Electrical stability of electrical insulating oils under limited oxidation

ELECTRICAL insulating oils find a wide application in the electrical industry in cables, transformers, capacitors, circuit-breakers, and other oil-insulated equipment. The present paper discusses the electrical stability as indicated by low power factor of an oil under limited-oxidation conditions, such as exist in certain service applications in oil-impregnated paper-insulated cables and sealed transformers. This test differs from the conventional “open-beaker” or unlimited-oxidation test. The limited amount of oxygen made available to the oil may be varied, from residual amounts due to contact materials or present in the oil, to the normal solubility of air in oil, and to larger amounts as desired. The limited-oxidation test indicates that, for a given duration of deterioration, oils may give power factors of 10 to 100 times those indicated in the conventional test. This fact is important in service applications for oil-paper cables (since these losses limit the rating of the cable and decrease its efficiency) and for oil-filled equipment (since increased dielectric losses may reduce directly or indirectly the useful life of the equipment). While the power factor of an oil is not of prime importance for many types of oil-filled electric equipment, it is one of the most sensitive known criteria for stability tests. Operating performance of oil-paper cables in service shows, in general, that the power factor of the insulation increases with time, even in certain cases to the extent of impairing the investment seriously. Hitherto, these changes have been unexplainable. It is believed that the limited-oxidation test, which to a certain extent simulates service conditions, will be useful in controlling this important problem.