Buss differntial

[Bugman1400]

Looking back at the OL, I do not see the CTs for the mains. Can I assume they are in the xfmr? Secondly, what is the operating procedure for taking a feeder breaker out of service? Is the load tied to the xfer bus and served from the middle breaker or is the feeder load tied to the xfer bus and then fed from another feeder breaker? If the middle breaker is used then, you should be able to set up two bus diff zones. The CT from the middle breaker will need to be wired to both bus diff relays with an Enable/Disable switch that turns off the middle breaker CT input to the relay that is not switched to the middle breaker. This would even allow the middle breaker to be sourced from one xfmr while supplying load from the other bus (via xfer bus).

 

[mbrooke]

Looking back at the OL, I do not see the CTs for the mains. Can I assume they are in the xfmr?

They are in the individual bay breakers, on the bushings the like this:

http://www.gpdgroup.com/wp-content/uploads/2014/01/SenecavilleSub1.jpg

Secondly, what is the operating procedure for taking a feeder breaker out of service?

When a breaker needs to be taken out of service, the normally open air break connected to the transfer buss is closed, and the transfer buss coupler breaker then has its relaying changed to that of the breaker being taken out of service. The breaker to be taken from service is then opened, (this step is skipped if stuck) and then the air breakers (isolators) are opened on each side of the breaker. When the breaker is ready to be put back into service the exact opposite switching procedure takes place. The switching procedure is such that the air breaks (idolators) never break load directly, rather simply break current in parallel with another switch. Because of this parallel action the voltage across the switch is under 1kv allowing the current to be interrupted without issue. Of note, the switches are specked as having no arc whiskers since (my understanding) that high current low voltage loop splitting can damage them.

Is the load tied to the xfer bus and served from the middle breaker or is the feeder load tied to the xfer bus and then fed from another feeder breaker?

The middle breaker under normal conditions feeds no load. Its purpose is to simply take the place of any other breaker when a particular bay needs to be serviced without interrupting the line. It is run normally closed and when not taking the place of another breaker its relaying is a low pickup, definite time zero sequence curve. The only purpose in doing this is to indicate when or if the transfer buss has faulted. Running the transfer bus normally de-energized is considered poor practice in that a fault would go unnoticed until the buss is needed down the road.

If the middle breaker is used then, you should be able to set up two bus diff zones. The CT from the middle breaker will need to be wired to both bus diff relays with an Enable/Disable switch that turns off the middle breaker CT input to the relay that is not switched to the middle breaker.

Middle breaker can end up on either side of the main buss when the main buss is split.

This would even allow the middle breaker to be sourced from one xfmr while supplying load from the other bus (via xfer bus).

Middle breaker is only for the transfer buss. Let me know if I am describing this correctly.

 

[JoeStillman]

Can the tie point skip the CT (not the transfer buss coupler but the "hls")? The transfer buss will simply have a very low zero sequence pickup when not in use, and when in use carry either line or transformer protection.

Yes, the tie could be left out of the 87 relay scheme. But if you put it in, it gives you an extra level of selectivity when it's closed.

 

[mbrooke]

Yes, the tie could be left out of the 87 relay scheme. But if you put it in, it gives you an extra level of selectivity when it's closed.

Id agree, but the "HLS" air breaks can not interrupt fault current. A buss fault must trip all breakers. The only advantage of the CT is determining which side of the buss faulted.

 

[JoeStillman]

I could in theory get away with one duss differential zone since if all the breakers are tripped on one side and the buss mid point is opened, all the CTs will read zero on the tripped breakers. But if one of those breakers is serviced, that could cause the relaying to mis-operate?

You could do this and a lot more with any of the modern electronic relays.

 

[mbrooke]

You could do this and a lot more with any of the modern electronic relays.

So two controls (primary + backup) will do the job instead of 4?


Here is the scenario I envision. A buss fault occurs west or east side of the buss, in this case Ill randomly pick west side. All breakers trip clearing the buss. Then a field crew arrives at the substation to check out the problem. The west side is determined to be faulted, say because of bad bus VT. The HLS (high line side) is opened, and then the breakers on the East side are closed. Half the station (east side) is now energized. Work is performed on the west side as needed. Several hours latter the HLS is closed and the west side breaker is closed. What would the relating procedure be for something like this?

 

[Bugman1400]

Id agree, but the "HLS" air breaks can not interrupt fault current. A buss fault must trip all breakers. The only advantage of the CT is determining which side of the buss faulted.

I don't see how you can eliminate the bus xfer bkr from the bus diff scheme. I've heard of desensitizing the diff to allow for small loads like station service but, not feeder load. I still think my scheme of wiring the xfer bkr CTs to each bus diff scheme and then using a toggle switch wired to the diff relay to indicate which bus the xfer bkr is fed from is best. This would also allow faults on the xfer bus to be cleared by the xfer bkr relay instead of the bus diff relay. In all cases you will have to account for all the current flowing through the mains.

 

[mbrooke]

I don't see how you can eliminate the bus xfer bkr from the bus diff scheme. I've heard of desensitizing the diff to allow for small loads like station service but, not feeder load. I still think my scheme of wiring the xfer bkr CTs to each bus diff scheme and then using a toggle switch wired to the diff relay to indicate which bus the xfer bkr is fed from is best. This would also allow faults on the xfer bus to be cleared by the xfer bkr relay instead of the bus diff relay. In all cases you will have to account for all the current flowing through the mains.

There is only on buss carrying load (the main buss) where the transformer feeds in and the outgoing circuits then feed out. Because multiple transformers feed the main buss under normal operation, IMO buss differntial is best. What gather is I need a system where CT inputs can selectively be ignored.

 

[Bugman1400]

There is only on buss carrying load (the main buss) where the transformer feeds in and the outgoing circuits then feed out. Because multiple transformers feed the main buss under normal operation, IMO buss differntial is best. What gather is I need a system where CT inputs can selectively be ignored.

Yes, an SEL-487B relay is a bit pricey but, can handle this with no problem. Many utilities are already using this relay as part of their standard bus diff scheme.

 

[mbrooke]

Yes, an SEL-487B relay is a bit pricey but, can handle this with no problem. Many utilities are already using this relay as part of their standard bus diff scheme.

What is the other option (though this will be considered) to selective input of CTs? Also, (and Im sure its silly to ask), the SEL-487 and ignore/acknowledge CT inputs via SCADA command?

 

[iwire]

As a side note, it is bus, not buss.

 

[Bugman1400]

What is the other option (though this will be considered) to selective input of CTs? Also, (and Im sure its silly to ask), the SEL-487 and ignore/acknowledge CT inputs via SCADA command?

Yes, you can set up the logic in the SEL-487B to change bus zones (via CT selection) via SCADA control. You will have to set up your SCADA HMI to show the active bus zone. I think that is easy to do. The other alternative is to have a single bus zone that wraps both buses. This would allow the xfer bkr to be fed from either mains.

 

[mbrooke]

Yes, you can set up the logic in the SEL-487B to change bus zones (via CT selection) via SCADA control. You will have to set up your SCADA HMI to show the active bus zone. I think that is easy to do. The other alternative is to have a single bus zone that wraps both buses. This would allow the xfer bkr to be fed from either mains.

Sounds good so far. Can you explain further what you mean by having a single bus zone wrapping both buses?