ABSTRACT
The transformer's insulation can withstand higher voltages for the shorter periods of time that are characteristic of lightning or switching disturbances. This chapter briefly discusses some of the proposed mechanisms of voltage breakdown in solids, liquids, and gases, with primary emphasis on transformer oil. Because in oil-filled transformers, due to the higher dielectric constant of the solid insulation, the highest electric stress tends to occur in the oil. One of the major results of the theory of gaseous breakdown is that the breakdown voltage across a uniform gap depends on the product of pressure and gap thickness or, more generally, on the product of gas density and gap thickness. This relationship is called the Paschen curve. The chapter establishes a phenomenological theory of breakdown in oil, which can be applied to all geometries. At short times, the breakdown voltage or stress versus time characteristic changes rapidly with time. Impulse wave shapes, are somewhat complicated functions of time.
