Avaliação da resposta transitória de aterramentos encapsulados por concreto

Abstract

Grounding system design ensures safety compliance for transmission lines under lightning conditions. In this regard, over time, efforts have been made to seek new techniques to improve the transient response of grounding, mainly for high resistivity soils. In this sense, one of the long-known techniques is the use of low resistivity materials around the grounding electrodes, such as bentonite and concrete. However, despite the elapsed time, few works in the literature carry out an in-depth study on the characterization of the transient response of concrete-encased groundings; which would have special importance in the evaluation of them for use in transmission line tower grounding. Given the above, in order to contribute to the state of the art, in this work, the transient response of concrete encapsulated electrodes is investigated. For this purpose, simulations were performed using the finite element method, considering a rigorous electromagnetic modeling. Concreteencapsulated groundings of the vertical electrode type (rod) with 3 and 30 m, as well as horizontal electrode (counterpoise) with 15, 30, 45 and 60 m, were analyzed. First and subsequent stroke impulsive current excitations were considered. The effect of frequency dependence of soil parameters was taken into account. In addition, soils with low, medium and high resistivities (300 Ω.m to 4000 Ω.m) were evaluated. Different lengths of concrete casing were taken into account in order to verify situations of partial or total encapsulation. For all these cases, the harmonic impedance (from 60 Hz to 4 MHz) was simulated, and the respective ground potential rise and impulsive impedance were calculated. In total, 150 different groundings were evaluated, each subjected to two discharges, totaling 300 impulsive responses. The results indicate that the concreting of the electrode in high resistivity soils produced average reductions in impulsive impedance of up to 40% for the 3 m vertical rod, considering first and subsequent stroke. For the horizontal electrodes, the reduction in impulsive impedance in high resistivity soils was up to 22% for first stroke, while for subsequent stroke the results varied considerably between 10% and 26%, depending on the electrode length and soil resistivity. Furthermore, it was found that the use of concrete in grounding reduces the effective length of the electrode.