La inclusión del Trastorno por Estrés Postraumático en el DSM

3.1 Physical Tests :

(a) Density : The density of an oil is defined as the ratio of the weight of a given volume of the oil to the weight of the same volume of water. The test can be conducted either by using a hydrometer or a density bottle if greater accuracy is desired. The specific gravity is used extensively to facilitate volume to weight conversions, especially while conducting oxidation stability, neutralisation number tests etc.

The transformer heat co-efficient is determined by the physical properties of the coolant, density, specific heat. thermal conductivity and viscosity in addition to other variable factors.

The density of transformer oil usually varies within a fairly narrow range, of 0.85 to 0.89. Higher density of oil results in higher viscosity which in turn affects the heat dissipation characteristics of the oil. Further, the prescribed value of the density (0.85-0.89) also facilitates easy separation into distinct layers of free oil and oil with dissolved moisture.

(b) Viscosity : The viscosity of an oil is a measure of its resistance to continuous flow without the effect of external forces. It is expressed as the kinematic viscosity (unit being a stoke).

Viscosity is measured by determining the time required for a fixed volume of liquid to flow through a long capillary tube, e.g., Ostwald U-type Viscometer.

The oil must be mobile as heat transformer in transformers occurs mainly by convection currents. A low viscosity is therefore, advantageous. The oil viscosity may have marked effect on the dielectric losses generated under A.C. voltages at the higher frequencies.

It is important that at low temperatures the viscosity of transformer oil be as low as possible. In other words, the curve characterizing the temperature dependence of oil viscosity should be as fault as possible. Otherwise, with a high viscosity in the cooled transformer, the removal of heat from its windings will be difficult in the initial period after energizing, leading to over eating. An increase in the viscosity of oil in transformer tap exchanger and oil circuit breakers impedes the movement of the moing pars of the apparatus and thus impairs normal operation.

In view of this, the British standard has prescribed viscosity - temperature characteristics for the oils used for transformers and switchgear characteristics for the oils used for transformers and switchgear. The Indian Standard IS : 335 (1973) prescribes the viscosity value only at ambient (the value at 40 degree is under consideration). It is, therefore, considered necessary that incorporation of temperature characteristics in line with British Standard should be explored for indigenous insulating oils also.

(c) Pour Point : The pour point may be defined as the lowest temperature at which the oil will just flow under the prescribed conditions of the test. This is determined with the aid of a cloud and pour point apparatusk in which a fixed volume of oil is chilled without disturbance by means of a freezing mixture (solid CO₂+Acetone mixture is usually employed).

The pour point of an oil is an important characteristics especially when the equipment is exposed to weather. If the oil becomes viscous or solidifies, it may hinder the formation of convection currents and thus proper cooling of the apparatus may not occur.

A pour point which is below the low temperature that may normally occur, is specified.

(d) Flash Point : The flash point of an oil is the minimum temperature to which an oil must be heated so that it gives enough vapour which can form a flammable mixture with the air under the prescribed conditions of the test. This is determined by employing a Pensky Martens closed cup type apparatus. A minimum value of flash point is specified in order to prevent the risk of a fire that might result by accidental ignition. Normally a flash point of 140o_{C is the minimum specified for good oils.}

(e) Interfacial Tension : The inter facial tension between the oil and water is a measure of the molecular attractive force between their unlike molecules at the interface. It is expressed in dynes / cm.

The interfacial tension is determined by means of suitable tensiometers (Torsion balance), in which the force required to lift a planar ring of platinum from the oiil/water interface into the oil is measured. The value varies with temperature and ageing of interface, and therefore, the prescribed test conditions should be strictly adhered to.

A minimum value of 40 dynes / cm for new oils is specified in Indian and Foreign Standards. This ensures freedom from all types of dissolved impurities.

