Avaliação experimental sobre o efeito de ionização do solo devido à injeção de correntes impulsivas

Abstract

This work presents the results of an experimental research on the behavior of soil samples subjected to lightning-patterned impulsive currents, with a view to assessing the dynamics of ionization soil phenomenon. The work involved a series of laboratorial tests, where impulsive currents of different intensities and front times were impressed to samples of soils at humidity condition varying from very dry to very wet. Four soil types and at least three moisture contents were used to obtain different values for resistivity. Two basic configurations of recipients were used to carry out the tests with the soil samples. In the configuration of parallel disks, a uniform electric field distribution was applied to the sample. In the coaxial configuration, a non-uniform field distribution was applied in the gap between the cylindrical conductors, with the field varying with the inverse of the distance to the axis of the internalelectrode. The values of critical field Ec, which are considered the onset of the ionization process, found for each configuration, have the order of magnitude indicated in literature, although they are much higher in the uniform-field condition than in the cylindrical configuration of field.The differential aspect of this work is that the critical fields were evaluated for each soil sample in two field-distribution conditions (given by the parallel disks and by coaxial cylinders), unlike the other works presented literature that carried out the assessment of Ec only for one of such conditions. This led to the original result of this research that is the identification of a dynamic behavior for the critical field, resulting from the influence of field non-uniformity. The value of this field evolves from a very low threshold (2 to 3 kV/cm), when the ionization process begins near the internal electrode. It increases while the ionization zone expands around the electrode, and reaches the typical order of magnitude of this field for uniform distribution (around 8 to 9 kV / cm). Thus, the process of ionization tends to start with low values of critical field. But, with itsdevelopment, higher values are required for Ec to ensure the process evolution. Taken this dynamic profile of Ec into account leads to reducing the relevance of ionization effect in relation to the usual assumption of a constant value parameter, around 3.0 kV/cm.