On Tower Impedances for Transient Analysis

Abstract

Abstract-The analysis of the dynamic behavior of power transmission line and telecommunication towers is of interest in protection and EMC studies related to lightning. Usually, time-domain surge impedance is used to characterize tower dynamic behavior. The main drawback in the definition of such surge impedance is that it is dependent on the excitation waveshape and there is no consensus on the current waveshape to be used. Also, there is no consensus on the unique definition of the involved voltage. This paper explores possibilities for a systematized approach to the analysis and uniquely defined quantities that characterize transient response of towers. Further, limitations associated with simplified approaches are emphasized by examining examples of direct comparison between computations based on transmission-line approach and antenna theory for a 100-m tall tower. It is pointed out that problems in the definition of voltages might occur above 100 kHz, especially near resonant frequencies, while differences in current distribution exist already at the lowest frequencies.

Investigation on the Potential Rise and Currents in Insulated Mast Scheme with Single Tower During Stroke Interception

“…A detailed analysis of the path dependency of the electric field line integral (potential rise) on down conductor has been made in [4]. However, with regard to the dielectric stress in the gap, the line integral of electric field over the shortest path across the gap would be an appropriate quantifier.…”

Numerical investigation of transient surge impedance of a vertical conductor over a perfect ground

“…In case of vertical conductors above a ground or the conductor systems in which non-transverse electromagnetic (TEM) fields are observed, the classic transmission line theory is generally not valid. Fortunately, for such conductors some of the wave propagation features observed in the TEM transmission line retains [2,4]. Surge impedance, therefore, is often used in the study of transients during a lightning stroke [3].…”

“…A number of theoretical formulas have been proposed, which were basically derived using basic circuit theory and transmission line theory [4]. These formulas are generally applicable to the conductors over a perfect ground.…”

“…In [4] a critical review of tower impedance for transient analysis was presented. Several important issues have been highlighted, such as surge impedance definition, source current waveform, and path-dependent voltage.…”

“…An equivalent circuit for vertical conductors can be developed using (3) and (4). In this model the mutual coupling between two cells is represented by either a controlled voltage source or controlled current source.…”