Oil impregnated electric device

#The present invention relates to oil impregnated electric devices containing polyolefin film as an insulating material impregnated with an impregnating agent which comprises a mixture of an arylalkane compound having a specific structure and an organic phosphorus compound.

#Conventionally, mineral oil type insulating oils and chlorinated diphenyl oils have been widely used as insulating oil for capacitors, transformers, cables, etc. However, recently, said oils have been used in combination with plastic materials, especially, polyolefins with increased demands for high voltage stresses and small-sized electric devices. However, being different from conventional use in combination with paper, the use in combination with plastic material results in improvement in insulation, but requires suitability with the oils and the following various problems have occurred when used with the conventional oils.

#1. The oils readily dissolve the plastic materials, especially polyolefin materials to cause reduction in insulation.

#FIG. 1 is a graph which shows the results of tests on solubility of polypropylene film in oils in a high temperature atmosphere. As is clear from the graph, the solubility decreased with increase in mixing ratio of the organic phosphorus compound in the oil and the solubility increased with decrease in aromatic content of the arylalkane compounds.

#FIG. 2 is a graph which shows the relation between tan .delta. and mixing ratio of the organic phosphorus compound. As is clear from the graph, tan .delta. has the tendency of increasing with an increase in the mixing ratio of the organic phosphorus compound and especially when the ratio was higher than 80%, tan .delta. increased abruptly.

#FIG. 3 is a graph which shows relation between corona starting voltage, mixing ratio of the organic phosphorus compound and aromatic content of the arylalkane compound. As is clear from the graph, sufficient effect was recognized when the corona starting voltage in the case of the mixing ratio being 0% is taken as 1. When aromatic content of arylalkane compounds was less than 50%, the effect was very small and some of these compounds had no compatibility with the organic phosphorus compounds. Therefore, the arylalkane compounds having an aromatic content of less than 50% were not suitable. On the other hand, when aromatic content of the arylalkane compounds was more than 90%, vapor pressure of the compounds became high and conversely pour point thereof became higher than normal temperature, whereby characteristics as an oil could not be sufficiently exhibited.

#From the results as shown in FIGS. 1, 2 and 3, mixtures of an arylalkane compound having an aromatic content of 50 - 90% with 20 - 70% of an organic phosphorus compound are most suitable.

#The present invention will be further illustrated by the following Examples.

#A capacitor of 1 KVA was produced by impregnating a capacitor element comprising aluminum foil as electrodes and a polypropylene film as a dielectrics with an impregnating agent comprising a mixture of diarylethane (e.g., 1,1-phenylxylylethane) and tricresyl phosphate in a volume ratio of 2 : 1. A capacitor of the same construction as above except that the capacitor element was impregnated with only diarylethane was produced. Change in corona discharge amount with lapse of time was measured on said capacitor. The results are shown in FIG. 4. As is clear from FIG. 4, when initial corona discharge amount was taken as 1, in the case of the capacitor of the present invention, the discharge amount began to decrease in a few minutes and no dielectric breakage occurred while in the case of the capacitor impregnated with only the arylalkane compound, the discharge amount increased and finally dielectric breakage occurred. Similar effects were obtained with other phosphorus compounds when they were used under optimum conditions depending upon kind of device and capacity. Conspicuous effect was obtained especially when potential gradient of the insulator was higher than 30 V/.mu..