Overvoltage analysis of transmission towers considering the influence of tower-footing impedance

Lightning is one of the main causes of outages due to backflashover mechanism in power systems. In this context, the tower-footing impedance presents a key role in the overvoltages caused by this phenomenon. Tower-footing impedance is traditionally modelled as a pure resistance, which is a good approximation, at low frequencies. However, the inductance and resistance of the tower-footing impedance strongly depend on the frequency at higher frequencies of the spectrum that is covered by lightning strokes. As such, their frequency dependence must be considered for accurate calculation of transient responses. Using the Vector Fitting technique, the admittance of grounding electrode buried in soil is represented by a lumped parameter circuit. The circuit model can be connected to transient transmission line models which enables the calculation of the voltages at the cross-arms of the tower. This paper shows the influence of tower-footing impedance of vertical electrodes on the overvoltage caused by a lightning striking at the tower top. The analysis will show how the soil characteristics and geometrical parameters affect the overvoltages at a typical transmission tower.

I. Introduction

Lightning strokes may cause overvoltages in transmission lines leading eventually to short-circuits, insulator damages and outages [1]. Lightning strikes hit either an overhead ground wires or directly at the tower top itself. Consequently, an impulsive lighting current will travel down to the bottom of the tower, causing a voltage raise on the tower body. A voltage wave travels back and forth, being reflected at the tower-footing and at the tower top, increasing the voltage at cross-arms and electrically stressing the insulator strings. If voltage difference between cross-arm and phase conductors exceeds its Critical Flashover Voltage (CFO), a backflashover (BF) occurs from the tower to the phase conductors and it may cause an outage [2].