How can the zero-sequence impedance of a three-winding transformer be theoretically calculated?

by Stewie Griffin   Last Updated September 26, 2018 10:25 AM

I have a three-winding YNyn(d) transformer, where measurements of zero-sequence impedance have been performed.

The zero-sequence impedance of the Primary-Tertiary, and Secondary-Tertiary has been measured, but Primary-Secondary has not, since the terminals of the tertiary delta winding are buried.

The Primary-Secondary zero-sequence impedance can't be measured when the tertiary terminals are buried, since the connection must be open.

I have received the following information about a transformer:

Z0 (P-S) = 43,6 Ω/ph - 10,0 %  <-- calculated theoretical value
Z0 (P-T) = 76,3 Ω/ph - 17,5 %
Z0 (S-T) =  6,5 Ω/ph - 6,0 %

How are the P-S values calculated? Is this explained in any literature? Is it based on the two other values or something else?

The transformer is rated: 132/66/22 kV, 40/40/10 MVA.

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