CT reversal / CTs in series 1

[Mbrooke]

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.

 

[davidbeach]

As a single zone you'd just ignore the bus coupler.

 

[Mbrooke]

As in opening the bus coupler test switches or just switching from bus to bus as per usual? In "normal" mode the bus coupler will have an SEL351 that provides bi-directional over current for the reserve bus when not being used.

 

[davidbeach]

If the station is to be a single zone, using a 587Z, then including the coupler buys you nothing. You could do two zones but the CT switching would be a mess.

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.

 

[Mbrooke]

But my understanding is you need to include the coupler in a single zone if you want to trip the auxiliary bus for an aux bus fault, otherwise a fault on the aux clears the whole sub?

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.

 

[davidbeach]

If you're saying aux bus, that says to me Main & Transfer, but then your bus coupler would be replacing one of the normal breakers and the connection from one of the buses would be on the line side of breaker. The way it's shown it isn't a configuration that I've ever worked with, but I wouldn't try high impedance bus diff on anything other than the most straight forward simple bus configuration; and probably never at transmission voltage levels. Low impedance bus diff with a relay that understands switching and multiple zones seems so much more secure while providing the necessary level of dependability.

 

[Mbrooke]

Technically this design is known as double bus, single breaker and it is normally operated with two bus differential zones with half the circuits on bus one and the other circuits on bus two. Both zones are dynamically assigned based on disconnect position. A fault on one bus clears only half the circuits.


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.

 

[Mbrooke]

I also want to let you know I evaluated your idea of not including the bus coupler and it works flawlessly, however I would need to program an 8-10 cycle delay into the bus differential relay in order to allow the bus coupler to clear the reserve bus first during a fault.

 

[davidbeach]

What I was thinking was having two zones, plus the overall check zone. During switching you could have currents showing up in the wrong zone, as the switches and switch status contacts won't precisely align. With currents in the wrong zone, you'd have both zones showing a differential; tripping both zones wouldn't be particularly desirable. That's where the check zone comes in, it would determine if there's a fault within the substation. With misalignment during switching you could have both zones seeing differential current but check zone wouldn't. You'd trip zone 1 if there's a zone 1 differential and the check zone, likewise you'd trip zone 2 if there's a differential in zone 2 and the check zone.

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.

 

[Mbrooke]

Effectively a check zone is simply fusing both zone 1 and zone 2 together so switching can take place?


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?

 

[davidbeach]

Our preference. Others may come to other conclusions. With a low impedance bus diff it is very easy to keep track of the performance of each CT at all times. Not so easy with high impedance.

 

[Mbrooke]

Ok, that makes sense. I've seen high impedance up to 345kv provided over 200 volts threshold and the same CT ratios and class are used, but yes you are correct low impedance makes things easier in several regards.