Throughout this paper data have been presented showing that the apparent inconsistency of the reported dielectric strength behavior of insulating liquids can be satisfactorily correlated if proper consideration be given to the state of the “purity” of the liquid itself. As a result it is suggested that insulating liquids should be classified as ( a) “pure,” indicating those liquids free from dissolved gases as primary “impurities”; and ( b) “impure,” including those liquids which contain dissolved gas. The breakdown mechanism depends on the distinctive behavior of these two general classes. “Pure” liquid breakdown is a function of charged particle formation. In part, this may be caused by the assumption of a charge by molecular aggregates, colloidal-like in nature. In part, the charge may arise from molecular ionization by collision. The latter occurs chiefly in the voltage range immediately preceding electrical rupture and is the chief cause of “pure” liquid insulation failure. The presence of the first type of charge—that is, the existence of a difference of potential between molecular aggregates and the liquid—is chiefly responsible for the variation in the time factor to breakdown. The breakdown of “impure” liquids is a function of dissolved gas elimination. This dissolved gas is eliminated as a result of changing solubility produced ( a) by electro-striction effects, or ( b) by changing pressure or temperature. The presence of secondary impurities such as dust particles and fibers, acts chiefly through the effect on increasing gassing tendencies. It is suggested further that the localization of dielectric breakdown in liquids, irrespective of the type or degree of “purity,” is chiefly in the “neutral membrane” located near the electrodes and formed by the discharge of particles. Such a “neutral membrane” results in a space charge effect giving marked drop in potential and as a result promoting ionization by collision effects in “pure” liquids and electro-striction effects in “impure” liquids.
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Jan 1, 1940
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