Unleashing the Power of Transformer Oil Testing: The Impactful Significance in Maintenance
Nasir Anwar BE, MBA, AM Consultant
GM (Services)- ENERPEC-Engineering Business Consultant
Analysis of the insulating oil in high-voltage equipment like transformers and tap changers is a key part of managing assets. It enables massive amounts of data to be collected without invasive and unnecessary maintenance. The asset owner can save a lot of money if they can improve the reliability of their assets and help them make the right decisions about how to manage them.
When it comes to insulating oil analysis, expertise and experience are essential for reviewing results, and ENERPEC has several industry experts on hand to help our clients
Benefits of Transformer Oil Testing In Maintenance
International Players in Pakistan
Transformer Oil Tests
1) Dissolved Gas Analysis (DGA)- IEC 60567
The essential purpose of dissolved gas analysis is to detect gases generated due to electrical activity, or thermal degradation of insulation components within the transformer. The main goal of gas analysis is to catch faults like local overheating, discharge activity, or arcing and sparking in the transformer before they get worse.
DGA is one of the best methods to detect internal faults within power transformers
It is normal practice to measure nine gases in an oil sample. Nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, and the four hydrocarbon gases methane, ethane, ethylene, and acetylene.
The four hydrocarbon gases and carbon dioxide and carbon monoxide can be formed as a result of a thermal decomposition of oil generated either through the ageing of oil, electrical activity, or thermal activity within the unit. These, therefore, provide the opportunity for identifying the presence of faults or developing faults that give rise to heating in the transformer.
When the temperature changes, the relative amounts of the different gases produced change. This means that the relative concentration of the different gases can be used to find different types of faults. For each of the gases, the absolute value is considered, and then the ratio of various gases is looked at in order to give information on the nature and severity of a potential fault.
The laboratory can also offer the separation and monitoring of an extra fourteen gases (three carbon and four carbon isomers).
1.1) Rate of Change Analysis
Gas rate of change is a critical assessment technique to determine the amount of gas generation present per day. The DGA results are used and compared with the previous history to gain an understanding of fault progression and fault activity.
1.2) Stray Gas Analysis
Stray gassing is the gas generated as a result of the oil experiencing normal operating temperatures in the presence of sensitizers and in the absence of faults, during the lifetime of the transformer from its construction materials and their degradation products. This is not an “abnormal” condition; it does, however, quite often result in misinterpretation of diagnostic analysis.
Under normal operating conditions, very little gas should be generated if any, and in particular, the “high energy” gasses as considered by the industry (i.e. Ethylene, Acetylene).
Stray gassing is usually the result of material incompatibilities, sensitiser internal or external chemical contamination and/or poor curing timing of paints, varnishes and glues prior to filling the tank with oil during the manufacturing process.
2) Oil Quality (Routine / Standard)
This includes moisture, acidity, and electrical breakdown strength. The main purpose of these parameters is to tell you about the condition of the oil, but they also indirectly tell you about the condition of the transformer.
The moisture content is particularly relevant to transformer conditions. Maintaining acceptable oil quality is critical in preventing premature ageing of the transformer and therefore can assist in determining appropriate life-enhancing measures.
3) Furan Analysis (Paper and Pressboard Insulation Assessment)- IEC 61198
Furan detection has become accepted as a means of monitoring the degradation of paper insulation in transformers paper consists of cellulose molecules which have very long carbon chains.
As the paper ages the carbon chains are progressively broken into shorter lengths with the subsequent reduction in the mechanical properties of the paper.
Bi-products of the chain breakdown are organic compounds known as the furanic (furans) family.
3.1) Methanol and Ethanol Analysis
Similarly to the Furan analysis, the breakdown of the paper insulation within the unit can be assessed by Methanol and Ethanol Analysis as these are also bi-products of the chain breakdown.
3.2) Phenol and Cresol Analysis
Similarly, when pressboard degradation is taking place the resin material used to bind paper into the pressboard breaks down to produce Phenol and Cresol. Monitoring their concentration provides a means of assessing the degree of degradation.
This information is extremely useful in determining the condition of the paper insulation and therefore the transformer as a whole.
Generally, once the paper insulation has reached the end of its life the transformer can also be considered to be at the end of its life.
This information can assist in making informed decisions regarding the replacement strategy for transformers.
4) PCB Analysis- IEC 61619
The polychlorinated biphenyl (PCB) content of the oil is purely an environmental issue. The presence (at any level) of PCB has no significance for the condition or performance of a transformer, PCB is a very effective insulating liquid and is entirely compatible with oil.
Therefore, the significance of the measured levels is determined by the local environmental regulations. Polychlorinated Biphenyls were widely used as a fire retardant and insulator in the manufacture of transformers and capacitors.
This was due to their ability to withstand exceptionally high temperatures. Because of their classification as a human carcinogen, the Environmental Protection Agency (EPA) banned their use in 1979. It should be noted that PCB content in HV transformers is stable, and only one test is required on the oil.
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5) Dissipation Factor DDF (Power Factor)- IEC 60247
The Dissipation Factor measures the leakage current through the oil, which can assist in understanding the presence of contamination or deterioration of the transformer within the oil.
6) Interfacial Tension (IFT)- ASTM D971
The interfacial tension between oil and water provides a means of detecting soluble polar contaminants and products of degradation. This property changes fairly rapidly during the initial stages of ageing but levels off when deterioration is still moderate.
