Figure 3
Upstream Contribution
Starting from the utility, combine MVAs writing each one above the arrows.
At Transformer 1:
MVAsc @ 33KV = 250 MVA
MVAsc @ 11KV = 1/ (1 / 250 + 1 /111.11) = 76.87 MVA
At Transformer 2:
MVAsc @ 11KV = 76.87 + 35.4 + 29.41 = 141.68 MVA
MVAsc @ 6.6KV = 1/ (1 / 141.68 + 1 / 71.43) = 47.49 MVA
At Transformer 3:
MVAsc @ 6.6KV = 47.49 + 40 = 87.49 MVA
MVAsc @ 400V = 1/ (1 / 87.49 + 1 / 33.33) = 24.14 MVA
At 400V Motors
Motor 3: MVAsc = 24.14 x 1.76 / ( 1.76 + 2.38 ) = 10.26 MVA
Motor 4: MVAsc = 24.14 x 2.38 / ( 1.76 + 2.38 ) = 13.88 MVA
Downstream Contribution
Starting from the bottom (400V Bus), I combined MVAs writing each one below the arrows. In this bus, the motor contribution to short circuit is the sum of the MVAs of the lumped motors Motor 3 and Motor 4.
At Transformer 3:
MVAsc @ 400V = 1.76 + 2.38 = 4.14 MVA
MVAsc @ 6.6KV = 1/ (1 / 4.14 + 1 / 33.33) = 3.68 MVA
At Transformer 2:
MVAsc @ 6.6KV = 3.68 + 40 = 43.68 MVA
MVAsc @ 11KV = 1/ (1 / 43.68 + 1 / 71.43) = 27.11 MVA
At Transformer 1:
MVAsc @ 11KV = 27.11 + 29.41 + 35.4 = 91.92 MVA
Note: Two downstream plus the generator contribution.
MVAsc @ 33KV = 1/ (1 / 91.92 + 1 /111.11) = 50.3 MVA
To determine the Faults Current at any bus on the power system, add the MVA values above and below the arrows. The sum should be the same on any branch.
Example:
11 KV Bus:
From Transformer 1: MVAsc = 76.87 + 91.92 = 168.79 MVA
From Generator : MVAsc = 35.4 + 133.39 = 168.79 MVA
From Transformer 2: MVAsc = 141.68 + 27.11 = 168.79MVA
From Motor 1: MVAsc = 139.38 + 29.41 = 168.79 MVA
This is a check that we have done the correct calculation.
Ifault @ 11KV = 168.79 / (1.732 x 11) = 8.86 kA
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how do you know that MVAsc utility is 250 MVA ?
@dian,
the 250MVA in this example is just assumed. Ask your utility for the actual fault level at your point of connection
Sir,
If my system is 4.16kv, is there any formula for BIL so I can specify the right cable BIL rating?
BIL is normally 3-4 times the KV rating of an equipment. Refer to standard to ensure you are using the right rating.
Can u pls put some details documents for eath and unearth cable.
can this method account for NGR grounded system say at 6.6kv level?
@rrphatak
.
Yes you can. The NGR is a resistor so it’s MVA will be
But remember that it does not contribute to the fault current but instead limits it.
Treatment of Resistor will be like a transmission line?
R will be dominant in the total Z.
So %/ MVA will be Z(R).100%/ Un^2.
In case of a single phase fault i shall add Z1+Z2 +Zo +3 * %/MVA of the NGR.
Is this treatment right?
paano po i compute ang MVAsc ng GEnerator at Motor 1
@norato:
Check these equations.
Hi, Mr Ver,
I have a doubt, how did you get MVAsc1 and MVAsc2 at 400V and 6.6kV?
Thanks for excellent work.
@Larroya, the MVAsc of each branch is inversely proportional to the MVAsc of each branch in the opposite direction.
Hello mr Ver
i want to asked about MVAsc Utility, can you sharing how to calculated ?
if i had different voltage little bit higher in 20kV, 66 kV or 115 kV, how much i must put MVAsc ?
@riza,
please refer to this article.
Thanks
thank you Mr Ver..
How did u calculate Downstream for generator. I can’t find out any supportive calculation for 133.32MVA as u mentioned in generator downstream calculated value.
@shozy
That is the sum of all currents towards the 11 kV bus of the generator (76.87+27.11+29.41)