## 6 Discussion  
In order to avoid the geometric influence of the insulation system depending on the transformer power, we performed an analysis of the comparison of the ratio of two capacitances against the determined parameters of paper humidity and oil conductivity. According to Figure 17, transformers T1, T2, T3, T4, and T7 are sorted according to the ratio of the two capacities, sequentially from the lowest value to the highest, and the values of humidity (%) and relative oil conductivity (pS/m) are compared to them. Transformers T6 and T5 are not shown graphically due to significant oil conductivity values (17,164 and 2145 pS/m) compared to significantly lower values of the other compared transformers. They also have the same trend of increasing \(\text{C}_{0.01\text{Hz}}/\text{C}_{50\text{Hz}}\) ratio with increasing oil conductivity.  
According to experimental measurements, the ratio of two capacitances \(\text{C}_{0.01\text{Hz}}/\text{C}_{50\text{Hz}}\), i.e., for frequencies of 0.01 Hz to 50 Hz, was sufficient for the reliability of the above analysis.  
Figure 16: Evaluation of transformer insulation condition based on capacitance ratios at two different frequencies and selected temperatures.  
Figure 17: Transformer parameter comparison analysis for T1, T2, T3, T4, and T7.  
Therefore, it was not necessary to address the ratio of capacitances to a lower frequency (e.g., for 0.001 Hz, i.e., \(C_{0.001 Hz}/C_{50 Hz}\)). These tests were performed at a constant ambient temperature of 7 \({}^{\circ}\)C, eliminating temperature as a variable.  
Another significant advantage of the ratio \(C_{0.01 Hz}/C_{50 Hz}\) over \(C_{0.001 Hz}/C_{50 Hz}\) is the lower temperature influence on the measurement result in the temperature range 0-23 \({}^{\circ}\)C (Figure 16). The higher the frequency with which we can reliably assess the quality of the insulation, the lower the demands on the accuracy of temperature determination.  
From the above comparison of results according to Figure 15, it is clear that when the ratio of capacitances to lower frequencies increases relative to 50 Hz, the relative conductivity of the oil also increases accordingly. On the contrary, a significant influence of the humidity content of the solid insulation on the provided capacitance ratio was not confirmed, as shown according to the theory in Figure 9. Even with the capacitance ratio \(C_{0.01 Hz}/C_{50 Hz}\) (5.08) of the T3 transformer at a lower humidity of 2.9%, the magnitude of the capacitance ratio was influenced by its higher oil conductivity (30.2 pS/m) than that of the T7 transformer with a humidity of 3.3% and an oil conductivity of only 10.1 pS/m (Figures 11 and 13). Thanks to this knowledge, the capacitance ratio method will only be used to investigate oil conductivity because it is not sensitive to paper-moisture analysis. In transformers T1 and T4 with the same power (630 kVA), there was a significant difference in paper moisture (by more than 100% - 1% and 2.2%) at almost the same oil conductivity value (1.7 pS/m). When analyzing the capacitance itself, there was an almost constant difference of 11 to 13% when analyzing the capacitance ratio, there was only a difference of 3% at \(C_{0.01 Hz}/C_{50 Hz}\).  
According to the above analysis, the ratio of two capacitances can only be used for oil-quality analysis with relatively high reliability.