## 1 Introduction  
Power transformers are vital components in electricity grids and should be maintained in healthy conditions along with their entire operational life. As such several transformer condition monitoring and fault diagnosis techniques have been developed to identify the transformer health condition criticality and its remnant life [1, 2, 3]. The tap changer is used in power transformers to adjust the turn's ratio of its windings in order to maintain the voltage at one side at the desired level. This process can be done online through an on-load tap changer (OLTC) or offline using a de-energized tap changer (DETC) [4, 5].  
The OLTC is an electromechanical system mounted on the transformer to monitor the variable load voltage levels without blocking the load current, while the DETC should shut down the load current before changing the tap manually. This mechanical system is the only moving part in the power transformer and is considered an essential and costly component. The tap changer contacts must be strong enough to withstand regular movement, friction, and mechanical stresses. The tap changer contacts must remain functioning over the transformer's operational lifetime since tap changer replacement is very costly and time-consuming. Around 40-56% of all transformer failures are caused by a tap changer malfunction [6, 7, 8]. Tap changer failures may be due to the aging effect due to the electrical, mechanical, thermal, and chemical stresses. The tap changer failure modes comprise of mechanical wear and coking of the contact.  
The mechanical stability of the transformer tap changer over its lifetime needs to be assured. There are different tap changer diagnostic methods to assess the tap changer's stability condition that can be classified into oil and insulation analysis, tap changer contact analysis, and mechanical analysis [9]. However, these methods comprise some drawbacks. For instance, dissolved gas analysis (DGA) cannot identify the tap changer damage's exact location. The dynamic resistance measurement (DRM) method can be used to check the tap changer contacts' condition. In this method, a DC power supply is utilized to inject a test voltage into the transformer winding. The vibroacoustic analysis is a new tap changer diagnostic technique that investigates the vibroacoustic waveform transmitted from the tap changer via the structural components to diagnose different tap changer problems [10].  
While the frequency response analysis (FRA) technique has been widely used to detect the mechanical deformations of transformer windings and core, not much attention was given to use it for tap changer diagnosis [11; 12]. In [13], FRA is used to check its sensitivity to tap changer coking fault. However, the impact of changing the tap position on the FRA signature is investigated. Fundamental analysis for the effect of tap changer coking and pitting on the FRA signature is presented in [14].  
Previously, the FRA method is applied for the diagnosis of the transformer core and winding faults. Due to few studies on using the FRA method for tap changer damage detection, this paper presents a further sensitivity analysis of the FRA signature to the transformer tap changer's physical faults. This study implemented faults that include tap changer contacts coking and pitting. The impact of such faults on the FRA signature is investigated through experimental and simulation analyses. Furthermore, the feasibility of using FRA to simultaneously detect winding deformation and tap changer faults is investigated.  
The following sections of this work are organized as follows: Section 2 presents the experimental setup, FRA open circuit measurements are introduced in Section 3, FRA short circuit test is expressed in Section 4, capacitive and inductive interwinding tests are presented in Sections 5 and 6 respectively, and final conclusions and recommendations are introduced in Section 7.