An Electromagnetic Model for the Calculation of Tower Surge Impedance Based on Thin Wire Approximation

Abstract

When lightning strikes a transmission line tower or shield wires, electromagnetic waves propagate through the tower back and forth, increasing the voltage across insulator strings. is can eventually lead to a back-ashover (BF), which may cause damage to equipment or costly power outages. To calculate the over-voltages and predict the probability of a BF, an accurate model of the tower and its grounding system is needed in electromagnetic transient (EMT) type simulators.ere are a number of theoretical models for the equivalent circuit of a transmission tower. However, they either are not accurate enough or they are derived for a certain type of transmission tower, which limits their applicability. Numerical electromagnetic analyses have less simpli cations compared to the theoretical solutions and are by far less expensive than eld measurements. ey also have the exibility to analyse any type of tower. In this paper, the direct method for the measurement of tower impedance is implemented by NEC4 and applied to a 400-kV double circuit tower with all its details. e process of obtaining the wire network of the tower used in this paper is completely automated and it can be applied to any other type of transmission tower. e results of the numerical simulations are compared to those obtained with existing tower models.e developed model in this paper is capable of considering all the details of the tower and including the nite resistance of the ground and grounding electrodes.

Tower Models for Power Systems Transients: A Review

“…In this regard, for example, the concept of surge impedance is used to describe different quantities in tower analysis, although it is well-defined in electromagnetic textbooks [28]. As confirmed in [29,30], researchers proposed four definitions of surge impedance: three are specified in the time domain (TD) (two are function of time, whereas one is constant), and are tied to the shape of the injected current; the fourth is defined as the Inverse Fourier Transform of the harmonic impedance computed in the frequency domain (FD) [28], and it is dependent on the geometry and on electromagnetic properties of the media.…”

“…What has been observed is that different models result in different transient responses to transient events [23]. Depending on the tower geometrical characteristics and conductors arrangement, and on the specific choice of its equivalent circuit, different voltages of the tower arms are computed [29], leading to different voltages across the insulators, affecting the probability of occurrence of backflashover and the lightning performance of the line, which also depends on the statistical distribution of lightning current characteristics [40], shielding failure, ground flash density, etc.…”

“…Figure29. Input impedance Z in ( f ) seen at the top of tower (b) in Figure16over the soil with ρ g = 600 Ωm, r g = 20, and Z gr ( f ) in Figure19.…”