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Posted by Ed, KI6R [ ki6rcm98@gmail.com ] on January 19, 2025 at 16:18:38.
In Reply to: Re: LED driver frequencies in LW bands? posted by John Davis on January 15, 2025 at 21:54:10.
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PLC systems are interesting. I've done field maintenance and testing on several different systems in the 1990s on 500kV transmission lines. PLC is very good at protecting HV transmission lines in remote areas, especially where microwave or network communications take a circuitous route and have long group delay. A system will typically signal from HV station to HV station (sometimes a reactor station) to provide end-to-end over-current or fault protection. The older systems are usually two carriers that are frequency shifted to indicate a fault. The carrier is placed on one or two phases of the transmission line. A fault signal will result in the HV circuit breaker opening as fast as possible at the distant end. Far end opening can happen as fast as 20ms or so after an event. The carriers are transmitting at all times. When a fault occurs they usually boost from 10W to perhaps 100W during the frequency shift. I've seen SSB available to the distant end via a handset. End-to-end ringing is done by transmitting a telephone ring. One ring for the first station, two rings for the next etc. The SSB was a comm link of last resort if the telephones were down. All this description regards PLC systems designed in the 1970s using discrete transistors. I'd guess this technology has since been replaced with digital transmission line protection systems. It is difficult to match the speed of PLC end-to-end fault clearing between stations many miles apart. One could find an old PLC system by Westinghouse or General Electric and make a very good VLF transmitter. Most were crystal controlled and could tune from about 100 kHz - 250 kHz.
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