Abridgment of the mechanism of breakdown of dielectrics

Abstract

In attempting to analyze various experimental data that have been obtained in researches on dielectric phenomena in high-voltage cable insulation and other dielectrics, the various existing theories of dielectric behavior have seemed inadequate. A critical study has therefore been made of these theories in an attempt to obtain a working hypothesis that more nearly meets the stringent requirements of experimental facts. The logarithmic formula is shown to give erroneous results if applied to high-voltage cables when they are operating under high stress. The gradient in a cable must be calculated from the volt-ampere characteristic of the dielectric when stresses above the elastic limit are used. For stresses below the elastic limit it makes no difference which method is used, but at high stresses an entirely different gradient distribution is obtained when calculated from the volt-ampere characteristic. Likewise, when an insulation is operated above the elastic limit the stress ceases to be a critical factor, but the strain is of utmost importance. In comparing cables that are operating under high voltages, therefore, the strain at the core should be considered rather than the stress at the core. Stress is given by the voltage gradient and strain by the polarization or the current density in the dielectric. Since there is always a conduction current flowing, there must be mobile or free ions present. It is assumed that these free ions or electrons come from the molecules of the dielectric and that the number that are present depends on a condition of equilibrium existing between the molecules and the free ions. There exists then a state of kinetic equilibrium between the molecules and the free or mobile ions. Any change in external or internal conditions will disturb the equilibrium, and thus change the electrical behavior of the dielectric. Thermal effects and corona effects are accounted for on this basis. Breakdown occurs when the equilibrium conditions are so disturbed that the insulation as a whole becomes unstable, electrically. High stress or strain and high temperature affect the conditions of equilibrium decidedly. Corona in gases, oils, and solids consists of minute disruptive discharges that are initiated by rapid changes in equilibrium conditions when the dielectric is over stressed. In solids, however, where the ion mobility is very low and the ion friction high, there will not in general be a corona effect observed because there can be no rapid readjustment of equilibrium conditions. If the insulation is not homogeneous, or if it is composite, there will likely be internal discharges, a corona effect, when the weaker dielectric is overstressed. Moreover, since the molecules of a solid can not readjust themselves quickly to the new conditions of equilibrium imposed by a high voltage suddenly applied, there will be high local stresses and strains set up which may result in mechanical deterioration of the dielectric, that is, chipping or cracking of the dielectric. Breakdown, therefore, will take place when the insulation is rendered unstable by disturbing the equilibrium conditions, regardless of whether it is due to mechanical strains, electrical strains, or to thermal effects. The tri-fold nature of the phenomenon must be considered in the complete analysis of the problem.