The basic significance of the interfacial tension value for electrical insulating oils lies in the fact that it provides a sensitive means for detection of polar contaminants in the oil including those deterioration products acquired in service as a result of oxidation. It is widely accepted that the drop in the interfacial tension value which accompanies oil oxidation can be used as a gauge of oil quality in commercial use. Mention is also made that interfacial tension values are established by the operators of oil filled transformers, below which the oil is considered unsatisfactory for continued commercial use because of contaminant formation of oil sludge and other degrading products in service as a result of oxidaiton.

The studies at the Central Poer Research Institute have also established that interfacial tension tests can be used as a guide to the quality of insulating oils in our equipment.

3.2. Chemical Tests :

(a) Neutralisation Number : Neutraliation number or the acid content of insulating oil is defined as the number of miligram of potassium hydroxide required to neutralise completely the acids present in one gram of the oil. The presence of acids in the oil is detrimental for besides corroding the various parts of the equipment, viz, transformers it also lowers dielectric strength of the oil and often polymer- ise to form insoluble sludge which can clog the cooling system.

Two types of acids are present in the oil, viz., Inorganic and organic. The inorganic acidity has to be nil in new insulating oils since mineral acids are highly corrosive.

The maximum permissible limit of 0.03 mg KOH/g for new oils is prescribed only to ensure that it is not detrimental for safe use in transformers in which oil comes in contact with many parts of the transformer. However, in service the oil deteriorates due to oxidation resuslting in the formation of acids and sludges and the acidity of the oil goes up. The acidity of oil has to be constantly monitored for the safety of the transformers and oil when it attains a maximum acidity value of 0.5 mg KOH/ g should not be used in transformers. CPRI has developed a quick test with which the acidity limits can be constantly monitored and precautionary measures taken. This has been discussed in detail in para four of this paper.

(b) Copper strip corrosion test : Crude petroleum usually contains sulphur compounds, most of which are removed during the refining process. This test is carried out to detect any traces of free or combined sulphur that may be present in an oil. A clean polished strip of pure copper metal of the specified dimensions is heated in a fixed quantity of the oil at a fixed temperature for the specified period of time.

A black, brown or grey tarnish indicates the presence of corrosive sulphur. The presence of sulphur promotes oxidation and also corrodes the copper and silver metal points of the equipment (Trans- former).'

(c) Oxidation stability test : The oxidation stability test is perhaps the most important test an oil must pass before it is accepted for service. This test submits the oil sample to accelerated oxidation i n a relatively short time than it is likely to encounter in service. The aim is to assess the behavious of the oil during its long service from the results of the severe laboratory oxidation.

The accelerated conditions in different standards are generally brought out by : (i) increase in temperature to the order of 100o_C-170o_C

(ii) inclusion of metals or soluble catalysts

(iii) bubbling of air or oxygen in the oil during oxidation.

The oxidation stability test as laid down by Indian Standard consists in ageing 25g of the oil at 100o_C

for 164 hours during which oxygen at the rate of one litre per hour is bubbled into the oil. A copper wire of specified dimensions, after being cleaned and rolled into a cylindrical shape, is placed in the oil to act as a catalyst. The oxidation is carried out in a test tube. After oxidation, the aged oil is diluted to four times its volume with normal heptane and the amount of sludge and acids formed are deter- mined by chemical analysis. The maximum limits fixed for new oils are acidity 0.4 mg KOH/gm and sludge 0.1 percent by weight. Formation of deterioration products above this level has been correlated by experience to a faster rate of deterioration in service.

(d) Moisture content : Water will dissolve in insulating oil and its presence contribute a hazard not only to the dielectric performance of the oil itself but also to insulations that are immersed in the oil. Normally in new oils the tendency to form an emulsion with water is small and the problem is not considered of major importance. However, as the acidity of the oil increases during use because of oxidation or contamination, the tendency to form a water emulsion (ability to dissolve moisture) as the result of temperature cycling of agitation, increases.

The water in insulating oils can be determined by Karl Fischer method. This method is based essentially upon reduction of iodine by sulphur dioxide in the presence of water. This reaction becomes quantitative only when pyridine and methyl alcohol are present to react with the sulphur trioxide.

