Circuit Model of Vertical Double-Circuit Transmission Tower and Line for Lightning Surge Analysis Considering TEM-mode Formation

The electromagnetic fields, which are produced by lightning currents flowing into a transmission tower and into overhead ground wires, spherically expand with time, and propagate as non-transverse electromagnetic (TEM) waves at least within the round trip time of the traveling wave along the tower. In this paper, a distributed-parameter line model of a vertical double-circuit transmission tower with an overhead line struck by lightning is composed considering the non-TEM characteristics. The surge impedances of the tower, ground wires, and phase conductors increase with time and reach their TEM-mode or steady state values with time constants determined from the configuration. In addition, the significant wave attenuation along the tower is included in the circuit model according to the propagation characteristic determined by the tower configuration. The virtual inductive grounding impedance or damping circuits for approximately representing the wave attenuation is no longer needed. The proposed model is applied to four types of transmission towers. Waveforms of voltages across insulator strings computed by the finite-difference time-domain method, which are regarded as the reference, are well reproduced by the proposed model. Computation efficiency is drastically improved by the proposed model comparing to the numerical electromagnetic field analysis.