementing this situation 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 will be a lot easier to identify the required parameters.
Health indexing of belongings is turning into a outstanding software 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 within the varied requirements.
The scope of oil analysis, interpretation of the information, and important values
At the start of this part, it is important to state that we take care of different measurement transformers within the trade. Transformers are divided into courses in accordance with the kV rankings of the gear. It is up to the reliability or asset manager to make use of the rules for bigger gear, thereby implementing shorter increment oil evaluation and electrical exams.
The upkeep engineer or manager wants to find out what sort of testing would benefit him in figuring out problem areas inside his fleet. Some of the analysis has been identified as to routine kind tests. Still, there may be an extensive range of exams that may assist in figuring out specific downside criteria inside the system, which could not be clear via the everyday day-to-day evaluation usually carried out.
Please see the score courses in TABLE 1
TABLE 1 FIG
TABLE 2 explains which oil analyses are recommended and how usually or under which circumstances.
TABLE 2 FIG
The taking of the oil sample is likely certainly one of the most important and significant influencers in the analysis outcome. If a sample is not taken to the prescribed process, then there is a important possibility that the evaluation carried out, and the outcomes acquired, are of no use.
Care should be taken to ensure high quality management procedures are applied in every step of the sampling process as set out by international standards. A good high quality sample taken by making use of the correct process is essential. A pattern could be contaminated by numerous components, all of which may affect the end result of the ends in a negative method.
All steps concerned in sample taking should adhere to high quality control procedures, together with the container that’s used, the sampling equipment, the taking of the sample, the marking of the sample, the closing of the sample, the storage and handling of the pattern, after which the delivery of the pattern to the laboratory.
If the sample identification and sample information are handwritten, the human factor can result in incorrect interpretation of the info. 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 put in writing on the label should be oil- and waterproof. Otherwise, some knowledge could be misplaced, making it extraordinarily difficult for the laboratory personnel to complete the report or allocate the sample to a particular transformer.
Now, let’s discuss the types of checks to determine the transformer’s condition, the important values, and the recommended actions in each case:
a) Colour and look
This is a routine inspection applied to each oil sample.
When an oil pattern arrives at the laboratory, one of the “tests” is a visual inspection of the oil pattern 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 is plenty of turbidity, it would indicate a excessive water content within the oil.
If the drain valve was not cleaned by the sampler, the dirt 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 confirm if that’s so.
Clear oils without contamination will indicate an excellent situation, and no action is recommended.
When oils are dark or turbid, additional evaluation will verify any issues. The oil analysis results will also decide the degree and sort 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 level application<40 kV for OLTC in delta or line-end software
This is a routine inspection.
Breakdown voltage will indicate the water content or the presence of international 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 have to be excessive.
If the values are Good, it is recommended to continue with the current pattern interval action plan.
If the values are Fair, extra frequent sampling is recommended in collaboration with different parameter outcomes just 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 different tests point out severe growing older, the oil may be changed with new or reclaimed oil. Another possibility could be to perform on-site oil reclamation utilizing a reclamation plant.
Reclamation of oil has the advantage that the colour of the oil is restored, and the polar elements are faraway from the oil. This course of removes acid and water as nicely as another compounds. Another advantage is that the oil could be re-used, and in most conditions, this can be accomplished with out switching off the unit, which contributes to cost-saving. If doubtful – instead swap off the unit during this therapy course of.
If the values are Poor, it is advisable to take action as quickly as potential and never delay the upkeep course of. Excess water within the transformer system decreases the projected transformer lifetime considerably; extraordinarily excessive water content material could cause flashover within the unit, resulting in loss 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 check
This is a routine take a look at for all lessons of electrical tools, besides class G
The outcomes of this test ought to all the time be thought-about at the side of the breakdown power. If it is discovered that the water content is high and the breakdown strength is low, further motion needs to be taken. It is really helpful that a second sample from the same unit is tested to confirm the results.
In the case of switching gear, where there isn’t a paper current, the breakdown voltage is the figuring out issue.
It ought to be famous that the boundaries indicated by IEC 60422 Edition four apply to transformers with working temperatures between forty and 70 °C. If it is discovered that the unit’s working temperature is outside this temperature vary, it is best to refer to Annex A of the standard.
