## 7 Conclusions  
Based on the conducted diagnostic analyses, the importance of frequency dielectric spectroscopy (FDS) in evaluating the insulation condition of high-voltage transformers has been clearly established. The FDS method allows for the independent evaluation of transformer oil quality and moisture content within solid insulation. Additionally, the research highlights those comparative analyses across multiple transformers significantly contribute to advancing diagnostic methods.  
A notable advantage of using capacitance measurements via FDS to evaluate insulation conditions is their lower sensitivity to temperature fluctuations, compared to loss factor measurements. Consequently, when temperature determination is uncertain, it is advisable to prioritize capacitance frequency analyses over loss-factor evaluations.  
However, a limitation of capacitance-based diagnostics is their sensitivity to insulation geometry, influenced by transformer type and power rating.  
A beneficial finding of the aforementioned analysis was that the capacitance ratio showed smaller differences when analysing the insulation properties of transformers with different geometric designs and ratings than when measuring capacitance alone. According to these findings, it will be recommended for the diagnosis of insulating parts that when measuring various types and powers of transformers at the voltage level of 22/0.4 kV, the capacitance ratio should be used when measuring by the FDS method.  
Another fact found was that it was not possible to identify a significant influence of paper humidity on the overall evaluation of transformer insulation using the capacitance ratio. Thanks to this knowledge, the capacitance ratio method will only be used for investigating oil conductivity because the paper's moisture sensitivity analysis was not detected.  
The experimental analyses also emphasized the critical role temperature determination plays in ensuring the accuracy and reliability of the FDS method for assessing insulation in oil-filled power transformers. To reduce the influence of temperature, a \(C_{\rm 2 Hz}/C_{\rm 50 Hz}\) ratio is recommended, as the differences for different temperatures in the range from 0 to 23 \({}^{\circ}\)C have been shown to be minimal.  
The analyses underline the importance of accurately measured capacitance and dissipation factor values at selected frequencies for dependable evaluations using frequency dielectric spectroscopy.  
Such diagnostic procedures facilitate predicting potential transformer failures due to insulation aging, enabling timely maintenance interventions (such as oil replacement, cleaning, or paper insulation drying) to ensure uninterrupted transformer operation.  
Conceptualization, M.G.; Methodology, M.G.; Formal analysis, D.K. and J.K.; Data curation, R.K.; Writing--original draft, M.G.; Writing--review & editing, J.K.; Visualization, D.K. and R.K. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the Slovak Research and Development Agency under the contract No. APVV-21-0449 and No. Vega 1/0640/24.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
This work was supported by the Slovak Research and Development Agency under the contract No. APVV-21-0449, APVV-21-0078 and No. Vega 1/0640/24.  
The authors declare no conflict of interest.