PLC (Power Line Carrier) is an old, established method for sending a Trip or Block signal over to the other side. It consists of using a carrier signal that rides on the main conductor along with the power flow. The information carried in the signal usually consists of a digital type of ON/OFF or TRIP signal. Since LD would require a much higher bandwidth of information, PLC cannot be used. PLC is being used less and less for new construction.Sorry, I meant LD. :ashamed1: Although I did not know PLC does not support LD. I guess I should ask what is PLC is used for?
Correct, and if using a communication (POTT) scheme once zone 1 is cleared the remote terminal can be tripped by communicating that a breaker has opened tripping instantly afterwards eliminating the time delay in none communicating schemes. However, where LD comes in is easy of setting, application, and possible faster tripping of all involved breakers. No need to do extensive impedance calculations on fault current in relation to distance and guessing what values to plug in.
Good point! That should have been obvious now that I think about it. Though to be fair to myself every breaker and half I've seen had a breaker failure scheme.
Well, that depends on the utility. Some utilities will tap directly off of 115kv lines many times, while others loop the lines in and out of each substation leaving only 2 and 3 terminal 115kv lines. In this discussion we assume that all 115 and 230 kv lines loop in and out of each transmission to distribution substation.
However, that is not to say it can not be done. Could the summed currents not take into account a inverse time current curve? SEL offer relays that can integrate line taps into the differential application. Both SEL and GE have papers on it, though I can not find the SEL at the moment.
They do, often primary is LD with secondary as POTT or DCUB. But would it not be possible to have two redundant relays each with their own communication channel doing LD?
You are correct that the tapped loads off a line can be summed (or compensated for) but, for this to happen, comm must exist between the tapped subs back to the LD relays. This is not always practical due to economics or feasibility. However, for new construction (including tapped loads), this is more technically feasible but may not be economical. I am not aware of using overcurrents to integrate into the LD. As has been done for many years, if the tapped load is light enough, it is possible for the LD to be de-sensitized enough to allow a non-compensated tapped load. However, this has been debated by relay engineers for many years since most engineers don't like the idea of de-sensitizing an LD.
Also, I think you should think about why a 500 or 345 or 230kV application would have an LD for the primary and a POTT or DCB for the secondary when comm is available for both relays to the other side. I think it is typical for most relay engineers to avoid a thing called Common Mode Failure (CMF). In the case where you would have LD on both the primary and secondary relays, the potential exist for CMF to exist. It is always best to apply several diverse layers of protection for the best result.