Breakdown in gases can be described satisfactorily by the streamer theory. This theory can also be applied to the AC breakdown in dry transformer oil; it is used for determining the breakdown voltage or the inception voltage for various uniform and non-uniform configurations. Conductor insulations and harriers disposed between the electrodes are studied in addition to the oil gap form between bare electrodes. The application of the equations indicated for AC voltage stress to impulse voltage stress is subject to certain qualifications. In the case of switching impulse voltages, the equations hold for uniform configurations and for slightly non-uniform ones (η> 0.1). In the case of lightning impulse voltages the validity is limited to very small clearances.
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