Double-end buss

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Razzap

Member
In automotive manufacturing body shops, it is common to have the weld line fed from a dedicated 3-phase 480-volt buss, fed from each end. If you have for example, a 100kVA transformer at each end of the buss, is the total available 200kVA, even though each transformer protected at 100kVA?
Also, is the total power balanced equally between each transformer because they are tied together?
 

cpal

Senior Member
Location
MA
My Guess
Power should add

if both xformers feed the buss from the center they are paralled. total buss capacity should be 200kVA. I'm not sure how the phasing of the two paly but would be interested to see a post form some one with the info.
 

ohmhead

Senior Member
Location
ORLANDO FLA
In automotive manufacturing body shops, it is common to have the weld line fed from a dedicated 3-phase 480-volt buss, fed from each end. If you have for example, a 100kVA transformer at each end of the buss, is the total available 200kVA, even though each transformer protected at 100kVA?
Also, is the total power balanced equally between each transformer because they are tied together?

Well if you parallel transformers you get 200 kva at 480 volts are they hook in parallel ?

Why i ask is why there on each end of buss ?

If parallel yes
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100213-1905 EST

If all the load was at one end of this buss, then no. The capability would not be 200 KVA, but it would be more than 100 KVA. If the loads are uniformly distributed along the buss, then 200 KVA would be about right.

If both transformers are at the center of the buss and the buss impedance is substantial relative to the transformer internal impedance, then the source impedance seen at one buss end is individual transformer impedance divided by 2 plus the buss impedance of 1/2 the total line length.

If the transformers are at the ends of the buss, a load is at the center of the buss (1/2 the total length), then the source impedance for this centered load is 1/2 the transformer impedance plus 1/2 of the buss impedance of 1/2 of the total buss length.

.
 

Razzap

Member
Each transformer and its switchgear are located in a rooftop substation. The two subs are located on either end of the building, and the secondaries feed the buss, one substation at each end. I think they use two transformers to increase the load capacity, because one transformer at one end of the buss would be a huge monstrosity! As far as phasing is concerned, both transformer primaries are fed from the same service, so A is hooked to A, B to B, C to C. Whenever the buss needs to be shut down, BOTH switchgears must be turned off and locked out. I guess this kind of answers my own question, I just wondered if I was thinking correctly. Still wonder if the current both transformers are balanced. They are identical in kVA, impedance, voltage, etc... Having two transformers must also reduce the voltage drop, because the buss runs for 400 - 500 feet, with a huge multiple welder load on it.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Each transformer and its switchgear are located in a rooftop substation. The two subs are located on either end of the building, and the secondaries feed the buss, one substation at each end. I think they use two transformers to increase the load capacity, because one transformer at one end of the buss would be a huge monstrosity! As far as phasing is concerned, both transformer primaries are fed from the same service, so A is hooked to A, B to B, C to C. Whenever the buss needs to be shut down, BOTH switchgears must be turned off and locked out. I guess this kind of answers my own question, I just wondered if I was thinking correctly. Still wonder if the current both transformers are balanced. They are identical in kVA, impedance, voltage, etc... Having two transformers must also reduce the voltage drop, because the buss runs for 400 - 500 feet, with a huge multiple welder load on it.

Well Gar is correct in his example but yes your 200 kva in my book at 500 feet you dont have a voltage drop on a buss to worrie about in that size and lenght if there on each side what voltage drop the drop would be from your loads .
 

Cold Fusion

Senior Member
Location
way north
... Still wonder if the current both transformers are balanced. ...
With what you described, yes.

... Having two transformers must also reduce the voltage drop, because the buss runs for 400 - 500 feet, with a huge multiple welder load on it.
Probably a little, but not from the long bus, but rather each xfm is only carrying half the load, for the Vd on the xfm internal impedance is lower. As ohmhead said, there is rarely any significant Vd on the bus.

However, I don't think I have ever seen a double ended buss fed with 100kva xfms - why would one bother for 200kva. 1000kva transformers is common.

cf
 

zog

Senior Member
Location
Charlotte, NC
Each transformer and its switchgear are located in a rooftop substation. The two subs are located on either end of the building, and the secondaries feed the buss, one substation at each end. I think they use two transformers to increase the load capacity, because one transformer at one end of the buss would be a huge monstrosity! As far as phasing is concerned, both transformer primaries are fed from the same service, so A is hooked to A, B to B, C to C. Whenever the buss needs to be shut down, BOTH switchgears must be turned off and locked out. I guess this kind of answers my own question, I just wondered if I was thinking correctly. Still wonder if the current both transformers are balanced. They are identical in kVA, impedance, voltage, etc... Having two transformers must also reduce the voltage drop, because the buss runs for 400 - 500 feet, with a huge multiple welder load on it.

Chances are I have worked in your subs. All the big 3 have used this method in the past, but most have stopped. First, to answer your questions. Yes, they do stay pretty much balanced for loading on each sub, and the cleaner sine wave for a higher quality weld is why they do it. But every automotive manufacturing plant I have worked in (Nearly all of them) has had more like 3000kVA transformers on each end.

This presents a serious issue with available fault current on each welding bus, your fault current levels can easily exceed 100kVA which exceed the AIC rating of any LVCB. But, they can use fused breakers (AKRU, KDON, RLF, etc..) that can have 200kAIC's thanks to the fusing and single phase protection.

Recently, as these plants conduct arc flash studies and find Ei's that are way more than 40cal/cm2(Un-protectable) and arc flash boundaries that are 100 feet or more, the trend has been to split the busses for safety concerns.
 

WastefulMiser

Senior Member
Location
ANSI World
This presents a serious issue with available fault current on each welding bus, your fault current levels can easily exceed 100kVA which exceed the AIC rating of any LVCB. But, they can use fused breakers (AKRU, KDON, RLF, etc..) that can have 200kAIC's thanks to the fusing and single phase protection.

What about SQD's Masterpact? It is a 200kAIC breaker.
 
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