Short Line (around 2 km) Protection

[bagbesh]

Hey,

I would like to know if there would be any technical problem in protecting a 115 kV short overhead line (around 2 km) with distance relay and over current as back up.

The line is dual circuits between a ring substation and 2 outdoor type SF6 circuit breakers feeding 2 step-down transformers 115kV / 13.8 kV vector group DY1 with 400A NGR. The bus tie on the 115 kV side is normally closed while the bus tie on the 13.8 kV side is normally open.

I know that line differential would be better but this is the design we have and the project would be delayed significantly if we would have to change the scheme to differential. I just would like to know if distance would do? Because some are skeptic that the line is too short to be protected by distance relay.

Thanks

 

[DTR2011]

There was a similar discussion here:

http://eng-tips.com/viewthread.cfm?qid=319845

 

[bagbesh]

Where?

There is no link in your post!

 

[DTR2011]

Try again

http://eng-tips.com/viewthread.cfm?qid=319845

 

[cranky108]

do the lines terminate in transformers? Or is there a tie between the two lines?
If there is no tie, but terminated in transformers, then there would be no problems,

 

[bagbesh]

The only tie between the lines is in the sending end substation.

There is no tie in the receiving end substation. The lines are directly terminated in the transformers via outdoor SF6 type circuit breakers.

The tie on the secondary side of the transformers is normally open.

So I guess the installation is ok.

The proposal is to set zone 1 to be over reaching and to delay it a bit to overcome any transients in the system.

The lines are treated as radial feeders.

 

[cranky108]

I assume that means you only have one zone available.
A transformer differential can be used to clear the transformer quickly.

The problem with an over reaching zone is there either needs communications, time delay, or expect a coordination problem. For either a line or transformer fault the same customers will be out, so while it won't matter, operations people will expect coordition. Or you could try to convence them it dosen't matter, but that never worked for me.

Beware that for the low side tied, there may be an issue of a fault feed through the secondary to a high side fault. So it should be set up to operate the remote end of the lines for a line (transformer goes without saying)fault. This again can be with communications, or directional relaying.
I just believe what can be done by operations, will be done. So plan for it.

 

[isquaredr]

We have many sites supplied in this way with the lower voltage side tied. The trick is to set the distance relay to look into the transformer and not beyond. Transformer diff, OC, BEF & REF will look after your TX via local and remote end intertripping with DOC as backup should the intertripping comms go down.

 

[isquaredr]

Forgot to mention zone 1 set to 0s with Zone 2 & 3 set to 400ms / 800ms respectively, all zones have the same distance setting, therefore zones 2 & 3 act as backup hitting the trip relay from different output relays within the distance relay.

 

[davidbeach]

Our practice with radial lines is to have the instantaneous distance element reach into the furthest distribution transformer, with the expectation that the transformer will be tripping at the same time and gone when the line recloses. That way we can minimize the duration of the voltage sag for the rest of our customers.

 

[cranky108]

David, interesting. But some of us have operations people who get confused very easy. How did you train your operations monkeys, I mean people?

 

[davidbeach]

Transformer faults are very rare. Overreach, trip, and reclose is far more common, but we are attempting to work as many of those out of the system as possible.