In the event of failure or a short circuit occurs, the transformer will experience mechanical stress, electrical stress, and thermal-induced. Frequency Response Analysis (FRA) is one of the diagnostic techniques used to determine the transformer condition after the failure event. It measures the transfer function of the transformer over a wide frequency range from 2Hz to 2MHz. Changes in measured response will indicate that mechanical deformation has occurred on transformer winding. However, not much research has been conducted on the severity of winding deformation as an established decision factor for the safe operation of in-service transformers. This research has analyzed the FRA response on several winding deformations on 33/11kV transformer and 11/0.433kV transformer. The winding has been modeled and the algorithm was developed to determine the allowable limits for the changes in FRA Signatures. Based on the experiment and research conducted, the magnitudes are allowed to change within 9 and the maximum change is 11 of its response. © 2023 Author(s).