What is busbar OC and EF

[veritas]

Hi

I am familiar with high and low z buszone schemes but have not come accross busbar overcurrent and earthfault scheme yet. I have a situation where 3 x 80MVA trfrs feed onto a 33kV bus, the bus being split into 3 sections (two bus-section breakers). Besides high z buszone protection being used there is also 3 x bus OC and EF with IDMT settings. See attached sketch.

Could someone perhaps explain how this is supposed to work and what it needs to grade with please? I assume, for example, that CT1 and CT3 currents are added???

Thanks in advance.

 

[dpc]

Looks like what we used to call a partial differential scheme. It provides selectivity between the bus segments for bus faults when the tie breaker is closed.



David Castor

http://www.cvoes.com

 

[ScottyUK]

Agree - it's a partial diff scheme.

The CTs are arranged so current leaving the bus at a coupler is vectorially subtracted from the current entering the bus at the incomer. In the event of a through-fault the magnitude of the current into the relay is the difference between the two CT currents. In the event of a busbar fault the CT currents add and the relay operates quickly.

It's a relatively cheap scheme for clients who can't justify a busbar differential scheme, perhaps on cost or criticality grounds. It's unlikely to be the first choice of a protection engineer, but is far better than just a plain OC/EF scheme.


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If we learn from our mistakes I'm getting a great education!

 

[MYTE]

thanx all for explanation, this is news to me.
i am familiar with another form called "stuck breaker prot." which is the same as the attachment in addition to CT of each incomer per busbar.
are they the same??

 

[veritas]

Thanks for all the info so far.

Scotty UK - why does the customer who use this scheme then call it a Bus OC and Bus EF and uses a SPAJ140C?

Thanks.

Veritas

 

[ScottyUK]

Hi veritas,

I guess that's just down to whatever the client wants to call it, or whatever the designer called it. I've only ever seen it described as a partial bus diff, but there's a whole lot of stuff I haven't seen!

MYTE,

It's not a conventional breaker fail scheme. Have you got an example you could share?


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If we learn from our mistakes I'm getting a great education!

 

[veritas]

Scotty

This is still puzzling. Let's say this partial diff scheme has the currents subtracting with normal throughload and adding with an inzone fault as you put it earlier.

Then why does it have an IDMT characteristic and equally important why do the outgoing feeders have to grade with this bus OC? I mean under a throughfault the partial diff scheme should see minimal current as the CT currents subtract. What would the feeder then grade with?

Also, the is no OC on the HV or LV side of the incoming trfrs. The customer just said the feeders need to grade with the BUS OC and EF. He himself does not know why - he's not a protection engineer.

Finally, how is this Bus OC and EF relay connected? In series with the CT circuits (like in a voltage balance scheme) or in shunt like in a normal biased or unbiased diff?

Thanks once again. all contributions are sincerely appreciated.

 

[dpc]

Summation of CTs into standard inverse time OC relays used to be quite common due to the expense of relays. If you have a copy of Blackburn's Protective Relaying book, I believe these schemes are described in there somewhere. Think of it as a more selective overcurrent relay scheme, not a differential scheme.

I would imagine that the "Art and Science of Protective Relaying" book that is available on-line also discusses partial differential schemes.



David Castor

http://www.cvoes.com

 

[MYTE]

AGAIN THANX ALL,
SCOTTYUK i am attaching a draft as an example, we sometimes use it for 13.8kV systems, as a backup for disconnecting a bar on which one of the outgoing feeders CB failed to clear fault. thus it is also graded with Trafo LV side oc and ef relays.
also take into consideration, that despite the fact that our bar doesnt have busbar diff prot, this scheme is not intended to be instead as it is IDMT.

 

[ScottyUK]

Ok, I can see how that works now. In the event of a breaker failing to clear, the partial bus diff effectively treats the fault on the outgoing feeder as an in-zone fault. That's something which a true bar differential wouldn't help with.

Thanks.


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If we learn from our mistakes I'm getting a great education!