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Please, can someone help me understand what the impedance correction iec factor the psse uses to calculate the zero sequence thevenin impedance.

using a theoretical system of only one generator (with impedance equal to zero) and a delta-wye grounded transformer, a short circuit In high side (wye side) in the ascc method the zero sequence thevenin impedance is equal to zero sequence impedance of transformer on the wye side

but in the iec method, thevenin impedance is different.

I tried using the formula kt = 0.95.cmax / 1 + 0.6xt but the result does not match.

thank you