## 4 Challenges in Assessing Transformer Insulation Conditions via Capacitance Measurements  
Measurements of capacitance and dissipation factor are valuable tools for evaluating the degree of insulation deterioration in power transformers [20].  
The dissipation factor reveals the presence of polar and ionic substances within transformer oil, typically associated with oil aging. Although temperature-dependent variations of the dissipation factor can indicate contamination by dissolved substances and humidity levels, its extreme sensitivity to temperature fluctuations poses a significant limitation, particularly for equipment lacking internal temperature-monitoring devices [24,25].  
Therefore, the advantage of measuring the capacity compared to the loss factor is a lower sensitivity to temperature changes, at normal ambient values up to 40 \({}^{\circ}\)C.  
Figure 7 illustrates the frequency-dependent capacitance of a high-voltage transformer measured at temperatures of 15, 6, and 0 \({}^{\circ}\)C are determined at 2% paper humidity. For frequencies up to 1 Hz, relative differences remain minimal, calculated at around 10%. However, at extremely low frequencies (0.001 Hz), the maximum observed capacitance variation between temperature curves reaches up to 30%.  
Compared to dissipation factor measurements (Figure 8), these capacitance measurements demonstrate significantly smaller temperature-induced relative variations. In contrast, the dissipation factor exhibits variations of 45-55% across the same temperature range.  
Moreover, capacitance curves can offer a straightforward method to assess and compare the insulation condition across different transformers, as depicted in Figure 9.  
When insulation is moist, absorption currents become dominated by larger, frequency-independent leakage currents. Thus, insulation status is commonly evaluated by calculating  
Figure 6: MEGGER IDAX-350 software v5.3.39 interface showing the measured (purple) and modeled (green) curves along with calculated transformer parameters.  
the capacitance ratio at two distinct frequencies. Ratios such as \(C_{\rm 2 Hz}/C_{\rm 50 Hz}\) are frequently employed to assess moisture levels in class A transformers, with pre-drying values typically ranging between 1.3 and 2.3, decreasing below 1.2 after drying [8; 9].  
Figure 8: Frequency-dependent dissipation factor curves of the tested high-voltage transformer at temperatures of 0–6–15 \({}^{\circ}\)C at 2% paper humidity.  
Figure 7: Frequency-dependent capacitance curves of the tested high-voltage transformer measured at temperatures of 0–6–15 \({}^{\circ}\)C at 2% paper humidity.  
Figure 9: Comparison of frequency-dependent capacitance curves for transformers with wet and dry insulation.  
Experimental analyses presented in Chapter III demonstrate that incorporating measurements at even lower frequencies (e.g., \(C_{0.1\mathrm{Hz}}/C_{50\mathrm{Hz}}\) or \(C_{0.01\mathrm{Hz}}/C_{50\mathrm{Hz}}\)) can enhance the reliability of this assessment method.  
This capacitance-based method yields reliable results without requiring complicated temperature adjustments, simplifying the insulation evaluation process significantly [10].  
However, the main limitation of capacitance-based diagnostics remains its strong dependence on transformer power ratings and geometric configurations. While directly comparing insulation states among transformers with different power ratings using capacitance measurements can be problematic, combining capacitance analysis with frequency dielectric spectroscopy (FDS) effectively addresses this drawback.