On the Transient Response and the Frequency Analysis of Transmission Line Towers

This paper proposes an approach to the study of the transient response and the frequency characteristics of power lines' towers, when subjected to lightening strikes. It starts with dividing the nonuniform line representing the tower into a number of sections. From the usually known dependence of the tower's characteristic impedance on the vertical coordinate and the application of a recursive circuit reduction technique, an s-domain expression for the tower input impedance can be obtained. This expression, followed by the numerical Laplace inversion, is utilized for the determination of the tower's transient response. The impedance expression can be also used to determine the tower's resonance frequencies. This was used to demonstrate some potentially critical situations in which the tower is hit by lightning strikes comprising multiple current pulses. The validity of the proposed technique is demonstrated by comparing the achieved results with those already available in the literature for the same case studies. 1. Introduction The proper insulation coordination and overvoltage protection of power networks and components will depend on their topology and their characterizing circuit parameters as well as on the time waveform of the input voltage and/or current stimuli. The accurate determination of the expected transient stresses will allow more effective protective measures and more efficient utilization of the network components. In the recent literature, considerable attention was paid to the transient phenomena involving overhead lines and underground cables [1–13]. Although most of these investigations are dealing with the active (live) line conductors, the ground wires, and the insulating equipment, many interesting studies dealing with the line towers and their steel structures were also conducted [6–13]. An important issue in this regard is the transient behaviour of the transmission lines’ towers after being hit by lightening strikes In terms of circuit analysis, the towers are treated as nonuniform transmission lines with location-dependent inductance and capacitances per unit length. There are three well-established techniques for the analyzing tower transients. (1)The time domain simulation based on dividing the tower into a number of sections represented by L-equivalent circuits. A set of simultaneous differential equations in the state variables (the inductor currents and capacitor voltages) are formulated and then solved numerically [1, 3]. This method allows the consideration of losses and nonlinear phenomena such as corona [4, 5,