A multistory transmission tower model to be used in multiconductor analysis by EMTP (Electromagnetic Transients Program) is proposed. The circuit parameters of the model are determined based on the measurement of voltages across the insulator strings on an actual 500 kV transmission tower. The multiconductor lightning surge analysis considered enables one to predict the waveform of each insulator voltage, which is useful in determining the phase and the instant of a back flashover. The new tower model is recommended in Japan to be used in the new multiconductor analysis of EHV and UHV class double circuit transmission lines. The new tower model and conventional tower models are compared in the analysis of minimum back flashover lightning currents at a UHV transmission line. The new model gives about 20% smaller current. >

Multistory transmission tower model for lightning surge analysis

Experimental and theoretical methods for determining the surge impedance of towers are presented. It is demonstrated that the values so obtained adequately describe the response of the tower when a transmission line is struck by lightning. A right cone representation for conventional double-circuit steel towers is proposed, and a theoretical analysis using field theory yields the surge impedance as Z= 60 ln (2/S) where S is the sine of the half-angle of the cone.

Tower Surge Impedance

Modelling of transmission towers is an essential part of the travelling-wave analysis of lightning surges in overhead power transmission lines. In this paper, an equivalent distributed constant line model of a transmission tower is developed. The model consists of three parts: main legs; bracings; and crossarms. The surge impedance of each part is expressed by the functions of their dimensions and geometry. This tower model is applied to a 500 kV transmission tower whose surge performance characteristics are measured. It is found that the tower voltage wave shapes, calculated from this model, closely agree with the measured ones. This proves that the authors' proposed tower model simulates well the surge performances of actual transmission towers.

Modelling of a transmission tower for lightning-surge analysis

In prediction of HV and EHV lightning outage rates, the tower surge response consists of two parameters, the tower impedance and the tower travel time. Of the two, tower travel time has a larger effect on the predictions. A new model for tower travel time is developed based on the relationship between tower capacitance and tower surface area. Effects of tower crossarms and other features are modelled correctly by the new theory. When the new model is used, improved lightning outage rate prediction accuracy is obtained for 500kV single circuit lines.

Travel Time of Transmission Towers

This paper presents results of field testing on towers whose heights vary from 26 to 214 meters according to the author's new measuring method. The data of each tower were consistently in agreement. Surge characteristics of the towers were made clear by these measurements.

A. J. Schultz

Papers

Field Studies of the Surge Response of a 345-Kv Transmission Tower and Ground Wire