Leakage Reactance Testing-way off nameplate impedance

I’m testing leakage reactance on 4 new transformers in -10C weather. Impedance values given by Doble’s M4110/M4100 combo differ greatly from the FAT results. FAT was performed at an apparatus temperature of 85C. Would this large discrepancy in temperature explain a 10-14% difference in impedance from nameplate?

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Short answer is Yes. Transformers are sensitive to temperature changes, and their impedance values are influenced by the temperature at which the measurements are taken. -10C is pretty cold. Are you using and comparing temperature corrected values? If not, you should calculate the TCF at 20C or 40C to accurately compare the impedance values.