This study focuses on the analysis of the non‐linear behaviour of magnetisation inductance during a transformer frequency response test in open‐circuit configuration. For this work, frequency response measurements were made on a three‐phase transformer and a transformer model was used to simulate the response curve and to estimate the magnetisation inductance curve as a function of frequency. It was observed that the magnetisation inductance increases with increase of the test voltage. Also observed was an initial decrease at a very low frequency of the magnetisation inductance, caused by the displacement of the work point by the first elbow of the magnetisation curve. Additionally, the magnetisation inductance acquires a linear behaviour when the input small signal is sufficiently low. Finally, it was observed that the remanent magnetisation level in the core reduces the value of the magnetisation inductance.