Phase To Phase Infinite Bus

[I-learns]

Is the amount of fault current that can flow phase to phase different than phase to ground in a simple infinite bus calculation? Here's how I was doing it. It differs from a number of reputable publications that suggest instead to start with xfmr FLA and then divide by percent impedance. However it gets the same result as what those publications publish:

For a 2000 KVA xfmr with 5.75%Z 277/480Y
*Since the xfmr is a percent short circuited, I find the full short circuit KVA by dividing it by the impedance, then divide that by voltage to get amperage.
(XFMR VA/XFMR%Z)/(V-Phase*3)
(2,000,000/0.0575)/(277*3)
41856 amps

The same result could be achieved by first dividing total KVA by 3 (amount of KVA per phase winding) then dividing that by the percent impedance, then dividing that by the line voltage.
((XFMR VA/3)/XFMR%Z))/(V-Phase*3)
((2,000,000/3)/0.0575))/277

If the fault were phase to phase, would it be to multiply total KVA by 2/3 (KW for 2 phases) and then divide that by percent impedance then divide that by line to line voltage? It seems that way to me at the moment but if I tried to consider the L-G and L-l shorts instead as just resistive circuits in a 3 phase system, then it doesn't seem this way. We know I line and I phase are the same for a 3 phase load but when trying to think of a source short circuit scenario I am getting confused a bit. Would this be a L-L short circuit calculation:
(XFMR VA*(2/3))/XFMR%Z/(V L-L)
2000000*(2/3)/0.0575/480
48309 amps

 

[jim dungar]

Is the amount of fault current that can flow phase to phase different than phase to ground in a simple infinite bus calculation? Here's how I was doing it. It differs from a number of reputable publications that suggest instead to start with xfmr FLA and then divide by percent impedance. However it gets the same result as what those publications publish:

For a 2000 KVA xfmr with 5.75%Z 277/480Y
*Since the xfmr is a percent short circuited, I find the full short circuit KVA by dividing it by the impedance, then divide that by voltage to get amperage.
(XFMR VA/XFMR%Z)/(V-Phase*3)
(2,000,000/0.0575)/(277*3)
41856 amps

The same result could be achieved by first dividing total KVA by 3 (amount of KVA per phase winding) then dividing that by the percent impedance, then dividing that by the line voltage.
((XFMR VA/3)/XFMR%Z))/(V-Phase*3)
((2,000,000/3)/0.0575))/277

If the fault were phase to phase, would it be to multiply total KVA by 2/3 (KW for 2 phases) and then divide that by percent impedance then divide that by line to line voltage? It seems that way to me at the moment but if I tried to consider the L-G and L-l shorts instead as just resistive circuits in a 3 phase system, then it doesn't seem this way. We know I line and I phase are the same for a 3 phase load but when trying to think of a source short circuit scenario I am getting confused a bit. Would this be a L-L short circuit calculation:
(XFMR VA*(2/3))/XFMR%Z/(V L-L)
2000000*(2/3)/0.0575/480
48309 amps

In general Single Line to Ground (SLG) faults are more complicated to cslculated than are 3-phase (LLL) ones.

However the further away from the source, like the generator or large transformer, feeding the fault the simpler the SLG calculation becomes. After a point it is not even commonly calculated.

 

[I-learns]

In general Single Line to Ground (SLG) faults are more complicated to cslculated than are 3-phase (LLL) ones.

However the further away from the source, like the generator or large transformer, feeding the fault the simpler the SLG calculation becomes. After a point it is not even commonly calculated.

Yes I understand there could be complex factors that come into play and which would make the infinite bus calculation possibly in many cases overly conservative. Usually it’s only the engineers who are allowed to make decisions based on those more complicated calculations. However infinite bus is a simple calculation compared to some of those. And I’m really just wanting to understand it well more than anything else right now.

 

[jim dungar]

In my experience, the use of FLA and %Z is the likely the most common method. I can't remember the last time I use kVA for a quick infinite bus calculation.

SLG is important for utility and other MV systems. It is not regularly calculated for industrial or commercial locations even though some single phase switching may be involved.

 

[kwired]

Still a little complicated but take a center tapped single phase winding - you have half the length of winding so basic impedance is going to be less, but you also are dealing with half the voltage you would have across the full winding as well.

At the source terminals the available current is higher than line to line current, but as you mover farther down the load conductors the impedance of those conductors eventually gets to a point where line to neutral available current is less than line to line.

Don't get confused by arc flash incident energy calculations - those involve not only current but voltage and time to come up with total incident energy. You can have a pretty high available fault current but if response time by overcurrent device is fast you could have less incident energy than for a lesser fault current level over longer time.

 

[I-learns]

I’m not getting into the complications of arc flash type calculations. What I’m pointing out is that if you do the math and just analyze it as a simple three phase resistance, the current that would flow line to line is less than the current that would flow phase to phase because the resistance is doubled while the voltage is only increased by the root of three. However if you calculate it as the amount of short circuit kVA per phase and then divide that by the line to line voltage you get a higher amperage.