D-C cleanup in insulating oils

Three unusual types of behavior of insulating oils have been subjected to a series of experiments which indicate that all 3 are closely related. These types of behavior are the reduction of conductivity and a-c dielectric loss following a sustained application of continuous potential (called “d-c cleanup”), the current-time relation under continuous potential, and the unequal potential distribution under continuous stress due to the accumulation of space charges toward the electrodes. Theories regarding a common cause of these different types of behavior are drawn from the experiments.