What is the definition of zero-sequence impedance for a three-phase winding transformer?

The zero-sequence impedance is between connected together line terminals of a three-phase star-connected or zigzag-connected transformer windings and its neutral terminal. Zero-sequence impedance expressed in ohms per phase at rated frequency.

NOTE 1. The zero-sequence impedance may have several values because it depends on how the terminals of the other winding or windings are connected and loaded.
NOTE 2. The zero-sequence impedance may be dependent on the value of the current and the temperature, particularly in transformers without any delta-connected winding.

NOTE 3. The zero-sequence impedance may also be expressed as a relative value in the same way as the short-circuit impedance.