## 3 Setup and Diagnostic Analysis of the Transformer Using the FDS Method  
Diagnostic systems utilizing the FDS method typically offer temperature compensation features to account for temperature effects on insulation parameters. Such systems adjust the measured values, providing more precise and reliable results. During the actual measurement, a sinusoidal test voltage (e.g., 140 V) at varying frequencies is applied to the terminals of the transformer, with the high-voltage winding short-circuited. Simultaneously, the low-voltage winding is connected to the diagnostic instrument's measurement input. Figures 3 and 4 illustrate the connection schemes for the IDAX-350 diagnostic system from MEGGER with two-winding (HV/LV) and three-winding transformers, respectively. These diagnostic systems effectively measure and assess transformer insulation characteristics via frequency dielectric spectroscopy [19,20]. As depicted in Figures 3 and 4, all high-voltage and low-voltage bushings of the transformer are interconnected. During measurements, a comprehensive insulation analysis is conducted by evaluating capacitances between high-voltage and low-voltage windings (CHL), between windings and transformer frame (ground) (CLG, CHG), and between these components and the tertiary winding (CLT, CHT, CTG), as shown in Figure 3.  
The MEGGER IDAX device analyzes measured curves by comparing them against theoretically modeled curves. Accurate measurement and recording of both ambient temperature and the transformer's temperature are critical during this diagnostic process. Figure 4: Connection of the diagnostic device MEGGER IDAX-350 to the tested 35/10 kV transformer. Figure 3: Wiring diagram of a three-winding transformer connected to the MEGGER IDAX-350 frequency dielectric spectroscopy measurement system. The measurement results yield frequency-dependent values for the loss factor, capacitance, and permittivity. These values enable the determination of moisture levels in the solid insulation and oil conductivity by precisely measuring extremely low currents (in the nanoampere to picoampere range) [18]. Typically, in transformers, the low-voltage winding is positioned close to the core, while the high-voltage winding surrounds it. To ensure adequate dielectric strength and effective cooling, these windings are separated by multiple cylindrical barriers made from pressed cardboard. These barriers are supported by spacers, creating channels through which insulating oil can flow freely. A simplified depiction of the arrangement between two transformer windings is presented in Figure 1.  
For modeling and analyzing frequency curves using the MEGGER IDAX system, the insulation structure between the windings is represented by the relative proportion of spacers and barriers within transformer channels (Figure 5). Parameter X is defined as the sum of all barriers in the channel, consolidated into one unit, and divided by the channel width. Spacer coverage Y is calculated as the combined width of all spacers divided by the total length of the transformer channel [14; 16; 19]. The description of the complex geometry of the barriers and spacers between the windings as one simple capacitance is a simplification of real conditions. This is one of the reasons why the modelled geometry may not exactly match the \"real\" geometry in the transformer. In most cases, the best fit/modelling is achieved by having the instrument software automatically calculate the geometry [14; 16; 19].