Frequency response analysis (FRA) is a technique used to diagnose the mechanical integrity of a transformer winding; such diagnostic tools can be of enormous value since power transformers are a critical asset within any electrical network. To minimize the probability of an unexpected outage, or prevent a catastrophic failure, maintenance and monitoring of power transformers is essential for utilities. Over the past couple of decades, FRA has been utilized as an off-line diagnosis method. However, with the recent development in smart grid systems, there is now a growing interest in the development of on-line FRA techniques. This paper proposes a technique for in-service monitoring of power transformer winding deformation, which uses a broad frequency sine wave voltage excitation signal and high frequency Current Transformers (CT) in conjunction with the bushings test taps. Experiments using this system were conducted and then validated on a single-phase 22kV/110V voltage transformer.