A rapid decrease in IFT may also be an indication of compatibility problems between the oil and some transformer materials (varnishes, gaskets), or of accidental contamination when filling with oil. However, oils with interfacial tension values at or near the lower limit value may or may not need to be investigated further, and is highly dependent on the Oil Quality results.
With overloaded transformers, the deterioration of materials is rapid and IFT is a tool for the detection of deterioration.
This test becomes more important the smaller the size of the equipment, with a limited amount of cooling. This is because of the energy loss it is converted in its entirety to heat. Since the energy loss is proportional to load overheating can reach high dangerous values for small equipment
7) Particle Assessment
The assessment of the particle sizes and volumes helps to understand the cleanliness of the oil to assist in remedial treatment requirements. The oil is sent through a particle counter, which will count the number of particles in pre-set size ranges and ultimately determine the ISO cleanliness code.
Particles in the oil affect other properties such as breakdown voltage and Power Factor.
8) Scanning Electron Microscopy & Energy Dispersive X-ray Analysis (EDAX)
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image.
The SEM is coupled with energy dispersive x-ray (EDX) spectroscopy to enable the detection analysis and quantification of elements with an atomic number 6 and above.
Wear materials can be found and assessed, as well as, their historical generation, thus understanding the process by which they have been produced. This gives clues to mechanical and/or chemical failures their prevention, or post-mortem understanding.
9) Viscosity - IEC 6186
Oil viscosity is an important controlling factor in the dissipation of heat. Ageing and oxidation of the oil tend to increase its viscosity. Viscosity is also affected by temperature, so in a cold climate it is important that the viscosity is sufficiently low to enable adequate oil circulation.
10) Density (Specific Gravity) - IEC 61868
The Density (Specific Gravity) of the oil is the ratio of the weights of equal volumes of oil and water. A high density indicates the oil's ability to suspend water and in extremely cold climates this can be used to determine whether ice will float on the oil potentially resulting in flashovers.
11) Colour & Appearance ASTM D1500
The Colour and Appearance of the oil indicate the level of contamination caused by the deterioration or ageing of the oil or insulating materials. The colour of the oil is compared to a specified spectrum of colours.
12) Inhibitor Content (DBPC) - IEC 60666
Inhibitors are added to the oil to replace the natural oxidation inhibitors. The inhibitors increase the resistance to the oxidation of the oil and assist in preventing degradation. As the oil is exposed to oxidation oil is protected by the inhibitors which are used up in the process.
Once the inhibitors are depleted the oxidation and degradation of the oil will occur at a faster rate. Monitoring the inhibitor content allows this degradation to be prevented by topping up the inhibitor as required before it reaches depletion. This has the effect of protecting paper degradation from the accelerating influence of acidic components generated from oil oxidation.
13) Corrosive Sulphur Assessment - IEC 62535
Copper in direct contact with the oil and at elevated temperatures, can cause a corrosive reaction. From this, the by-product copper sulphide will be formed, which will float in the oil and deposit between the windings. Copper Sulphide will reduce the electric strength of the insulation causing inter-turn faults.
14) Dibenzyl Disulfide (DBDS)
Dibenzyl Disulfide (DBDS) is one of several sulphur compounds known to cause copper corrosion in transformers under certain circumstances. Remedial processes, such as chemical additives known as passivation can be added to the oil to protect from copper sulphide formation. These additives are chemicals that are adsorbed onto the copper surface and stop the reactions between the sulphur-bearing chemicals within the oil and copper.
A more expensive remedial process involves oil change-out in order to reduce the concentration of DBDS, or sulphur-bearing chemicals capable of producing copper sulphide. in the oil. However, if not destroyed or removed below several mg/kg (ppm), the breakdown of the DBDS can still cause corrosion of the copper and the formation of copper sulphide.
15) Passivator Content
Passivator in Oil Whether in the oil originally or after it has been added, this test provides information on the amount of passivator present in the oil by detecting Irgamet 39 along with similar type products. This test does not determine if there is corrosive sulphur. Passivators are used to retard the formation of copper sulphide.
16) Fourier-transform Infrared Spectroscopy (FTIR)
Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.
This technique, coupled with microscopy becomes very powerful in assessing chemical changes on surfaces of solid materials. The acquisition of infrared microscopy has also enhanced our capability in forensic analysis.
Standards for DGA and Transformer Oil Analysis:
There are several standards available for DGA and transformer oil testing and analysis, including IEEE, IEC, and ASTM.
IEEE Standards:
IEEE provides several standards related to transformer oil testing and analysis, such as IEEE C57.104-2019, which covers the interpretation of DGA for transformers, and IEEE C57.106-2015, which covers a guide for the acceptance and maintenance of insulating oil in equipment.
IEC Standards
IEC also provides standards related to transformer oil testing and analysis, such as IEC 60599-2015, which covers the interpretation of DGA for transformers, and IEC 60422-2013, which covers the guide for sampling, testing, and diagnosis of mineral insulating oil in equipment.
ASTM Standards:
ASTM provides standards related to transformer oil testing and analysis, such as ASTM D3612-02(2015), which covers the test method for analysis of gases dissolved in electrical insulating oil by gas chromatography, and ASTM D5837-17, which covers the test method for Furanic Compounds in Electrical Insulating Liquids by High-Resolution Gas Chromatography and Spectrometry.
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