## 2 Frequency Response (FR) Interpretation  
During the FRA measurement, the low voltage sinusoidal signal was injected into the one of the transformer terminals in a wide frequency range and measured on the other terminal, with respect to the ground (transformer tank). There were several FRA test configurations; the main was end-to-end open configuration [4], where the signal was applied to the input of the transformer winding and registered at its end. During this measurement, the remaining windings stayed open. This measurement allowed us to examine each winding separately, with some visible impact of the capacitances of the remaining windings [25]. The FRA results were presented as a signal dumping in dB (denoted as FRA) in the frequency domain. A typical spectrum of the FRA amplitude is shown in Figure 2.  
Interpretation of the results consists of comparing the differences between the measured curve and the reference curve. As shown in Figure 2, the measurement results can be also compared between phases of the same unit. In these cases, when the damage occurred in one of the windings, the FR of this winding was different from the FR of the other windings. For the aim of the results of interpretation, the FRA spectrum was normally split into three frequency subranges: low (LF range), medium (MF range), and high (HF range). The division made it easier to evaluate the results, because different types of windings deformations affected the behaviour of the FR curve in different frequency ranges.  
The shape of the FR curve in the low frequency (LF) range depended mainly on the magnetizing inductance and less on the bulk capacitance of HV-winding. The FR curve in the LF range was sensitive to short circuits between coils and wires. The slope of the curve corresponded to the inductive character of coil impedance in this range. The descending of the curve stopped when the first parallel resonance was reached. After the first resonance, the curve started to increase due to dominant influence of the winding capacitances, and the LF range ended at the inflection point of the curve slope [26].  
In end-to-end test configuration on the FR amplitude curve, the first parallel resonance could always be seen. First resonance occurred in the low frequency range and usually had the biggest damping value in the measured FR curve. Generally, the position of the first resonance depends on the winding self-inductance and total capacitance; however, several other factors can influence its position on the FR curve. The position of this resonance on the frequency axis was similar for all coils, despite the fact that there were other types of power transformer windings, which differed in design and number of wires. Resonance phenomenon is associated with the results presented in  
Figure 2: Typical frequency response (FR) curves measured on a three-phase transformer.  
this paper, because the simulated resonance visible on the frequency-dependent inductance curves had the same background. Research conducted on this subject shows that the position of the first resonance, especially in the FR results for LV windings, depended highly on the mutual inductances and capacitances of the HV windings, even from other phases not directly related to the performed measurement [26; 27].