Methods for Field Measurement of the Frequency-Dependent Soil Electrical Properties: Evaluation of Electrode Arrangements Through FEM Computations

摘要

The low radio-frequency electrical properties of soil (up to 3-5 MHz) are important for lightning transient and grounding system studies. Measurements of soil electrical properties are most commonly performed on undisturbed or reconstituted soil samples even though it is difficult to use these results for the representation of undisturbed ground in field conditions. This work presents an evaluation of several electrode arrangements for field measurement (in situ) of the frequency-dependent soil electrical properties at low radio-frequencies through FEM computations. The developed model is validated against results obtained theoretically and using grounding analysis software. A parametric analysis is performed by varying electrode dimensions, separation distances between electrodes and the excitation frequency (up to 100 kHz); several uniform as well as two-layer soil models were investigated. Results are compared and discussed based on deviations from the assumed hemispherical symmetry of the potential distribution in the ground around the injecting electrode taking into account the effects of excitation frequency and soil model parameters. The most appropriate arrangement for field measurements comprises a hemispherical and a ring electrode for current injection as well as two potential probes; for practical applications, the ring current-return electrode can be replaced by four ground rods in order to minimize the time and effort required for the setup of the electrodes. A preliminary investigation is conducted to assess the applicability of the measured apparent electrical properties of non-uniform soils to the computation of the impedance of ground electrodes. It is shown that using the apparent soil electrical properties as determined for an arbitrary distance between potential probes, may result in significant errors in the computed impedance of ground electrodes.