When the worth obtained through analyses is GOOD, the traditional sampling interval could be maintained, requiring no additional action.
When the value returns a FAIR result, more frequent sampling is really helpful. It can additionally be useful to suppose about different parameters like the breakdown voltage, particle content material and DDF/resistivity, and acidity to decide on the motion to be implemented.
pressure gauge วัด แรง ดัน will require quick action from the asset supervisor. This would possibly include taking one other pattern to confirm the results from the primary evaluation. If it’s confirmed that the water content is high, the oil could be filtered; this course of ought to take away a big portion of the moisture from the oil if utilized appropriately. Follow-up samples must be taken to make sure that the moisture content remains to be throughout the required limits. The reason is that probably 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 beneath circumstances that favour this motion. It might be found later that the oil within the water has increased once more with none apparent cause, however the supply would be the paper in the transformer.
A visible inspection can be really helpful to find out if any water may move into the transformer or electrical equipment through leaks. This downside may be more severe if the transformer or electrical tools is outdoors and not in a coated area.
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.one 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 test for all classes except F and G
The acids in oils are shaped due to chemical reactions between the oil, water, and paper. Higher temperatures or load increases will assist in the formation of the acids. Because acids are polar compounds, it’s going to adversely affect the insulation properties of the oil and can enhance paper degradation. If left untreated in transformers, this will lead to sludge formation, often across the decrease parts of the transformer core. The sludge will eventually form a semi-solid substance that’s extremely tough to take away.
If the result is GOOD, the common sampling interval can continue.
In case of a FAIR result, the sampling interval should be decreased to fit the state of affairs. Future evaluation should include a visual inspection of the oil for sediment and sludge.
If the result’s POOR based on the prescribed values in IEC 60422 Edition four.zero, the asset manager might resolve to reclaim the oil or substitute it with new or reclaimed oil, whichever choice would possibly swimsuit their necessities one of the best.
e) Dielectric Dissipation factor 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 zero.30>0.03
F, GNot a routine check
This is a routine test for all classes of electrical equipment, besides F and G
The dielectric dissipation issue or tan delta of this check offers info 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, leading to part displacement and dielectric losses. Other impurities which may affect the dissipation issue embrace water, dissolved insulating resin, and paper.
When the result’s FAIR, extra frequent sampling and checking additional parameters is beneficial.
When the result’s POOR, reclamation or an oil change is beneficial. The structure of the oil is damaged, in impact that the chemical bonds between the molecules have broken 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 take a look at
DC resistivity of the oil is probably considered one of the key parameters to assess the transformer insulation situation; this is based on the reality that DC resistance is sensitive to grease degradation.
When the result is FAIR, extra frequent sampling and checking further parameters is recommended.
When the result’s POOR, reclamation or an oil change is really helpful.
g) Inhibitor content percent
Inhibitor Content %AllRestricted to Inhibited oils,Consult oil supplier40% to 60% Of original value<40% of original value
This check is restricted to oils with this additive.
It can be advisable to contact the oil supplier to confirm the details relating to components.
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 function of the inhibitor is to forestall oxygen from reacting with the oil. This considerably slows the growing older process within the oil and the stable insulation.
If the result is FAIR, it’s suggested to top up the inhibitor stage to the prescribed stage per provider instructions. It is suggested to use a area skilled educated in the procedure to carry out this activity.
If the result obtained is POOR, the advice for this state of affairs would recommend that the tip person continues to make use of the oil “uninhibited,” however this will lead to more fast degradation of each the liquid and strong insulation.
It ought to be famous that some transformers already have a built-in oil preservation system; that 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 geared up with a rubber diaphragm (bladder). Using inhibited oils beneath these circumstances isn’t required, though it might add further protection towards oil oxidation if the preservation system ever fails. 
h) Passivator content material
Passivator Content (mg/kg)O,A,B,C,D,E,F.>70 and secure, (rate of lower < 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 referred to as steel deactivators, react with reactive metal surfaces and dissolved metals similar to copper and silver and reduce their fee of reaction with compounds in the oil. This consists of oxidation reactions with organic compounds and reactions with corrosive sulfur. Passivators are composed of two primary 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.