(e) Presence of oxidation inhibitor : The test is most important to check whether the insulating oils, presumed uninhibited, do not contain auti-oxidation additives. Further this test would also confirm the presence and nature of the anti-oxidant additives in inhibited oils (presence of amine or phenolic type). The Indian Standard IS : 335 (1993, is applicable to only uninhibited oils, and therefore oils con- forming to the specification should not be accepted if the oil contains any amine or phenolic type of inhibitor.

3.3. Electrical Tests :

(a) Dielectric Strength : Defined as the measure of the stress in kilo volts required to breakdown the oil across a specified gap under certain conditions. For determining this a 90 KV transformer and

The electric strength terst serves to indicate the presence of moisture, dirt and other foreign / conducting particles in the oil.

The specified electric strength of an oil must always be

Maintained during its service , if it has to perform the main functions satisfactory i.e. , insulate the live parts within the equipment.

standards.New oils should have minimum dielectric breakdown stress of 30 KV for 2.5 mm gap according to Indian

This test was considered to be and is still the most important test being carried out by the operation and maintenance staff to check the quality of oil. This practice is . not a healthy one as other important electrical characteristics like power factor and Resistivity have come into vogue which throws more light on the quality characteristics of the oil. Further, the oil is in contact with many insulating materials inside the transformer and hence electrical tests like resistivity, power factor are now con consider to be impor- tant to check the quality of oil for efficient running of the systems. There are instances wherein the oil though has

Requisite dielectric strength, has not been found suitable for use in transformers.

(b) Resistivity: The measurement of the resistivity of insulating oils is a sensitive test for the detection of conducting impurities on mineral insulating oils. It is especially valuable in that its greatest range of sensitivity is particularly applicable to the evaluation of the purity of new and unused insulating liquids. Even small traces of contaminating materials will give a marked lowering of the oil resistivity value. Using proper testing techniques and cleanliness together with suitable testing equipment, the test is reproducible within the required limits and gives a rapid means for gauging the purity of new insulating oils.

To determine resistivity normally 500 v DC is applied using a test gap of 2 mm. The temperature, time of electrification are fixed and the test cell should be cleaned thoroughly before use. It is expressed in ohm-Cm.

A low resistivity indicates the p presence of moisture and conductive contaminants.

For new oil, Indian Standard prescribes minimum value of 1500 x 1012 _{and 35 x 10}12 _{ohm-cm at 27}o_C

and 90o_{C respectively.}

The minimum resistivity value of 1500 x 1012 _{ohm-cm at 27}o_{C is prescribed for acceptance}

purposes in the case of new oil only to achieve the desired IR value for different category of power transformers. Thus, the new oil when filled in trasnformers comes into contact with many insulating materials particularly paper and varnish used for coil winding, etc. which would result in lowering of the IR value of the winding of the transformer. Hence, this phenomena should not be seriously viewed. However, it is considered necessary that the desired IR value as prescribed in IS : 1886 "Maintenance of Transformers" should be achieved for different rating of power transformers whenever the trans- formers are filled with new oil complying to the standards for efficient running of power systems. For any given oil, there is generally a relationship between dissipation factor and resistivity : if the dissipation factor increases, there is a reduction in resistivity. Useful additional information can be obtained if resistivity is carried out at ambient and at 90o_{C. A satisfactory result at 90}o_{C coupled with}

an unsatisfactory value at the lower temperature is an indication of the presence of water or cold precipitale material, without undue chemical deterioration or general contamination. The measurement of resistivity can be carried out at site but not that of dielectric dissipation factor. Figure I illustrates quality standard to be strictly adhered to where resistivity value of a transformer oil has to be used as a criteria for the continued useability of the oil in commercial transformer in service.

