IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 1, JANUARY 2012 205 The Use of Underbuilt Wires to Improve the Lightning Performance of Transmission Lines Silverio Visacro, Member, IEEE, Fernando H. Silveira, and Alberto De Conti Abstract—An unconventional technique to improve the lightning performance of transmission lines, consisting of the addition of grounded wires properly positioned below the phase conductors, is assessed. Overvoltages developed across insulator strings of ex- isting 230-kV lines in response to direct strikes to the towers were simulated using an electromagnetic model. Minimum reductions of these voltages in the ranges of 19%–32% and 26%–44% were achieved for 20- to 80- tower-footing grounding resistances due to the addition of one and two underbuilt wires, respectively. This practice was shown to be more efficient than reducing the tower grounding resistance to its half value. Index Terms—Lightning, lightning overvoltage, lightning per- formance of transmission lines, transmission lines. I. INTRODUCTION L IGHTNING is a frequent cause of transmission-line out- ages, reducing the availability of electrical networks. Ba- sically, three mechanisms are responsible for developing light- ning overvoltages in high-voltage lines that might result in elec- trical discharges across insulator strings, leading to faults: the flashover, associated with strikes to phase conductors; the back- flashover, associated with strikes to the tower or to shield wires at the tower vicinities; and the midspan flashover, as detailed in [1] and [2]. For transmission lines below 500 kV installed in regions of unfavorable soil-resistivity conditions, the backflashover largely prevails [3]. Therefore, this mechanism, which is mainly gov- erned by the value of tower-footing grounding impedance, is the main focus of techniques intended to improve the lightning performance of such lines. The conventional practices to decrease the frequency of back- flashover events comprise basically two actions: 1) the reduc- tion of tower footing resistance and 2) the installation of surge arresters to prevent flashovers. Most power utilities concentrate their efforts in the first practice due to the costs associated with the installation and maintenance of line surge arresters. It is worth mentioning that reducing grounding resistance is an in- direct way to achieve the actual goal, which is to decrease the tower-footing grounding impedance. As explained in [4] and [5], the low-frequency grounding resistance can be quite dif- ferent from impulsive grounding impedance, which is the pa- Manuscript received April 03, 2011; revised July 12, 2011; accepted August 27, 2011. Date of publication October 18, 2011; date of current version De- cember 23, 2011. Paper no. TPWRD-00260-2011. The authors are with the Lightning Research Center (LRC) of Federal Uni- versity of Minas Gerais (UFMG), Belo Horizonte 31270-901, Brazil (e-mail: LRC@cpdee.ufmg.br). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2011.2168546 rameter typically used to characterize the response of electrodes subjected to lightning currents. Nevertheless, the measurement of this impedance is not feasible in field conditions. Since, under certain circumstances, both parameters are related, the mea- sured low-frequency resistance is used to verify the grounding quality instead of the impedance, as discussed in [4]. The literature refers to some nonconventional practices to improve the lightning performance of transmission lines. These practices explore effects of parameters that influence the amplitude of overvoltages developed across insulator strings when lightning strikes the line, notably the tower surge impedance, and the electromagnetic coupling between shield wires and phases. It suggests the possibility of using guy wires, continuous counterpoise, or metallic wires placed below phase conductors, as alternative solutions to improve the line response to direct strikes [6], [7]. Despite these suggestions, the literature deals very poorly with the problems involved in their implementation, and no quantitative evaluation of their efficiency is provided. In this perspective, this paper addresses an unconventional technique and quantifies the improvement it might produce on the lightning performance of lines, taking as reference an ex- isting 230-kV line, used as a case study. II. DEVELOPMENTS A. Describing the Technique The technique consists of the addition of metallic wires installed below the phase conductors at proper distances and connected to the tower that requires lightning performance improvement. The underbuilt wires are extended only along the two spans adjacent to the tower. In this paper, steel wires were tested, though any metallic wire could be used. This technique, explored by the authors in a sequence of works since 2002 [8]–[12], has provided encouraging results in terms of reducing lightning overvoltages developed in response to direct strikes to the line, and it is here refined. Two arrangements of practical interest were identified in sim- ulations, corresponding to the addition of one and two under- built wires as indicated in Fig. 1. Observing a minimum distance “ ” is required to avoid diminishing the electrical insulation of the transmission line. B. Methodology of Development The proposed arrangements and the efficiency of this tech- nique were accessed by systematic simulation using the hybrid electromagnetic model (HEM). This frequency-domain model, described in detail in [13], has been widely employed to deter- mine voltage and current distributions in lightning-related prob- 0885-8977/$26.00 © 2011 IEEE