Single and Three Phase MVA

[Inder76]

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.

 

[dpc]

As for #1, the equation you give is correct for SLG fault duty. That is just the way the fault MVA is defined.

As for #2, if there is no zero sequence path, there is no ground fault current.

 

[Inder76]

Thanks for your reply.

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

 

[jghrist]

sqrt(3)*V(L-L)*ifault(1-Phase ground fault) is the same as 3*V(L-G)*ifault(1-Phase ground fault). Does stating it this way make more sense to you?

 

[dpc]

I don't think there is ground fault current, just phase current. Zero sequence current should be zero, but you will have positive and negative sequence current.

 

[stevenal]

I prefer to use current rather than MVA for describing fault levels, since you can directly look at fuse and relay curves. When using MVA for SLG faults, though, the tradition is to state the level as 3* the actual SLG value so that it can be compared to 3 phase faults. Sort of like you had three simultaneous SLG faults, one per phase, but with no current cancelling.

 

[Eleceng01]

stevenal,

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?

 

[stevenal]

Eleceng01,

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.

 

[Eleceng01]

Sorry, my typo - so what the utility gives you is actaully a psuedo MVAsc value for SLG then, huh?

Any idea why they do this?

 

[dpc]

Describing fault duty in MVA was much more common in the past when circuit breakers were actually rated in MVA. The SLG MVA was just a way of describing it using the same variables used for three-phase MVA.

Just convert it to amps and forget about it. I think you're overthinking this.

 

[stevenal]

I give out current values, since this is generally how breakers, panels, etc. are rated. I believe the use of short circuit MVA is old school, and that older equipment was rated this way. To stay within the rating, make sure the SLG and three phase available MVA is less than the equipment rating. In this way a single equipment rating can be used, and the division by three to find a per pole rating is avoided.