The resistivity value for insulating oils is affected by changes in voltage, the period of electrification and the temperature. For this reason, standardised test conditions are usually adopted for the quality evaluation of new oils and for the study of the purity of oils in practice.

or percent, as, for example 0.010 per unit or 1.0 percent.

The power factor characteric of insulating oil gives the engineer a valuable tool for evaluating the dielectric degradaiton in commercial use. As such, the study of insulation power factor is of real assistance and value. Not only the test responds readily to the presence of contaminating material which might have been picked up by the insulating oil or derived from the oil, due to improper manufacture or use of the e equipment, but also indicate presence of soluble varnishes, resins and moisture.

The power factor of a properly refined oil is extremely low, less than 0.002 when tested at power frequency (50 Hz) at 90o_{C. The test is influenced considerably by the temperature, testing voltage and}

frequency. Standard, specifications, therefore, define the test conditions. The Indian Standard pre- scribes an average stress 200 V/mm for power factor measurements at power frequency. The Standard also specifies a maximum value of 0.002 when measured at 90o_C.

The main effects of increased power factor are increased heating, increased corrosion, increased water solubility and emulsifying power and faster rate of oxidation. A lower value of power factor ensures freedom from moisture, polar compounds and other soluble impurities. The power factor is measured by using a Schering Bridge and suitably designed cell.

To select quality oils and to maintain these oils at a suitable level of dielectric quality during commercial use, has long been an engineering problem inevitably a sociated with the use of an insulating liquid in electrical equipment. Quality control has been generally exercised by the imposition of chemical and dielectric test requirements. More recently, the use of the power factor test has been applied not only for the acceptance of good quality during the commercial use of the oil filled appa- ratus.

As a quality control for new oils, the power factor test is applied to detect the presence of moisture and oil soluble electrolytic and polar compounds present as the result of improper refining techniques. As a control test for oils in service, the power factor value is used to protect against the accumu- lation of excessive oxiation products in oil soluble and to detect the presence of moisure and organic materials including resins and varnishes which may or may not have been sufficiently cured in the factory manufacture of the equipment.

The application of the power factor test is also intended to detect the presence of other types of polar materials which may have been introduced in the oil due to improper manufacturing techniques or to the selection and use of structural or dielectric materials which are not completely inert when in contact with the oil. Such materials include the various waxes, bitumens and asphalt compositions, solder and brazing fluxes, core plating materials, etc. The presence of such materials is frequently not detected by the use of the usual oil testing techniques such as dielectric strength, colour and oil acidity. The application of the power factor test to transformer oil is considered an excellent protection against the acceptance of an inferior grade of new transformer oil and also for the continuance of a highly degraded oil in commercial use.

3.4. S. K. Value :

In our country, all the indigenous manufacturers still adopt the conventional acid treatment process for manufacture of transformer oil from the available indigenous base stocks.

The behaviour of insulating oils in the presence of concentrated sulphuric acid furnishes information on the refining degree of mineral oils, particularly insulating oils. The S. K. value is the increase in volume of concentrated sulphuric acid on adding a given test sample.

VDE (German) Standard prescribes a minimum value of 4% for value for acceptance purposes. This test would certainly give insight to the extent of acid treatment for manufacture transformer oils from the base-stocks. This is particular important as the optimum dosage treatment of sulphuric acid should be given to the base-stocks for production of good quality transformer oil.

4. Reconditioning :

4.1. General Considerations - The physical means that are used for removing water and solids from oil include several types of filter, centrifuge and vacuum dehydrator.

The best choice of temperature for purification depends on circumstances. If vacuum treatment is employed to remove water, it is advisable to limit the temperature to 60o_{C to prevent oxidation. If it}

is desirable to reduce precipitable sludge (or free water without vacuum treatment), cold treatment may be appropriate.

4.2. Reconditioning Process :

4.2.1. Filters - Filter devices are generally based upon the principle of forcing oil under pressure through absorbing material such as paper. Filters of this type are capable of removing contaminants in suspension, but they cannot remove them effectively when they are dissolved or in colloidal form.