Single and Three Phase MVA
Single and Three Phase MVA
(OP)
Hi,...
This is my second. question on this forum. I am happy with my first question response.
Thanks to all who contributed.
# 1 : I am using ETAP for a short circuit calcs. I am pluging the info from utility.
As per the help file,
3 Phase Short Circuit MVA= sqrt(3) * V(L-L)* Ifault(3 Phase)
This makes sense to me;
Now
Single phase short circuit MVA= sqrt(3)*V(L-L)*ifault(1-Phase ground fault)
It did not make sense to me. I spoke to the tech support but looks like these people are more trained for software using issues and cannot answer any theortical issues like this.
Could you advise?
#2 After three phase analysis, I will be looking in to ground fault for grounding design;
Fotunately or unfotunately, the ground fault current is zero:
The transformer can be fed from two feeders:
So I have to consider the worst case contribution of ground fault for grid design. Either one of the feeders will feed at one time.
Feeder #1 is fed from Delta/Delta. That means no ground fault on my transformer (Delta/Star grounded) primary.
Feeder#2 is fed from Star(open)/Star(solidly grounded).
Again no ground fault current, I guess zero sequence for an ungrounded star does not lead to ground fault.
Now what value do I use for my ground fault ?
Here is the catch, when I do a double line to ground fault(on my primary), it gives me ground fault. Why is that ?
Thanks in anticipation for your effort.
This is my second. question on this forum. I am happy with my first question response.
Thanks to all who contributed.
# 1 : I am using ETAP for a short circuit calcs. I am pluging the info from utility.
As per the help file,
3 Phase Short Circuit MVA= sqrt(3) * V(L-L)* Ifault(3 Phase)
This makes sense to me;
Now
Single phase short circuit MVA= sqrt(3)*V(L-L)*ifault(1-Phase ground fault)
It did not make sense to me. I spoke to the tech support but looks like these people are more trained for software using issues and cannot answer any theortical issues like this.
Could you advise?
#2 After three phase analysis, I will be looking in to ground fault for grounding design;
Fotunately or unfotunately, the ground fault current is zero:
The transformer can be fed from two feeders:
So I have to consider the worst case contribution of ground fault for grid design. Either one of the feeders will feed at one time.
Feeder #1 is fed from Delta/Delta. That means no ground fault on my transformer (Delta/Star grounded) primary.
Feeder#2 is fed from Star(open)/Star(solidly grounded).
Again no ground fault current, I guess zero sequence for an ungrounded star does not lead to ground fault.
Now what value do I use for my ground fault ?
Here is the catch, when I do a double line to ground fault(on my primary), it gives me ground fault. Why is that ?
Thanks in anticipation for your effort.






RE: Single and Three Phase MVA
As for #2, if there is no zero sequence path, there is no ground fault current.
RE: Single and Three Phase MVA
dpc,
Q#1, I dont have much experience but I have never seen, sqrt(3) and V(LL) in any expression or formula in EE that talks about single phas entities, so how it can be justfied in this case. But if it an exception and everybody has accepted in industry than I will not question.
Q#2, I looked at my power system text as they are still fresh in my desk... did not graduate that long ago, I agree with your statement then How there is current flow for a double line to ground fault ?
If a ground fault current can flow in Double L-G fault, how come a open star primary is allowing zero sequencing
current flow?
\
Should
RE: Single and Three Phase MVA
RE: Single and Three Phase MVA
RE: Single and Three Phase MVA
RE: Single and Three Phase MVA
If I understand correctly, then you calculate SLG current as:
Ifault(SLG) = 3 Phase Short Circuit MVA /(sqrt(3) * V(L-L))
Is this correct?
RE: Single and Three Phase MVA
No, you cannot use the three phase fault MVA value. SLG fault current (and MVA)can be either more or less than the three phase fault value. Put SLG MVA in your formula and you've got it. Just remember that the SLG MVA is a pseudo three phase value.
RE: Single and Three Phase MVA
Any idea why they do this?
RE: Single and Three Phase MVA
Just convert it to amps and forget about it. I think you're overthinking this.
RE: Single and Three Phase MVA