Communication systems using bundle conductor overhead power lines

Abstract

The main properties of power line carrier (p.l.c.) systems, used conventionally for internal communication purposes in electricity supply undertakings, are examined, and an alternative method for use on bundle-conductor lines is proposed. Earlier work, departing from the use of conventional phase-ground or interphase carrier, proposed the electrical separation of the subconductors of a bundle for signal frequencies, but retained the remaining aspects of conventional p.l.c. These include particularly the expensive filter components required to limit the signal, as far as possible, to the desired parts of the power network, together with the frequency and bandwidth limitation imposed on the signal by these filters. The intrabundle communication system proposed uses a simple, inexpensive but highly efficient broadband coupling method which permits signal-isolation between subconductors without inserting additional impedance at power frequency or reducing the power transmission capability of the line. The signal characteristics of the new system are such that interference is reduced by orders of magnitude in comparison to p.l.c., avoiding need for frequency assignment even between adjacent phases. A bandwidth of 1.6 MHz is available. This may be exploited in a variety of ways, including the transmission of data at faster rates, to enhance the reliability and speed of protection signalling, or to provide up to 180 duplex voice channels. Even where one voice channel only is required, the cost of an intrabundle communication system is estimated to be less than that of a p.l.c, system for route lengths of at least 29 km, and allows subsequent increase in the voice channel numbers at low extra cost.