## 6 Conclusions  
At present, the main problem of the FRA method is the reliable interpretation of the obtained results. Due to the high sensitivity of the method, any variation in the values in the RLC circuit of the tested winding causes changes in the FRA spectrum. The modelling of the various mechanical winding's faults using RLC networks and field models can lead to the development of the fault classification on the basis of FRA and increase serviceableness of this diagnostic method. The presented research shows an approach to the development of a simplified field model, which allows to recreate the inductance of the winding in good accuracy.  
When conducting high-frequency modelling of the transformer windings, one should take into consideration that the magnetic field in the ferromagnetic core during the FRA measurements behaves differently than when the transformer is operating under rated conditions. The signal used in FRA measurements is a low-voltage signal, which, in relation to the B-H magnetization curve of the ferromagnetic material of the core, means that, during the FRA measurement, the magnetization of the material is at the beginning of this curve (on its linear section). Furthermore, to handle the eddy-current effects accurately and efficiently, it is necessary to implement the equivalent parameters into the model. The use of the complex relative permeability and the equivalent conductivity cause a significant acceleration of field calculations, as it eliminates a very dense FEM mesh necessary at high frequencies.  
The approach presented in the article describes how to create the simple 2D field model, which provides the accurate RLC parameters of the transformer windings. Simplifying the winding model, while not losing accuracy, is important when simulating the frequency response of multilayer windings with a large number of turns, reaching several thousand. Reducing the time-consuming nature of the field analysis can significantly influence the modelling of various mechanical deformations of windings using the lumped parameter circuit models.  
The development of a simplified model requires the application of several principles presented in this article. Because of different coordinate systems used for the 2D and 3D analyses, the 2D and 3D models differ in the final model geometry and their parameters. To obtain the equivalent results from 2D and 3D models, the correction factors for the 2D model are presented. The relative permeability, magnetic coupling in the active part, and the additional capacity associated with the windings remaining on the tested transformer should be changed accordingly. As a result of all the above-mentioned principles, the inductance in the wide frequency range of the 2D and 3D FEM models is shown.  
Conceptualization, K.T.; methodology, K.M.G., K.T.; software, K.T.; validation, K.T.; formal analysis, K.M.G.; investigation, K.T.; resources, K.M.G. and K.T.; writing--original draft preparation, K.T.; writing--review and editing, K.M.G. and K.T.; visualization, K.T.; supervision, K.M.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.