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Deterioration of Insulating Oil
Effect of Oxygen on Oil
Moisture contamination is one of the most common causes of deterioration in the insulating quality of oil. This contamination can be eliminated by purification. A less rapid but more serious characteristic deterioration, the formation of acids and sludge, is caused by oxidation. Thus, the exclusion of oxygen is of prime importance. In free-breathing transformers, the oxygen supply is virtually unlimited and oxidative deterioration is consequently faster than in sealed transformers. Atmospheric oxygen is not the only source of oxygen available for the oxidation of insulating oils; water also serves as a source of oxygen and, therefore, leaky gaskets constitute a very real hazard due to both oxidation and moisture contamination. The rate of oxidation also depends on the temperate of the oil; the higher the temperature, the faster the oxidative breakdown. This fact points to the importance of avoiding overloading of transformers, especially in the summertime.

Oxidation results in :
(a) the formation of acids in the insulating oil, and
(b) the formation of sludge.

Moisture in Oil

Water can be present in oil :
(a) in a dissolved form,
(b) as tiny droplets mixed with the oil (emulsion), or
(c) in a free state at the bottom of the container holding the oil.

Demulsification occurs when the tiny droplets unite to form larger drops which sink to the bottom and form a pool of free water. Emulsified water or water in the free state may be readily removed by filtering or centrifugal treatment; the filtration process can partially remove dissolved water if the filter papers are thoroughly dried before filtration and are replaced frequently.
Effect of temperature on moisture
The amount of moisture which can be dissolved in oil increases rapidly as the oil temperature increases. Therefore, an insulating oil purified at too high a temperature may lose a large percentage of its dielectric strength on cooling because the dissolved moisture is then changed to an emulsion.
Oil Deterioration in Transformers
In transformers, sludge sticks to the surfaces through which heat should be dissipated; the sludge forms a blanket barrier to the flow of heat from the oil to the coolant and from the core and coils to the cool oil. If allowed to continue long enough, the sludge may even block off the flow of oil through the cooling ducts. As a result, the transformer insulation gets too hot and is damaged, particularly between turns of the windings. Deterioration of the turns insulation may eventually lead to short circuits between turns and the breakdown of the transformer. When oxidation progresses to the point where sludge is being precipitated, the first step should be to remove the sludge from the transformer by a high-pressure stream of oil and to either replace the sludged oil or treat it with activated clay to remove the acid and sludge precursors. Complete treatment of the oil is normally less costly than replacing it with new oil.
WATER CONTENT
The water content of an insulating fluid in high voltage devices can adversely affect the physical, chemical, and electrical properties of the fluid. Water and oil are not mutually soluble in each other due to their large difference in polarity. Mineral oil is essentially non-polar while water is highly polar. However, up to a certain limit a small amount of water will dissolve in the oil. This limit is a function of the temperature of the system and the solubility of water in oil increases exponentially with the temperature. Some typical values for the solubility of water in oil are 52, 82, 132, and 206 ppm at 20, 30, 40, and 50 oC respectively. When the amount of water present exceeds the solubility limit of the oil, a separation into two layers occurs. Since the density of water is greater than that of the oil, the water will separate out at the bottom of the unit and appear as free water. The oil above this free water will be saturated with respect to dissolved water. The above values relate to the saturation values for new oil; however, as the oil becomes oxidized in service, the more polar oxidation products now in the oil allow more water to be dissolved in the oil.

PCB TESTING
Polychlorinated Biphenyls (PCBs) have been recognized by the EPA as an environmental hazard. The EPA has concluded that PCBs are toxic and persistent. Once released into the environment, PCBs do not readily break down. Instead, they may accumulate in the environment and have the potential to migrate through the food chain.
PCBs are produced by the chlorination of "biphenyl." One to ten hydrogen atoms of biphenyl can be replaced with chlorine atoms. Given all the possible arrangements of chlorine atoms, there are 209 compounds that are classed as chlorinated biphenyls.
As insulating fluids in electrical equipment, Aroclors were seldom used in pure form, but were frequently mixed with fluids such as trichlorobenzene or tetrachlorobenzene. These Aroclor-fluid mixtures are generically called Askarels. Askarels means most of Anti Oxidant used in transformer oil
I. LUBRICATING OIL CHARACTERISTICS
All lubricating oil products are made by blending and compounding base oils and additives together to form a specific lubricant. However, in some cases the product can consist of base oil only, without additive, usually referred to as mineral oil.

