Single phase short circuit current calculation
Single phase short circuit current calculation
(OP)
I have a single phase 75kVA transformer with 1.7 %Z. See attached file for oneline. Can someone please show me how to calculate the short circuit current on the secondary side (240V) of the tranformer?
Thank you for your time.
Regards.
Thank you for your time.
Regards.






RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
FLC of the transformer is 312.5A.
Bolted S/C fault current is 312.5/0.017 = 18.4kA.
Upstream network impedance will reduce the result slightly but not to any extent worth noting.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Single phase short circuit current calculation
@Cuky: The mathcad implementation is pretty slick. I am assuming the 0.9 is the powerfactor.
@Scotty: Lol. Not for school.
RE: Single phase short circuit current calculation
0.9 may be considered the voltage fluctuation @ -10% that provides the highest SC current. In other word, 216V < Vnom < 264V.
If the load of the system has low power factor, this also will be amplified accordingly.
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
On small dry type transformers designed to run quite hot and with a fairly high X:R ratio, the actual current will be greater with a cold transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
When calculating manually, it may be easiest to convert all impedances to the 240V level. Note that the 240V voltage is a phase-neutral voltage, but the other voltages are phase-phase voltages. For example, the impedance of the source is about 5.1Ohms at the 115kV level. This can be converted to the 240V level as: 5.1*(240*sqrt(3)/115000)^2. The short circuit current at 240V is 16.9kA.
If a three-phase program is used, it may be simplest to think the single-phase transformer as one-third of a three-phase YY transformer, and to calculate the short circuit current for a three-phase fault. The programs generally use phase-phase voltages as inputs. This means that the single phase transformer must be entered as a 24.9kV/416V-transformer. The apparent power of the single phase transformer must also be multiplied by three. The short circuit current will again be 16.9kA.
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
You would need a positive, negative, and zero sequence network (which connect in series) to calculate the single line to ground (SLG) fault current. Current for a three phase fault can be calculated using just a positive sequence network. Subsequently, higher impedance in a SLG network should give current magnitudes lower than say a three phase fault. I understand a three phase network is decoupled into three single phases in the symmetical component world but I dont understand how that equates to equivalent SLG and 3Ph fault currents.
Thanks.
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
Now if I want to connect a single phase 277V (L-N) breaker at one of the phases of the same busbar, what should be the rating of this single phase breaker? Should it also be at least 50KA?
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation
RE: Single phase short circuit current calculation