3 winding transformers

[wonderchi]

I have got a problem with fault study relating to 3 winding transformers:

For my simulation, a 415v bus connected to the tertiary winding is being supplied by 11kV supply as secondary winding, and a 12.5kV generator as the primary winding. For most cases, the 415V bus gets its voltages from the secondary winding which is the 11kV, if I were to model this scenario:

1. Can I use a two winding transformer to represent the secondary to tertiary arrangments for the fault level at 415V bus and ignoring the three winding arrangment (when the primary side is being open circuited?

2. In relation to 3 winding transformers, is the step down operation in turns (in other words, primary to tertiary first? or secondary to tertiary or both at the same time)

 

[dpc]

It's just one big bunch of resultant flux in the core. So the conversions are all happening at the same time.

If your modeling software can't deal with three-winders, you can do a reasonable approximation using two-winding equivalents (you don't have much choice).

 

[KJvR]

I am more familiar with the Australian Standards (AS3851 - Calc 3 phase short circuit currents) but it is based on the IEC 60909. AS 3851 (and I assume IEC 60909) provides a very simple guide on how to represent a three winding transformer using three two winding transformers in either a star or delta configuration. You end up solving three simultanuous equations with three variables (impedance of each two winding transformer).

I support dpc in that your software should be able to handle at least a three winding transformer.