Study of the Dielectric Breakdown of Insulating Mineral Oils under Impulse Voltages

The impulse withstand of insulating liquids in divergent fields is presently being studied with particular atten tion in regards foreseeable changes in the nature of mineral oils, and in regards the behavior of equipment subjected to lightning impulses. A draft standard is presently under study by the IEC. Tests based on this draft standard were undertaken on ten traditional naphthenic oils, and on three paraffinic oils developed in France. These tests indicated the following conclusions: the measurement cell must be adequately defined to obtain repeatable and reproducible results; it is unnecessary to treat new oils before testing; the value of the dielectric strength under negative impulse is always notably higher than the value under positive impulse (50% on average); the latter can therefore condition the behavior of equipment in operation; thermal ageing lowers the withstand under negative voltage (20% on average); under positive impulse, it may induce either a reduction or an increase of the withstand; paraffinic oils have a withstand voltage that is equal to or higher than that of naphthenic oils, depending on refining.