You can measure the three impedances only by getting the three-phase terminals of the tertiary winding. What is brought out is one corner of the tertiary delta winding. When you close those two terminals, the delta tertiary is formed, and it is to be grounded in service. You can also work the transformer without tertiary by keeping these terminals open and earthing one terminal.

To measure the leakage impedances Zps, Zpt, and Zst of a YnYn-D transformer with a buried tertiary winding, conduct tests by short-circuiting each winding individually. Apply rated voltage to the non-short-circuited winding and measure voltage and current. Calculate Zps, Zpt, and Zst using the formula: Impedance = √(3 × Voltage / Current). Ensure measurements are referenced to a common base for accurate system analysis. By following these steps, you can determine the leakage impedances of the transformer's primary-secondary, primary-tertiary, and secondary-tertiary windings, even when only Zps data is provided on the transformer plate.