MODELING TALL TRANSMISSION TOWERS FOR PREDICTING OCCURRENCE OF THE BACKFLASHOVER IN POWER SYSTEMS

Abstract

This project proposes a new transmission line model that will be used in transient analysis resulting from lightning discharges in these metal structures. The model will consider the tall transmission towers in the Amazon forest in Brazil north region. Based on this model, the currents and voltages are obtained by using the Numerical Electromagnetic Code (NEC-2) and/or by ANSYS MAXWELL, at the top of the tower (in frequency domain). Thus, it is possible to obtain the impedance of the tower "view" from the top of the structure, called surge impedance tower. This surge impedance is approximated by a rational function which can be represented in the time domain, as an association of resistors, capacitors and inductors. With the electric circuit, the current and voltages in any part can be written by state equations and solved by numerical methods in any computational language. Once with this representation, it can be inserted in the main software tools used for the simulation electromagnetic transients such as ATP, EMTP or PSCAD. The advantage of this model is that it takes into account the distribution of current in the various metal segments that constitutes the transmission tower, beyond the inclusion of the cross arms and slant bars. Thus, the model will estimated more precisely the level of surges resulting from lightning that strikes the transmission towers; consequently it can be predict the occurrence of failures in insulators strings due to backflashovers and reduces the outages in power systems.