Investigation Into Variation of Resistivity and Permittivity of Aqueous Solutions and Soils With Frequency and Current Density

Soil parameter characterization and the variation of permittivity and resistivity with frequency (dispersion) and current density have been the subject of many experimental studies but significant differences in measured values are found in the literature due to different testing approaches, apparatus, and test sample composition. This article first presents a comprehensive review of this previous body of work. Then, new experiments on soils and electrolytes with test frequencies in the range 1 Hz to 10 MHz and with current densities from 1 to 35 mA/m² are described. Such results help clarify the effects of frequency, soil moisture, electrolyte concentration, and electrode material on the measured test medium parameters. The contribution of the electrode-electrolyte interface (EEI) and the influence of current density are particularly highlighted. These findings indicate that some previous measurements may have overestimated the measured values of resistivity and permittivity due to the EEI effect. Finally, the test results are compared with soil parameter equations from CIGRE TB781.

I. Introduction

Grounding is an important aspect of power systems from the viewpoint of power system reliability, equipment protection, and human safety. Designing a suitable grounding system requires at least a reasonably accurate quantification of soil parameter values; particularly conductivity σ, (usually expressed as resistivity ρ in grounding literature), and for high frequency (transients) the relative permittivity ε_r. For most soils, relative magnetic permeability can be assumed to be unity [1]. The variation of resistivity and permittivity of soils (and electrolytes) with frequency, also known as dispersion, and with current-density has been studied by several investigators over many years [2]–[20]. Lightning currents cover a wide range of magnitudes (∼1 –300 kA) [21] and frequency components up to 2 [22] or 10 MHz [23]. Power system ground return fault currents can reach magnitudes of several tens of kA [24]. Therefore, the effect of frequency and current magnitude on ground impedance over these ranges should be accounted for in grounding system design. Practical verification of grounding installations is commonly achieved by measuring ground resistance or ground impedance using low voltage ac or switched dc test equipment. Such equipment may inject only a few amperes of current into the grounding system, sometimes in the order of mA, which represents a small fraction of the actual current that may flow under fault conditions. IEEE standards [25], [26] recognize the effect of current magnitude on soil resistivity and grounding impedance in terms of thermal effects at high current, causing drying out and soil ionization; however, no reference is made to the nonlinear characteristics of resistivity (or permittivity) and electrode impedance over a low magnitude current range as will be shown later in this study.