Through fault current of transformer
Through fault current of transformer
(OP)
Considering a single transformer feeding a switchgear,
for a fault in the downstream of the Switchgear, what will be the through fault current of the upstream transformer?
Will that be equal to the fault current at the switchboard?
for a fault in the downstream of the Switchgear, what will be the through fault current of the upstream transformer?
Will that be equal to the fault current at the switchboard?






RE: Through fault current of transformer
FLA = 1203A
1/0.0575 = 17.39
Ifault(3-phase) = 1203A x 17.39 = 20921A
This calculation obviously does not take into account any downstream motor contributions.
The fault current at the downstream switchboard may be slightly less depending on the impedance of the cables/buses in between the transformer and the switchboard, or slightly more depending on the above-mentioned motor contributions.
RE: Through fault current of transformer
This leaves me with another question on relay setting whether the relay at the trfr incomer has to be set for the value of the fault current at the Switchgear or less than that? (70 % ?????)
RE: Through fault current of transformer
RE: Through fault current of transformer
Note that DanDel has assumed an infinite primary source, hence ignoring any impedance behind the transformer. This is the most conservative assumption if you are trying to calculate the maximum fault current, for example, if you were trying to adequately rate equipment.
If you are trying to protect the transformer then you should be more interested in the minimum fault current to ensure that your protection will operate for all faults.
Cheers!
RE: Through fault current of transformer
In ANSI regions, a customary reference is IEEE Std C57.109, indexed at http://standards.ieee.org/reading/ieee/std_public/...
RE: Through fault current of transformer
For instance, primary protection can usually be set for a minimum value as long as it will allow inrush current for transformer energization.
Secondary protection is usually set for overload protection.
The IEEE Buff Book (Std 242) can be very helpful for issues such as these.
RE: Through fault current of transformer
About the terminology of "inrush current" of transformers,
I have a colleague with me who says that inrush current is referred as through fault current of an equipment (transformer, motor)???? Does this inrush current has in it 2nd and 5th harmonic currents and the transformer differential protection should be insensible to that?
Am I on track or missed something ?
RE: Through fault current of transformer
Through-fault current for a transformer is based on the %Z as mentioned before, considering the actual available primary current, which, in turn, is based on the primary system impedance.
Please refer to some of the references suggested for protection before trying to set your relays, especially a differential.