A. Base Oil
Base oils have inherent characteristics, depending on the crude oil used and the refining method. These base oil characteristics are very important in determining the quality of the finished product.
1. Base Oil Characteristics

a) Viscosity
Viscosity is the most important characteristic of almost any lubrication product. It is the measure of the fluidity (flowability) at definite temperatures. If the viscosity is too thin, the lubricant film will be squeezed out from between the moving metal surfaces allowing them to come into contact. If the viscosity is too thick, it will not travel into the small areas where it is needed. It will require excessive pumping force, causing undue wear on pumps and excessive heat built-up, and it will not permit easy cranking of any engine.
Viscosity of Base Oils is most commonly stated in terms of Saybolt Viscosity. This state the time in seconds it takes 60 milliliters of oil to flow through a small diameter tube at a certain temperature. This is expressed in Saybolt Universal Seconds (SUS) at either 37.70C (1000F) or 98.80C (2100F), such as 200 SUS @ 37.70C or 45 SUS @ 98.80C.
The metric system expresses viscosity in centistokes (cSt) or in SI units (mm 2/s) at Celsius temperature. This will, no doubt become the standard in the near future but, Saybolt Viscosity is still the most widely used system. The exception in the measurement of oil viscosity is at low temperatures. In this case, a “Cold Crank Simulator” ‘CCS’ is used to determine the viscosity which is usually reported in centipoises at -10 to -350C.
b) Viscosity Index
All lubricants change viscosity with temperature change, that is, lubricants become thinner as temperature increases and thicker as temperature decreases. Oil that was 100 cSt @ 400C will have a lower viscosity at 1000C and still lower viscosity again at 1500C. Different type of oils makes this viscosity change at varying rates. This rate of change is stated as “VISCOSITY INDEX”. In short, the oil is said to be of a certain “VI”. The Viscosity Index scale is an entirely arbitrary one. By measuring the amount of VI change from 400C to 1000C, an oils VI is determined. When the scale was established, the very best oil (the one that changed the least) was assigned the value of 100. While the oil that changed the most was given the value of 0. It was thought that all other oils would fall between these two limits. However today with improved refining techniques and “VI Improver Chemistry” It is now possible to make oils considerably above 100 VI.
It is worth nothing that, the viscosity index of base oil is directly related to the type of crude oil and the refining methods used. In general, the lower VI base oils will be from 15 to 30 VI, intermediate VI, from 30 to 85; and high VI from 85 to 100 VI.
c) Flash Point
The flash point is the temperature at which approximately 70ml of oil will “flash” when exposed to an open flame. This can be anywhere from 1320C to 3270C. This is usually an indicator to the volatility of the oil and is a very important factor in engine oils and their consumption rate.
d) Pour Point
The pout point is the lowest temperature at which the oil will pour. This is, of course, very important for engine oils and other lubricants operating at low temperature and extremely low temperatures. The pour point is directly related to the type of crude used and its wax content.
Other characteristics
There are other characteristics of base oils such as gravity, color, carbon demulsibility etc. These all comprise the physical specifications that are considered when using a certain base oil to make a certain lubricant, but the characteristics mentioned above however, are the most important.
2. Types of Base Oil

Base oils are mainly categorized by the type of crude: Paraffinic Crude Naphthenic Crude, and Mixed Crude.

a) Paraffinic Crude is the most important crude in the manufacture of lubricants. They have a certain amount of wax content which is extracted and sold to the wax market. But some waxy content in the oil is good for lubrication purposes and is left behind the base oil. Most paraffin crude’s

come from the Mid-Continent region or Pennsylvania region. Most lubricants come from either the Pennsylvania crude or the Mid-Continent Crude.

The American Petroleum Institute (API) classifies base oils using the following characteristics:

• Viscosity Index

• Saturate Level

• Sulfur Content

The three API categories for base oils are :

b) Naphthenic Crude’s usually come from Arkansas or the coastal areas of Texas and California The lube cuts from this latter area are sometimes called “coastal “ or “pale” oils because they are somewhat lighter in color than Arkansas oils.

AMERICAN PETROLEUM INSTITUTE PARAFFINIC BASE STOCK GROUPS Requirements :
API group     Sulfur, % wt     Saturates,% wt.    Viscosity Index
I     > 0.03     <90     80-120
II     <0.03     >90     80-120
III     <0.03     >90     >120