Step One - First Analysis:
DGA is use as a tool to identify incipient faults or problems in oil filled power transformers.
The use of DGA as a powerful diagnosis tool is a critical and important aspect of determining the condition and health of a transformer.
There are different methods for the interpretation of the results:
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Typical faults described in UNE EN 60599
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Gas ratios method, including Rogers Code
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Duval Triangle method
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And sometimes, Gas Pattern Method
-Sulfur contained in insulating oil may range from .001 to 0.5% - Forms ( in increasingorder of corrosivity)
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1. Thiophens
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2. Disulfides and poly sulfides
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3. Thio-esters
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4. Mercaptans
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5. Elemental sulfur
- Disulfides appear to be the most commonly occurring corrosivity causing compounds.
- DBDS has been identified as culprit in number of cases where failures have beenattributed to corrosive sulfur in oil.
- Can measure with a gas chromatograph with an elemental specific detector based onatomic absorption spectroscopy (GC-AED).
Triazoles (BZT&TT)
- Passivation is a term used in corrosion science to describe the formation of nonpermeableprotective layers on metal surfaces.
- The formation of such layers prevents corrosion and dissolution by decreasing theexchange currents between the electrolyte and the metal.
- Passivators - thus create a chemical layer that protects the copper from the oil, andthe oil from the copper.
- This prevents formation of oil soluble copper complexes that can catalyse oiloxidation.
- The protective layer thus fills two functions; to prevent direct copper transport andto prevent formation of corrosive oil oxidation products.
- In the electrical industry the organic passivators most commonly used arederivatives of benzotriazole (BTA). The methyl-substituted variant known astolytriazole (TTA) is slightly more oil soluble than BTA. But only when it is madesignificantly less hydrophilic by N-aminomethylation does it becomes fully misciblewith oils (One such example is Irgamet 39™ -CIBA Specialty Chemicals).
- Sulphur present refined as sulphur-containing is in oil sulphur molecules.
- The amount depends on oil refining processes, degree of refining and crude oiltype.
- Due to poor refining or contamination, reactive compounds giving corrosion atnormal operating temperatures may be present.
- They are detected by standard tests:
* ASTM 1275 Method B
* IEC 62535.
Furfural and Related Compounds (IEC 61198)
- Specifies test methods for the analysis of 2-furfural and related furan compoundsresulting from the degradation of cellulosic insulation and found in mineral insulatingoil samples taken from electrical equipment.
- The compounds are analysed by high performance liquid chromatography. Prior toHPLC analysis, furan derivatives are extracted from the oil by use of a suitable extraction method such as liquid-liquid extraction (method A) or solid-phaseseparation on a silica cartridge (method B).
Step Two- Secondary Analysis:
Oxidation Stability (IEC 61125)
- The ability of mineral electrical insulating oil to withstand oxidation underthermal stress and in the presence of oxygen and a copper catalyst is calledoxidation stability.
- It gives general information about the life expectancy of the oil under serviceconditions in electrical equipment.
- The property is defined as resistance to formation of acidic compounds, sludgeand compounds influencing the Dielectric Dissipation Factor (DDF) under givenconditions.
- For oils complying with IEC 60296, these conditions are detailed in IEC 61125method C and the limits acceptable performance in IEC 60296.
Sediment &Sludges
- Sediment insoluble material present in the oil - is oil. Sediment includes:
Insoluble oxidation or degradation products of solid or liquid insulating materials;
Solid products arising from the conditions of service of the equipment: carbon, metal, metallic oxides;
Fibres, and other foreign matter, of diverse origins.
- Sludge is a polymerised degradation product of solid and liquid insulating material.Sludge is soluble in oil up to a certain limit, depending on the oil solubilitycharacteristics and temperature.
- At sludge levels above this, the sludge is precipitated, contributing an additionalcomponent to the sediment. The presence of sediment and/or sludge may change theelectrical properties of the oil, and in addition, deposits may hinder heat-exchange,thus encouraging thermal degradation of the insulating materials.
- Sediment and sludge should be measured according to the method described in IEC 61125 Method C without submitting the oil sample to the oxidation process, by filtration of the oil for sediment content and by adding n-heptane and filtration of sludge.
Interfacial Tension (ASTM D 971)
- The interfacial tension between oil and water provides a means of detectingsoluble polar contaminants and products of degradation.
- This characteristic changes fairly rapidly during the initial stages of ageing butlevels off when deterioration is still moderate.
- A rapid decrease of IFT may also be an indication of compatibility problemsbetween the oil and some transformer materials (varnishes, gaskets), or of anaccidental contamination when filling with oil.
Flash Point (ISO 2719)
- Breakdown of the oil caused by electrical discharges or prolonged exposure tovery high temperatures may produce sufficient quantities of low molecularweight hydrocarbons to cause a lowering of the flash-point of the oil.
- A low flash-point may be an indication of the presence of volatile combustionproducts in the oil.
- This may result from contamination by a solvent but, in some cases, the causehas been observed to the extensive sparking discharges.
Compatibility
- Oil complying with IEC 60296 and the same type shall be used for topping upand/or refilling.
- Unused oils conforming to IEC and containing the same or no additivesare considered to be compatible with each other and can be mixed in anyproportion.
- Field experience indicates that problems are not normally encountered whenunused oils is added in small percentage, e.g. Less than 5%, to used oils classifiedas "good" though larger additions to heavily aged oil may cause sludge toprecipitate.
Pour Point (ISO 3016)
- Pour point is a measure of the ability of the oil to flow at low temperature.
- There is no evidence to suggest that this property is affected by normal oildeterioration.
- Changes in pour point can normally be interpreted as the result of topping-upwith a different type of oil.
Density (ISO 3675)
- Density may be useful for type identification.
- In cold climates, the density of the oil may be importantin determining its suitability for use.
- For example, ice crystals formed from separated watermay float on oil of high density and lead to flashover onsubsequent melting.
- However, density is not significant in comparing thequality of different samples of oil.
- There is no evidence that density is affected by normal oildeterioration.
Viscosity (ISO 3104)
- It is an important controlling factor in the dissipation of heat.
- Ageing and oxidation of the oil tend to increase viscosity.
- Viscosity is also affected by temperature.
- Normal ageing and oxidation of the oil will not significantly affect its viscosity.
- Only under extreme conditions of corona discharges or oxidation may this occur.
PCB (IEC 61619)
- Polychlorinated biphenyls (PCB) area a family of synthetic chlorinated aromatichydrocarbons, which have good thermal and electrical properties.
- These properties combined with excellent chemical stability made them useful innumerous commercial applications.
- However, their chemical stability and resistance to biodegradation has given causefor concern in term of environmental pollution.
- This increasing concern over the environmental impact of PCBs has progressivelyrestricted their use since the early 1970´s and their use in new plants andequipment was banned by international agreement in 1986.
- The PCB content of oil in new equipment should be measured to confirm that theoil is PCB free.
- Thereafter, whenever there is a risk of potential contamination (oil treatment,transformer repairs, etc.).
Filtergraphy (OSP)
Metals AES-ICP (OSP)
The ICP measures the wear and contamination metals present in the oil/paper: Cu, Fe, Al, Ag, Sn, Pb, Zn, Si, S
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