Electric Power Systems Research 118 (2015) 76–82 Contents lists available at ScienceDirect Electric Power Systems Research j o ur nal ho me page: www.elsevier.com/lo cate/epsr The inﬂuence of seasonal soil moisture on the behavior of soil resistivity and power distribution grounding systems Vilson Luiz Coelho a , Alexandre Piantini b,∗ , Hugo A.D. Almaguer c , Rafael A. Coelho c , Wallace do C. Boaventura d , José Osvaldo S. Paulino d a Faculty SATC (FASATC), Rua Pascoal Meler, 73, 88805-380 Criciúma, Brazil b University of São Paulo, Institute of Energy and Environment (CENDAT – IEE/USP), Av. Prof. Luciano Gualberto, 1289, 05508-010 São Paulo, São Paulo, Brazil c Regional University of Blumenau (FURB), Rua São Paulo, 3250, 89030-000 Blumenau, Brazil d Federal University of Minas Gerais (UFMG), Av. Antonio Carlos, 6627, Campus UFMG, Pampulha, 31270-901 Belo Horizonte, Minas Gerais, Brazil a r t i c l e i n f o Article history: Available online 22 August 2014 Keywords: Distribution systems Grounding Ground resistance Soil moisture Soil resistivity Soil behavior a b s t r a c t This paper discusses the inﬂuence of the moisture content on the seasonal behavior of both soil resistivity and ground resistance typical of power distribution grounding systems. Although ﬁeld surveys indicate a wide variation in the moisture content only for the topmost soil layer, laboratory tests show that the resistivities of several types of soil are greatly affected. As a consequence, the ground resistance of systems with small dimensions may vary signiﬁcantly and compromise the lightning performance of protection systems. The strong correlation, identiﬁed from measurements in this work, between soil moisture and, thus, soil resistivity, and monthly rainfall allows extrapolating resistivity measurements for the most critical conditions providing data that can be used to improve the design and performance of grounding systems. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In a typical electric power system, the number of distribution lines surpasses dozens of times (or even a hundred) the num- ber of transmission lines. Thus, differently from transmission line projects, which are developed on a case by case basis, the design of distribution lines uses pre-established standards. In this sense, utilities have standards for materials, equipment, structures, etc. which were designed to be applied to typical distribution systems. Distribution grounding systems are characterized by small dimen- sions due both to the lack of space for installation and high cost resulting from the large number of grounding structures. In order not to impair the performance of the overcurrent protection sys- tem, high values of ground resistances observed in critical, high resistivity soil conditions, are compensated through an increase in the number of grounding points [1]. However, this approach is not enough to prevent ﬂashovers caused by lightning overvoltages. ∗ Corresponding author. Tel.: +55 11 3091 2580; fax: +55 11 3812 9251. E-mail addresses: vilson.coelho@vlc.eng.br (V.L. Coelho), piantini@iee.usp.br (A. Piantini), hugo@furb.br (H.A.D. Almaguer), vgt.rac@gmail.com (R.A. Coelho), wventura@cpdee.ufmg.br (W.d.C. Boaventura), josvaldo@cpdee.ufmg.br (J.O.S. Paulino). Due to the inherent high frequency content of the lightning surges, the performance of distribution systems is strongly affected by the ground resistance locally [1–5], and, in this case, high values cannot be compensated by increasing the number of grounding points. Thus, to achieve a proper protection, it is essential to implement technical procedures to ensure not only that the ground resistance values measured during the implementation of the grounding sys- tem are within the limits set by the standards, but also that they will remain within these limits in all seasons of the year. In this regard, the Brazilian standard ABNT NBR 7117 [6] instructs that soil resis- tivity measurements are made in a dry period after at least 7 days without rain. However, in some regions professionals may have to wait months to get this condition, which hinders the applicability of the recommendation. IEEE Std. 81 [7] points out the signiﬁcant inﬂuence of moisture content, temperature, and soil compaction on the resistivity, but it does not suggest measurement procedures to minimize the problem. In the case of substations, IEEE Std. 80 [8] recommends performing periodic measurements for the veriﬁ- cation of maximum values of the ground resistance. In distribution systems this procedure is not economically feasible considering the thousands of existing grounding points. Aiming at ﬁnding solutions to this problem, this paper presents the results of an investigation on the inﬂuence of seasonal moisture on soil resistivity and its effect on distribution grounding systems. http://dx.doi.org/10.1016/j.epsr.2014.07.027 0378-7796/© 2014 Elsevier B.V. All rights reserved.