An intra-bundle coaxial PLC system for HVDC transmission lines

Despite its limitations, Power Line Carrier (PLC) systems often provide the only cost effective form of communication for long distance point-to-point HVDC transmission systems in regions having little or no infrastructure. Innovations such as that described in this paper may reduce the number of repeater stations and thereby reduce cost and maintenance requirements. At high DC voltages bundles of up to eight conductors per pole provide a useful compromise between the need for high ampacity and moderate conductor surface gradient. Various ideas have been put forward for 'intra-bundle" methods of PLC propagation [1,2,3]. In the limit, since the optimum sub conductor count is generally not sharply defined, one conductor could be placed at the centre of the bundle to gain the advantages inherent in co-axial cables, i.e. low attenuation, low external coupling of the radio-frequency signal, and relative invulnerability to external noise (e.g. corona and atmospheric discharges). Since skin effect does not influence dc current distribution among conductors, the centre conductor in a "coaxial" conductor configuration serves both to carry its share of dc current and its radio frequency signal. RF attenuation for an intra-bundle system is shown to be roughly half that of a traditional open-wire system. The system would have orders of magnitude higher signal to noise ratio, would permit much higher rf signal levels than would an inter-conductor system and would achieve a major reduction in the need for repeater stations. The paper quantifies the above technical advantages and shows they extend down to as few as three conductors per bundle.