MVA Short Circuit from ETAP

[Nald]

Hi.

As per one example in ETAP, Utility Information is given below

MVA 3phase is 1500 MVA
KV L-L is 34.5kV
Current Short Circuit is 25.1kA
computed as MVA 3phase / (1.732*kV L-L)

for this one I have no problem. The problem that confuse me is the MVA L-G, as per data given below

MVA 1phase is 1640 MVA
Current Short Circuit is 27.4kA
computed as MVA 1phase / (1.732*kV L-L)

Now the question is as follows:
1. How come that the MVA 1phase is 1640 MVA? MVA 1phase is just MVA 3phase /3 and suppose to be 1500/3 = 500 MVA.
2. Why is the formula for single phase short circuit above has sq rt of 3 (this is for 3 phase only)?

Hope you could help me

[etapexpert]

Is this from ETAP example ? Take it just as a example, like any data.
If they are from vendor.clarify with them (also with us ;-) )

The formula for single phase is wrong.

[Nald]

Yes this is an Etap example. You mean the formula use by etap to calculate the short ckt for 1 phase is not correct? then why did engineers approved it in the first place? etap was been use in many years now with the same formula for 1 phase,

[surgeArrester]

Hi,

The values are correct. Getting the MVA short circuit whether in 3phase, single-to-ground, line-line, or double-line-to ground fault are calculated the same. Never forget that you are calculating the MVA in a three phase system, which is:

MVA(3phase) = sqrt(3) * current * line-line voltage

or

MVA(3phase) = 3*current*line-ground voltage


In my opinion, you are wrong in taking the idea of MVA(1phase) as a single phase. But, in ETAP, MVA(1phase) for GRID/UTILITY means the MVA(3phase) during a single line-to-ground fault.

We can also think of it like this, "Why would there be single phase if the Grid is three phase?". Get what I mean?

Check out the user guide of ETAP, I bet that you haven't read it. Go to section Power GRID and it reads like this:

MVAsc
Specify the short-circuit MVA for three-phase and single-phase (line-to-ground) faults.

Hope it helps. Please ask me if you do have other questions.

God bless!!

[etapexpert]

Nald,
I said the formula "computed as MVA 1phase / (1.732*kV L-L)" is wrong.
It shall be MVA 1phase / (kV* L-G) .