## 5 Experimental Measurement and Analysis
### Comparison of Frequency-Dependent Capacitance for Transformers with Normal Insulation Humidity  
Figure 10 shows the frequency-dependent capacitance curves for two transformers rated at 630 kVA and two at 1000 kVA, each with different insulation conditions. Comparing transformers T1 and T2, which exhibit similar loss factor values at 50 Hz (0.22 and 0.23) and identical moisture levels (1%), reveals significant capacitance differences at frequencies below 0.1 Hz. This discrepancy arises primarily from the larger geometric dimensions of insulation in the 1000 kVA transformer T2, affecting the steady-state region  
Figure 10: Frequency-dependent capacitance curves for transformers rated at 630 kVA and 1000 kVA exhibiting different insulation conditions. of the capacitance curve. This highlights a limitation when comparing capacitance among transformers of different sizes. In comparing transformers T3 and T4, the differences in insulation geometry directly reflect their insulation conditions. Transformer T3 exhibits poorer insulation parameters (loss factor: 0.461, paper moisture: 2.9%, oil conductivity: 30.2 pS/m), correlating to lower capacitance values at frequencies up to 0.1 Hz. Analysis of oil conductivity among these transformers reveals a correlation between increased capacitance and higher specific oil conductivity. Transformer T3, having the highest conductivity (30.2 pS/m), shows a sharp increase in capacitance beginning at higher frequencies (around 0.1 Hz). Transformers T1 and T4, with significantly lower conductivity (-1.7 pS/m), only exhibit a notable increase at lower frequencies (-0.01 Hz). Transformer T2's capacitance begins rising at an intermediate frequency (0.02 Hz), reflecting its intermediate insulation condition. Capacitance ratio methods at two frequencies were applied for insulation condition evaluation (Figure 11). For the most accurate assessment, the capacitance ratio at the lowest frequency of 0.01 Hz relative to 50 Hz was used. Transformer T3 demonstrated the poorest insulation with a ratio of 5.08, clearly distinguishing it from the others. Comparison of the Frequency Dependence of High-Voltage Transformer Capacities with Extremely Wet Insulation  
Figure 12 displays frequency-dependent capacitance curves for three transformers (T5, T6, and T7), each rated 100 kVA with extremely high insulation moisture, and one transformer (T4) at 630 kVA with normal insulation. Significant capacitance increases at lower frequencies (100-1000 Hz) in transformers T5, T6, and T7, relative to T4, reflect elevated oil conductivity, notably T6's extremely high conductivity (17,164 pS/m). These findings are confirmed using capacitance ratios at frequencies from 2 Hz to  
relative conductivity levels of their insulating oils. Conversely, the reference transformer T4, having normal insulation conditions, shows the lowest capacitance ratio. When comparing the capacitance values of transformer T4 (630 kVA) with those of transformers T5, T6, and T7 (each 100 kVA), it is no longer necessary to consider the differences in insulation geometry (oil-paper systems). In addition, it is possible to follow the trend line at three points and thus analyse the further development of the increase at lower frequencies of capacity relative to the capacity at 50 Hz. Figure 13 shows an example of adding a trend line to transformer T6 with extremely high humidity. Insulation Analysis of the Selected Transformer by the Capacity Ratio Method at Various Temperatures  
In addition to evaluating insulation conditions, the described capacitance ratio method can also compare capacitance measurements at multiple temperatures. For this analysis, frequency-dependent capacitance and dissipation factor curves of a selected transformer measured at four temperatures (0-4-16-23 \({}^{\circ}\)C) were used, as depicted in Figures 14 and 15. Figure 12: Comparison of frequency-dependent capacitance curves for transformers T5, T6, and T7 (100 kVA) with high moisture content and transformer T4 exhibiting normal insulation conditions.