Load Summary for double ended substation

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skeshesh

Senior Member
Location
Los Angeles, Ca
I'm putting together a load summary for a client who wants to obtain a city permit to add in some floor-standing distribution panels. There's going to be two 800A distribution panels added, both lightly loaded (100-150kVA each) with a few spare breakers for possible future expansion.
These panels will be fed from an existing double-ended unit substation with a normaly-open tie breaker. The bus is rated at 3000A and each end is tied in with a 3000A breaker to individual 2000/2666kVA, 4160V-480/277V, 3ph transformers fed from 4.16kV main switchgear.
The question is should I consider only the loads to one side of the tie breaker (the bus we'll be connecting the new load to) while still using the full 3000A capacity? It seems to me that for the new load to be allowable I need to look at the unit while the tie breaker is closed in case of one of the transformers failing/OCPD at secondary tripping, in order to consider the worst case. I ran it by one of the senior engineers and he said just do it with the tie normally open, and mumbled something about them having load shedding - but I havent seen anything mentioned on the as-built single lines I have... Any comment/advise is much appreciated.
 

ron

Senior Member
Is it an automated M-T-M transition or manual. If it is automated, then you must consider worst case with tie closed. If it is manual, then you can talk with the client about manually load shedding, but be sure you have sufficient meters in the right places, that they can make that decision if needed.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Is it an automated M-T-M transition or manual. If it is automated, then you must consider worst case with tie closed. If it is manual, then you can talk with the client about manually load shedding, but be sure you have sufficient meters in the right places, that they can make that decision if needed.

It's automated so that if either main breaker for each end fails the tie which is normally open transitions to closed. I kind of agree with you, but I still fail to see if they have some sort of automatic (or even manual means of) load shedding, how it would allow for me to consider the load summary with a normally open tie breaker?
 

ron

Senior Member
When the tie is open, does one source serve half the load and the other is fed by the other source?
It is only during a transition, that the worst case load will be carried by the 3000A mains.
If some of the load is shed, then the load is less than if it wasn't shed.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
You're right, but that's in case they have PLCs and appropriate circuit breakers for automatic load shedding before the transition. I'm afraid I haven't seen evidence of that, and a lot of facilities have manual load shedding. But I guess the point is clear anyway - as I thought there's no justification otherwise for not counting the load on the other bus since worst case is in fact when the tie is closed.
 
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