Three-Winding Transformer / SKM PowerTools Representation

[saladhawks]

It seems like every software I use has different entry parameters for three-winding transformers. I think I have an analysis setup correctly in SKM PowerTools for the following three-winding transformer, but wanted to check to make sure:

Primary: Wye-Grounded
Nominal KVA: 25000
Rated Voltage: 34400

Secondary: Wye-Grounded
Nominal KVA: 25000
Rated Voltage: 17200

Tertiary: Delta
Nominal KVA: 3750
Rated Voltage: 2400

I have entered the following Positive Sequence Impedance:

Primary-Secondary = 0.34 (%Rpos), 7.8 (%Xpos)
Primary-Tertiary = 1.24 (%Rpos), 28.46 (%Xpos)
Secondary-Tertiary = 0.81 (%Rpos), 18.59 (%Xpos)

My specific question involves the entry of the Zero Sequence Impedance data. I checked the Westinghouse T&D Book and it states that the Zero Sequence Impedance is equal to the Positive Sequence Impedance for this 3-winding transformer configuration. Thus, in SKM PowerTools do I simply enter the same impedance information from the positive sequence for the below zero sequence values?

Primary-Secondary = (%Rzero), (%Xzero)
Primary-Tertiary = (%Rzero), (%Xzero)
Secondary-Tertiary = (%Rzero), (%Xzero)

Thanks in advance.

 

[davidbeach]

First you have to know shell or core type construction. For shell construction Z0=Z1, also applies to a bank of three single phase transformers. For core type construction Conrad St. Pierre suggests that:
Z0_H-L=0.85*Z1_H-L
Z0_H-T=0.75*Z1_H-T
Z0_L-T=0.9*Z1_L-T

Based on real world fault data I would suggest that St. Pierre may be on to something.