Power Line Communication (PLC) over high or medium voltage power lines can be used to transmit specific (tele-)protection commands over channels of small bandwidth, for example a bandwidth of 2 kHz located at the low end of the frequency band available for power line communication services. Typically, older PLC equipment is provided with a limited bandwidth of 2 kHz for transmitting command signals, used for coding (tele-)protection commands, as well as a guard frequency. Modern PLC equipment, on the other hand, offers typically a bandwidth of 4 kHz; with the guard frequency being preferably located in the upper half of this available frequency band. The guard frequency can be shifted within the upper half; however, moving the guard frequency to the lower half would require hardware filters to be changed, what is to be avoided. In existing networks, frequencies adjacent to the formerly used 2 kHz band are often occupied by other services. Consequently, in this case the modern PLC equipment cannot be used as if the network were designed from scratch. Nevertheless, even in new designs of PLC networks having no historical restraints and having a bandwidth of 40-500 kHz reserved for protection applications, it is generally advantageous not to waste bandwidth but to use 4 kHz PLC equipment and save the remaining bandwidth for other purposes.
USPTO Backgrounds
USPTO Backgrounds