The appearance of oil may show cloudiness or sediments which may indicate the presence of free water, insoluble sludge, carbon, fibres, dirt, etc.
The breakdown voltage is of importance as a measure of the suitability of an oil to withstand electric stress. Dry and clean oil exhibits an inherently high breakdown voltage. Free water and solid particles, the latter particularly in combination with high levels of dissolved water, tend to migrate to regions of high electric stress and reduce breakdown voltage dramatically. The measurement of breakdown voltage, therefore serves primarily to indicate the presence of contaminants such as water or conducting particles, one or more of which can be present when low breakdown voltage values are found by test. However, a high breakdown voltage does not indicate the absence of contaminants.
Water may originate from the atmosphere or be produced by the deterioration of the insulating materials. At comparatively low water content, the water remains in solution and does not alter the appearance of the oil. Dissolved water shall therefore be detected by chemical or physical methods. Dissolved water may or may not affect the electrical properties of the oil.
The solubility of water in transformer oil increases with increasing temperature and neutralization value. Above a certain water content (i.e. the saturation water content), all the water cannot remain in solution and free water may be seen in the form of cloudiness or water droplets. Free water invariably results in decreased dielectric strength and resistivity and increased dielectric dissipation factor. In a transformer, the total water content is distributed between the paper and the oil in a ratio that is predominantly in favour of the paper. Small changes in temperature significantly modify the water content of the oil but only slightly that of paper.
The neutralization value of oil is a measure of the acidic constituents or contaminants in the oil. Its value, negligible in an unused oil, increases as a result of oxidative ageing and is used as a general guide for determining when an oil should be replaced or reclaimed, provided suitable rejection limits have been established and confirmation is received from other tests.
Sediment and Sludge
This test distinguishes between sediment and total sludge that is oil insoluble sludge plus sludge which is precipitated by adding heptanes. Solid matter comprises insoluble oxidation or degradation products of insulating materials, fibres of various origins, carbon, metallic oxides, etc, arising from the conditions of service of the equipment. The presence of solid particles may reduce the electric strength of the oil and, in addition, deposits may hinder heat transfer, thus promoting further deterioration of insulation. Sludge consists of products formed at an advanced stage of oxidation and is forewarning of sludge deposits in the equipment.
Dielectric Dissipation Factor (DDF) and Resistivity
These characteristics are very sensitive to the presence in the oil of soluble polar contaminants, ageing products or colloids. Changes may be motivated even when contamination is so slight, so as to be undetectable by chemical methods. Acceptable limits for these characteristics depend largely upon the type of apparatus and application.
However, high values of dissipation factor may influence the power factor and/or the insulation resistance of transformer windings. There is generally a relationship between DDF and resistivity at elevated temperature with resistivity decreasing as DDF increases. It is normally not required to conduct both tests on the same oil.
Useful additional information can be obtained by measuring resistivity or DDF at both ambient and at higher temperature such as 90 °C. A satisfactory result at 90 °C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products perceptible in the cold, but generally at a tolerable level. Unsatisfactory results at both temperatures indicate a greater extent of contamination and that it may not be possible to restore the oil to a satisfactory level, by reconditioning.
The interfacial tension between oil and water provides a means of detecting soluble polar contaminants and products of deterioration. This characteristics changes fairly rapidly during the initial stages of ageing but levels off when deterioration is still moderate. For this reason, results are rather difficult to interpret in terms of oil maintenance.
A low flash point is an indication of the presence of volatile combustible products in the oil. Prolonged exposure of the oil to very high temperature under fault conditions may produce sufficient quantities of low molecular weight hydrocarbons to cause a lowering of the flash point of the oil.
Pour point is a measure of the ability of the oil to flow at low temperature. There is no evidence to suggest that the property is affected by oil deterioration. Changes in pour point may normally be interpreted as the result of topping-up with a different type of oil.
Density is not significant in determining the quality of oil but may be useful for type identification or to suggest marked compositional changes. In cold climates density may be pertinent in determining the suitability for use, for examples ice crystals formed from separated water, may float on oil of high density and lead to flash over on subsequent melting.
Viscosity is a controlling factor in the dissipation of heat. Ageing and oxidation of the oil tend to increase viscosity but the effect is not discernible at the deterioration levels considered in this guide. Viscosity measurements may be useful for oil type identification.
Dissolved Gas Analysis (DGA)
Mineral insulating oils are made of a blend of different hydrocarbon molecules containing CH3, CH2 and CH chemical groups linked together by carbon-carbon molecular bonds. Scission of some of the C-H and C-C bonds may occur as a result of electrical and thermal faults, with the formation of small unstable fragments, in radical or ionic form, such as H*, CH~, CH~, CH* or C* (among many other more complex forms), which recombine rapidly, through complex reactions, into gas molecules such as hydrogen (H-H), methane (CH3-H), ethane (CH3-CH3), ethylene (CH2 = CH2) or acetylene (-CH = CH). C3 and C4 hydrocarbon gases, as well as solid particles of carbon and hydrocarbon polymers (X-wax), are other possible recombination products.
The gases formed then dissolve in oil, or accumulate as free gases, if produced rapidly in large quantities. Low-energy faults, such as partial discharges of the cold plasma type (corona discharges), favour the scission of the weakest C-H bonds through ionization reactions and the accumulation of hydrogen as the main recombination gas. More and more energy and/or higher temperatures are needed for the scission of the C-C bonds and their recombination into gases with a C-C single bond, C-C double bond or C-C triple bond, following processes bearing some similarities with those observed in the petroleum oil-cracking industry.
Ethylene is thus favoured over ethane and methane above temperatures of approximately 500 °C (although still present in lower quantities below). Acetylene requires temperatures of at least 800 °C to 1200 °C, and a rapid quenching to lower temperatures, in order to accumulate as a stable recombination product. Acetylene is thus formed in significant quantities mainly in arcs, where the conductive ionized channel is at several thousands of °C, and the interface with the surrounding liquid oil necessarily below 400 °C (above which oil vaporizes completely), with a layer of oil vapour / decomposition gases in between.
Acetylene may still be formed at lower temperatures (< 800 °C), but in very minor quantities. Carbon particles form at 500 °C to 800 °C and are indeed observed after arcing in oil or around very hot spots. Analysis of DGA is well established and described in national / international standards. However, interpretation of DGA results is significant and needs expertise.
Furan is a chemical family. Atleast five types of furan are produced due to ageing of paper and they get dissolved in oil. The lower the furan content, indicates lesser deterioration of cellulosic paper. For this test, dissolved furans are extracted from the oil then analysed by HPLC. This test is a good indication of remnant life of cellulosic insulation in the transformer.
Degree of Polymerisation
The degree of polymerization test is conducted on cellulosic paper insulation. The average number of monomer (cellulose molecule, C6 H10 O5) in the cellulose molecule is known as degree of polymerisation of paper. Within a sample of paper, not all the cellulose molecules have the same degree of polymerization so that the mean value measured by viscometric methods is not necessarily the same as that which may be obtained by other methods. Normally new paper can have DP value more than 1000 which decreases with ageing.