Lightning Wave Propagation Characteristics of an Overhead Line with Insulated Phase Conductors

Compact overhead distribution lines are composed of insulated phase conductors and bare grounded cables. The use of an insulating layer allows a greater proximity between phase conductors and modifies the per-unit-length matrix and propagation characteristics associated with the line. However, it is not clear whether it is possible to accurately calculate its per-unit-length capacitance matrix using simplified formulae available in the literature. Also, it is unknown to what extent the propagation characteristics of the line will be affected. This paper makes an attempt to clarify these aspects by using a rigorous formulation based on the Method of Moments for calculating the external parameters of a three-phase compact distribution line. The influence of considering covered phase conductors on the transient response of the line is also investigated by considering the injection of a typical lightning current at one of the line terminations. The analysis is performed using the Finite-Difference Time-Domain (FDTD) method. It concluded that considering insulated phase conductors reduces the associated phase velocity so that it approaches a value that is lower than the speed of light at high frequencies. Consequently, a distortion is observed on the tail of the calculated voltage waveforms due to a relative increase in the propagation time associated with the line, but with minimal effect on the calculated peak voltages. It is also shown that the proximity effect has minimal influence on the calculated per-unit-length capacitance matrix.