25kV Feeder breaker U/F prot.

[Sn00ze]

I am adding a switchgear with 4 25kV feeder breakers and Transformer. The existing 25kV Bus had 4 feeder breakers. We are using one of the breakers as a Tie to connect to the new addition.

my question is:
1. there's an existing U/F relay for the feeder breakers, but i am not sure if the TIE would require to be connected to the U/F relay, this TIE is N.O.

Client says U/F is for feeders themselves usually so he is saying no to connecting the tie. Is this correct? and why? wouldn't you want to protect the TIE either way? It does look like the TIE will never be N.C.

Follow up question, would my new feeder breakers have to be tripped by this existing U/F prot? or would we need a new one?

2.- About TX protection; do I need a 50NN function on my relay for a NGR? or is this only when it is high impedance?

Silly follow up question, how do i know if the NGR is high impedance? I don't actually have a lot of documentation on the NGR other than it is dry-type air core NGR 150A continuous current for 27.5kV.


I would appreciate some input.

Thanks

 

[DTR2011]

NGR sounds low impedance, by amp rating.

For UFLS, these are generally selective items. Some feeders (hospitals, etc) may not have any tripping, while other loads will.

UFLS is not protecting any equipment in the substation, it is protecting the system -generation / load balance.

There are generally studies performed / commercial agreements with customers on connected load.

I just put into service today 3 13kV feeders with UFLS. Each new feeder had its own Schweitzer relay, with its own UF Pick up setting. I found the relay logic for UFLS did not block reclosing (corrected), so watch out for these types of things.

Older schemes, may have a single UF relaying scheme tripping a lock out relay, connected to each feeder (with the possibility to open/disable tripping via a switch or link). The LOR will also block reclosing after a load shedding event.

I would not trip the BT breaker, as there are many emergency scenarios where one would want the BT to close (loss of XFMR, etc). Trip / block the individual feeders.

 

[Sn00ze]

Thanks for your reply DTR2011.

I had a meeting with the client to talk these issues out. You are correct. It sounds like we need input from the Transmission line guys to tell us if we need U/F for feeders or not. And since, as you say, UFLS is not for protecting substation equipment but rather system / load balance, it makes sense that the TIE would not need U/F. The old feeder breakers had a centralized U/F relay but now each line has a SEL-351S with 81 function. So, i imagine this U/F relay will be obsolete in the near future.

Thanks for the heads up on the block reclose. The client likes doing the block close though hardwire. from the transformer protection's LOR.

Just to keep this thread up to date. I found out more about the NGR, it is 1ohm. and 4.7kA for 10secs. Which would be low impedance and would not require 50NN according to the settings engineer. Which, explains why you wouldn't use instantaneous tripping.

 

[DTR2011]

The NGR may have a time delayed OC element (as a back-up), if so equipped with a CT.

"The client likes doing the block close though hardwire. from the transformer protection's LOR."

I would ask for more information on this. 86T should be for faults specific to its zone of protection.

Block Close / Block Reclose (Drive to Lockout) are different, but similar items. Block close is a hardwired scheme, generally from "b" contacts on a lock out relay - this is equipment protection related. Block Reclose (Drive to Lockout) can be implemented via hardwire, however I believe it is much better to implement in the SEL relay logic. SEL relays have a setting called ~ 79DTL where the 81U element can be added.

I'm guessing this is a utility application (with overhead lines), therefore reclosing function would be normal. If it is an industrial facility with on-site generation, its a different story. Critical loads need to be identified.

 

[Sn00ze]

Yes i am sorry i didn't explain it very well now that i am re-reading it.

Yes block close is hardwired but there is also settings/logic to block 79.

No on-site generation just a substation.