Through fault current of transformer

[jasl]

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?

 

[DanDel]

The basic calculation, assuming an infinite primary source(most conservative assumption), is the FLA of the transformer multiplied by the inverse of the Percent Impedance (%Z). For instance, 1000kVA 3-phase transformer, 480V secondary, 5.75%Z.
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.

 

[jasl]

Thanks Dandel.
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 % ?????)

 

[jghrist]

If the switchgear is fed by only the single transformer and no motor contribution is included in the downstream fault calculation, then all of the fault current will flow through the transformer.

 

[flickstar]

jasl,

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!

 

[busbar]

In ANSI regions, a customary reference is IEEE Std C57.109, indexed at

http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.109-1993_desc.html

 

[DanDel]

Protective relays are usually set for protection of the transformer and cables. There are damage curves(frequent and infrequent fault) and inrush points for the transformer which are needed to accurately provide correct settings for primary and secondary protection.
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.

 

[jasl]

Valuable posts.
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 ?

 

[DanDel]

Inrush and through-fault currents are not the same. Inrush currents are high in 2nd harmonics, and are usually 10x FLA for 0.1s for 2500kVA and below, and 12x FLA for above 2500kVA.
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.