Transient Responses of Overhead Cables Due to Mode Transition in High Frequencies

This paper investigates the effect of mode transition from low-frequency transverse electromagnetic (earth-return wave) to high-frequency Sommerfeld-Goubau (surface wave) propagation on switching surges in gas-insulated buses (overhead cables) by adopting complete formulas for earth-return impedance and admittance using the extended transmission line (TL) approach. The lower attenuation of the surface wave shows a significant difference in the propagation functions evaluated by the classical TL approach with Carson/;Pollaczeck's impedance and space admittance. The lower attenuation in the high-frequency region leads to a spike-like voltage at the wavefront of a step response in time domain, and results in switching surges with frequency components higher than 10 MHz. This phenomenon explains the reason why measured transient voltages and currents in gas-insulated substations show frequency components ranging from some MHz to about 100 MHz, and the transient voltages and currents are sustained for more than a few microseconds. This phenomenon cannot be reproduced by the classical TL approach. It is also verified in this paper by numerical Laplace transform and by electro-magnetic transients program simulations using the extended TL approach.