4. ESTUDIO DE LA SIMULACIÓN DENTRO DEL ÁMBITO LEGAL
4.3. Diferentes clasificaciones de la simulación
Power Research Institute, Central Water and Power Commission (Power Wing).
Mixing of different brands of transformer oils has always posed a problem to the maintenance engineers. In india, where bulk of the oils are imported from various sources, shortages of one brand of oil and the need to top up he oil - filled equipment are the usual causes which necessitate such mixing. Experimental work has been carried out by the authors an different types of oils having varied composition with thd without additives. Various properties of oils and their mixtures are discussed. Results obtained on accelerated life tests on oils and their mixtures alone and in contact with trans- former constructional materials are presented. It is concluded that new oils without any additives. i.e. un inlibited oils, can be mixed with each other irrespective of their composition and degree of refine- ment. With inhibited oils, the compatibility of the additives must be evaluated prior to mixing.
1. Introduction :
During recent years, the question of mixing of different types of transformer oils has gained considerable importance and doubts are often expressed as to whether the mixed oil would be abail to give satisfactory performance during its service life. The State Electricity Boards and various electricity supply undertakings who form the main consumers of transformer ils have normally to fulfill their requirements by purchasing oil from different sources, and therefore mixing of different oils is a problem for a maintenance engineer, who has to top up a transformer with oil of a different brand when the brand of oil he uses is in short supply. This very often occurs since the bulk of insulating oil is imported from various sources.
The IS : 335 (1963) (Specification for insulating oil for Transformers and switchery)1 lays down in its caluse 3.3. "Unused oils considered to be compatible with one another and can be mixed in any proportion. This provision does not, however supply to inhibited oils". Ths trend, these days, is to add antioxidations or inhibitors to the oil for prolongine its service life. Every oil supplier usually develops his own inh ibitors and the formulation of these is a carefully guarded secret. The question then arises whether oils containing various inhibitors of unknown composition can be mixed without any serious damage. The paper reports the results of the experimental work conducted on this subject.
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CH3 One Module of DBPC reacts With Two Free Radicals CH3 C H3 C 3 H C H3C C H3C 2 +RO C CH3 CH3 C CH3 CH3 C H3 CH3 H3C CH3
2. TRANSFORMER OIL.
Transformer oil is essential a derivative of crude petroleum (Petra-rock, Oleum -oil) from which it is obtained by various processes of refining. The nature and characteristics of an oil depend largely on the parent crude from which it is derived and the extent of refining which is has undergone. The crudes are of different varieties and complexity, but it is possible to classify them broadly in our categories namely.
(i) Parallinic (ii) Naphthenic (iii) Olefenic (iv) Aromatic
While these are the four main simple groups, the constitution of transformer oil is often more complex with different proportions of components belonging to more than one - category.
However, irrespectie of the nature of the crude and the refining methos, the final product is essentially a mixture of hydrocarbons. These hydrocarbons are inert and do not react with each other, the oil being merely a physical mixture.
During service, the deterioration of the oil is due to the oxidation of thse hydrocabons in which each hydrocarbon as a separate entity undergoes a definite set of reactions. The overall oxidation charac- teristics of the oil is thus the combined result of the various reactions and this is normally assccssed in practice by means of standard oxidation stability test...
The IS : 335 (1963) oxidation stability test prescribes that 100 gm. of the oil he oxidized contiously at 150oC in the presence of a sheet of supper metal... an at the rate of 2 litres ... is
troubted through the oil. After this period, the ... and acide formed are determined by chemical analysis. The limits fixed for the acceptance of new oils are :
(i) Sludge ... 1.2% (maximum) (ii) Acidity ... 2.5 (maximum) (iii) ... mg . of KOH per gm.
It has been observed that oils conforming to these alues have given trouble-free service for a long time.
In evaluating oils for their compatibility during fixing, this oxidation test has been taken as the main riteration indicating the service life of the oil.
Another standard oxidation test utilized for the experimental work is the ASTM-D 1934/62T2
(method or oxidative ageing of electrical insulating petroleum .... by open-breaker method). This method briefly consisists in ageing 300 ml. of the oil in a 400 ml. open caker at 115oC. with 15 cm2
surface area of metal ... for a period of 96 hr. Thereafter, relevant .... ropertics of the oil before and after ageing are compared. A feature of this test is that by replacing the metal ... by various materials, the catalytic activity of .... same during oxidation can be determined.
The main objects of the experimental work are as follows. (i) To determine the composition of oils and their mixtures.
(ii) To compare various physical, chemical and electrical properties of the oils and their mixtures. (iii) To study the ageing characteristics of inhibited and uninhibited oils and ascertain their comparatibility.
(iv) To determine the rate of deteioration of oils and ... their mixtures.
(v) To study the effect of transformer constructional .. materials, viz, metals and insulating materials .... on the deterioration of oils and their mixtures.
TABLE -1
Composition of oil samples and their mixtures
Property OilA OilB OilC OilD OilE OilA OilA OilA OilA OilB OilB OilB OilC OilC OilD
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OilB Oil C OilD OilER OilC OilD OilE OilD OilE OilE
Carbon aromatic% 5 7 -14 8 6 2 10 6 3 8 7 5 4 12
Carbon naphthenic% 56 46 45 40 48 51 45 46 52 49 41 48 42 47 42 Carbon parafinic % 39 47 55 46 44 43 53 44 42 48 51 45 52 49 42 Average molccular 255 263 310 283 279 259 300 278 265 305 265 270 294 292 281
TABLE - 2
Properties of Oils and oil-mixtures
Property Oil A oil B oil C Oil D Oil E Oil A Oil A Oil A Oil A
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Oil B Oil C Oil D Oil B
.. nsity gm. per cm.2 at 0.8823 0.8772 0.8737 0.8968 0.8954 0.8807 0.8782 0.8896 0.8887 20oC cosity, centistokes, at 29.58 28.91 33.42 35.22 32.84 29.36 33.15 33.85 32.11 35oC fractive index 1.4806 1.4808 1.4777 1.458 1.4913 1.4897 1.4796 1.4885 1.4862 ur point, oC - 51 - 48 - 51 - 51 - 40 - 51 - 48 - 48 - 45 sh point, oC 147 144 153 147 162 147 147 144 156
crfncial tensions, dyne 35.2 25.0 38.5 33.0 45 .0 30.0 30.0 28.0 37.0 per cm.
utralization number 0.00 0.004 0.008 0.0013 0.00 0.004 0.004 0.008 0.00 ng. of KOH per gm.
onification value, mg 0.06 0.06 0.03 0.12 0.05 0,.10 0.10 0.12 0.08
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