What reference does ETAP consider to assume typical impedance values in transformers?

[Dvhez]

After entering transformer voltage level and power rating, when you click on "Typical Z & X/R" it assumes an impedance (depending on standard ANSI/IEC), but where does it come from? Is it taken from a standard? NEC?.

 

[cuky2000]

The link below provides an inside where electrical data coming from to perform SC calc.

http://apps.geindustrial.com/publibrary/checkout/GET-3550F?TNR=White Papers|GET-3550F|generic

 

[dpc]

There are nominal impedances for larger transformers in the ANSI standards. For transformers 500 kVA and below, there is no standard. Also, transformers can be specified with non-standard impedance. IMO, it is poor practice to use a default impedance for an existing transformer. The impedance will always be on the nameplate. Transformer impedance is a major factor in determining short circuit currents, so the fewer assumptions the better. The X/R ratio can be estimated. The IEEE Red Book has a graph of typical X/R ratios based on the transformer size.

 

[prc]

IEC World: Minimum short circuit impedance required for various ratings of transformers is given in IEC 60076-5 -2006 "Transformers-Ability to withstand short circuit" Table 1. But a much more exhaustive typical impedance values in various countries + typical X/R ratios are given in IEC/TR 60909-2:2008 Technical Report -Data of equipment for SC current calculation.
IEE World-Preferred standard values of impedances are given in product standards (for various types of transformers )in C57 series standards. Remember IEE values on the lowest cooling rating (ON) while IEC values are based on maximum ratings. Typical X/R values are given in Fig 30 of C37.010-2016 AC circuit Breakers