As the oil ages, the passivator may deplete extra rapidly; this depletion might speed up when the oil is un-inhibited.
With GOOD results, common sample intervals can be maintained.
With FAIR outcomes, maintain regular monitoring.
When POOR, it’s advised to remove 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 check is performed when the oil outcomes point out a high acid value and the dissipation factor is close to the unacceptable restrict.
The results must be lower than 0.02% by mass to be negligible. If the results return a price of greater than zero.02% by mass, it’s suggested that it be reclaimed; otherwise, an oil change is beneficial.
j) Interfacial pressure
This isn’t a routine test
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 during the getting older course of. What this implies in practical phrases is there is extra polar compound current within the oil, decreasing the flexibility of an oil to function an insulator within the transformer system. There is a direct correlation between interfacial tension and neutralisation number. Therefore, the interfacial pressure becomes a top quality criterion: the oil should be changed below a predefined restrict.
If results are GOOD, proceed the common sampling interval.
If outcomes are FAIR, decrease the sampling interval.
If outcomes are POOR, verify the oil for sediment and/or sludge.
k) Corrosive sulfur
This is not a routine check.
Oil is either corrosive or non-corrosive.
The presence of corrosive sulfur in transformer oil and its effect on the transformer system can be important. The extent of the corrosion harm attributable to the sulfur could be so severe that it would cause failure of the equipment if not checked. The addition of a copper passivator can cut back the influence of this compound on the transformer system.
In a research by Doble, it has been discovered that rubber merchandise used in transformers may add to the corrosivity of the oil. Nitrile rubber gaskets and hoses not permitted for oil filtration may contaminate the oil with corrosive sulfur.
CIGRE Brochure no 378, 2009 stipulates the necessity of corrective actions based on this institute’s threat assessment study. 
l) Particle counting and sizing
Table 3: Particles
Table B.1 – Typical contamination ranges (particles) encountered on energy transformer insulating oil as measured using IEC 60970 
m) Flashpoint ° C
Not a routine check
If there’s a most decrease in flashpoints by 10%, the tools may require additional inspection. This worth might differ in several countries.
It is suggested to perform this test when an unusual odour is seen, the unit has been refilled, or an internal fault has occurred.
n) PCB (Polychlorinated Biphenyls)
This test is to not decide the situation of the transformer; this is a well being and safety influence test. PCB is hazardous to each people and the surroundings; it is important to test for PCBs after the retro fill of a transformer. It is also required whenever any upkeep has been carried out on the unit, and the risk of contamination is present. If PCB content exceeds the beneficial limits, the suitable motion must be taken.
Units with a PCB content of greater than 50ppm require a fire security plan, environmental protection plan, and further precautionary measures when upkeep is done. This oil needs to be changed, and the oil disposed of as hazardous waste, with a certificate of secure disposal issued to the tools owner.
Local regulatory bodies outline the boundaries.
o) DGA (Dissolved Gas Analysis)
As DGA is an intricate science with lots of data and interpretation, we’ll focus on this phenomenon partially II of the article. The limits for the totally different gases and the interpretation of this knowledge based on international requirements shall be discussed in detail, forming part of the general well being score determination of the transformer.
Transformer condition monitoring is an interlaced, extremely thrilling field of study. In this article, we targeted on the types of tests to discover out the condition of the transformer, the crucial values, and the recommended 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 possible to ensure best follow utility and optimised maintenance. It also make it easier to attract up a maintenance plan and motion plan.
1. IEC 60422 Edition 4.0 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
three. Passivators, what they are 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 power.”
6. Article initially revealed by Transformer Technology Magazine Issue 16 – December 2021 https://www.transformer-technology.com
Corné Dames is a WearCheck transformer advisor. She has 20+ years’ experience in the business, having previously worked as laboratory supervisor for a significant industrial laboratory group, focusing on transformer health. She has been intrigued by transformer chemistry proper from the start of her career, significantly in the evaluation of test data. Corné has vast sensible and theoretical information of reliability maintenance programmes.