A Short Circuit Study is an important tool in determining the ratings of electrical equipment to be installed in a project. It is also used as a basis in setting protection devices. Computer software simplifies this process however, in cases where it is not available, alternative methods should be used. The per-unit and ohmic method are very tedious manual calculation. These hand calculations are very prone to errors due to so many conversion required. In per unit, base conversion is a normal part of the calculation method while in ohmic method, complex entities conversion.The easy way to do hand calculation is the MVA method.
In this example, we shall be presenting a short circuit study of a power system. Motors are are already lumped with ratings 37kW and below assigned an impedance value of 25% while larger motors are 17%. A 4MVA generator is also included into the system to augment the utility.
Figure 1
Utility: 33KV, 250 MVAsc
Transformer 1: 10 MVA, 33/11KV, 9% Z
11KV Bus
Generator: 3MVA, X"d = 0.113
Transformer 2: 5 MVA, 11/6.6KV, 7% Z
Motor 1: 5MVA (Lumped), 17% Z
6.6KV Bus
Transformer 3: 2 MVA, 6.6KV/400V, 6% Z
Motor 3: 6.8 MVA (Lumped), 17% Z
400V Bus
Motor 4: 300 KVA (Lumped), 17% Z
Motor 5: 596 KVA (Lumped), 25% Z
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thanks a lot and more power & blessings
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)
Hi, where can I find a book/standard where the MVA method is referenced? Thanks you.
Try using this search: http://www.google.com/search?q=MVA+Method&btnG=Search+Books&tbm=bks
Hi Ver,
Correct me if I’m wrong, but should the 33kV 3ph bus fault be 5.25kA?
i.e. fault = (250MVA + 50.3MVA) / (?3 x 33kV) = 5.25kA?
Thanks!
Ian
@Ian, Yes you are correct. Thanks for pointing the error.
Hi, Sir:
May I ask how did you get the upstream values for Motors? and why is it that the given MVAsc of the Motors are placed at the downstream side? Just like to know if my understanding is right.
Thanks
Hill
@Hill Portez
If I understand right, your question may be similar to a previous comment by Pio A. Lozano.
These are derived from industry standard values. Using these values in your MVA SC calculations wil provide a comparable result using computer based softwares.