CT reversal / CTs in series
CT reversal / CTs in series
(OP)
Where only a single zone of protection is needed (Ie main and reserve) is it possible to protect a double bus bar substation with a standard high impedance relay (587Z)?
Under normal conditions the setup appears doable, but when when switching from on bus to another, the bus coupler CT is reversed in polarity; or a feeder CT ends up being in series with the bus coupler.
Under normal conditions the setup appears doable, but when when switching from on bus to another, the bus coupler CT is reversed in polarity; or a feeder CT ends up being in series with the bus coupler.






RE: CT reversal / CTs in series
RE: CT reversal / CTs in series
RE: CT reversal / CTs in series
Using the 487B, you could have have two zones that could accommodate that scheme. In that case I'd prefer CTs on both sides of the coupler, but if there's just the one I'd wire it in twice. That way one zone would have positive polarity and the other would have negative polarity. To keep the scheme stable during switching I'd add a check zone that doesn't include the coupler and only trip if the check zone shows a fault.
RE: CT reversal / CTs in series
I like the 487B idea, by the looks of it I can safely eliminate the 387 and let the 487B do over current and BF for the coupler.
RE: CT reversal / CTs in series
RE: CT reversal / CTs in series
In this case reliability requirements are low enough such that they do not require that level of selectivity. One bus is operated as the normal bus with all circuits normal connected to that while the other is simply a spare or reserve bus. Technically under normal operation there is only a single bus zone on the main bus. The reserve bus is protected via over current from the coupler and trips for a fault on the reserve bus.
Ultimately Id like to eliminate the complexity associated with two dynamic zones, but if it is a must I will go that route.
RE: CT reversal / CTs in series
RE: CT reversal / CTs in series
During switching, with no fault in the station, you'd see differential currents in both zones but not the check zone, no trip.
During switching, should an actual fault occur, you'd see differential currents in at least one of the zones and in the check zone, tripping of at least one zone and probably both would occur.
During normal operation, any fault should show up in only one of the zones and simultaneously in the check zone, tripping would occur with no appreciable delay.
I don't think you'd need separate overcurrents or any time delay on the bus coupler.
RE: CT reversal / CTs in series
This is my current game plan: Because this station has no MODs and no disconnect position switches on the bus disconnects, my plan is to implement a 487B, while having a 3 position selector switch which will basically give the option of all 5 elements on Bus 1 zone, all 5 elements on bus zone two, or bus zone one and two fused into a single zone. The 3rd option of combining zones is used to facilitate the necessary switching of bays from bus one to bus two and visa versa, while the other two are used so a fault on the reserve bus will clear the coupler via differential zone. Disconnect position switches would of course be ideal, but in my eyes the cost and complexity for such a little station is not worth it.
This is a bit off topic, but you mention that high impedance relaying should not be used as the 115 and 345kv levels even for a straight bus?
RE: CT reversal / CTs in series
RE: CT reversal / CTs in series