## 3 Results
When measuring the electrical parameters that characterise the electrical insulators of PRESP (5, 8, 9 Figure 2; 2, 3 Figure 3), the UNILAP ISO 5 kV megohmmeter (test voltages up to 5 kV) and LEM were used, which can apply DC test voltages of 0.5, 1, 2.5, and 5 kV. Table 1 shows the experimental measurements (before and after cleaning; R\({}_{\mathrm{iso}}\)--isolation resistance) on the lower side of the electrical insulator (9, Figure 2a), which must be cleaned periodically. The experiments were carried out for three hours during which a fire was started in the boiler with dry wood (humidity < 10%). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test Voltage**} & \multicolumn{2}{c}{**Before Cleaning the Insulator**} & \multicolumn{2}{c}{**After Cleaning the Insulator**} \\ \cline{2-5}  & **R\({}_{\mathrm{iso}}\) (M\(\Omega\))** & **P\({}_{\mathrm{i}}\) (–)** & **R\({}_{\mathrm{iso}}\) (M\(\Omega\))** & **P\({}_{\mathrm{i}}\) (–)** \\ \hline
0.5 & 52.7 & 1.45 & 508 & 1.59 \\
1 & 54.1 & 1.41 & 956 & 1.68 \\
2.5 & 56.6 & 1.64 & 1486 & 1.54 \\
5 & 57.4 & 1.15 & 2169 & 1.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measurements before and after cleaning the lower side insulator (textolite, round plate) from the PRESP. For type 1 upper-side electrical insulator (5, Figure 2), measurements were performed before using the gas flow from the boiler. The measurements are presented in Tables 2 and 3, where the insulation resistances are very high (of the order of G\(\Omega\) when the electrical insulator is clean) and decrease after it has been used. For the upper-side type 2 configuration (5, 9, Figure 3a), three groups of single insulators connected in series were used for safety in operation and to increase the operation. As shown in Figure 6, experiments were carried out on insulators connected in series (3 \(\times\) 18 kV). Each graph was created for a time of 60 min. Error bars have been added to the graphs. For the upper type 2 electrical insulator with the configuration shown in Figure 3a. The insulation resistance and dielectric absorption ratio were measured at different test voltages (1 kV and 2.5 kV), and the results are listed in Table 4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test Voltage**} & \multicolumn{2}{c}{**R\({}_{\text{iso}}\) (M\(\Omega\)) After Time (s)**} & \multicolumn{2}{c}{**R\({}_{\text{ad}}\) (\(-\))**} & **Obs.** \\ \cline{2-5}
**(kV)** & **0** & **30** & **60** & \\ \hline
1 & 662 & 771 & 779 & 1.02 & Without fire \\
2.5 & 538 & 619 & 631 & 1.02 & \\
1 & 613 & 643 & 750 & 1.16 & \\
2.5 & 508 & 601 & 630 & 1.05 & After 2 h \\
1 & 501 & 647 & 667 & 1.03 & After 4 h \\
2.5 & 501 & 621 & 628 & 1.01 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measurements taken on the new type 2 upper-side medium-voltage insulator when mounted on the chimney. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Ut (kV)**} & **0.5** & **1** & **2.5** \\ \hline \hline \(\text{R}_{\text{ISO}}\) (M\(\Omega\)) & 908 & 735 & 589 \\ \(\text{P}_{\text{i}}\) (\(-\)) & 1.01 & 0.98 & 0.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Measurements made on the used type 1 upper-side medium-voltage insulator after approximately 14 days of operation. Figure 6: Three insulators connected in series, new insulators (D\({}_{\text{ar}}\) = 1.04), used (approximately 2 months; D\({}_{\text{ar}}\) = 1.1), and cleaned (D\({}_{\text{ar}}\) = 1.08), 5 kV test voltage: (**a**) insulation resistance; (**b**) polarisation index. For the chemical analysis of dust, measurements were made with a spectrometer (sequential X-ray fluorescence spectrometer, Thermo Scientific ARL X900 XRF spectrometer, UniQuant Software, Thermo Fischer, Waltham, MA, USA) on the dust deposited on the electro-insulating supports of the chimney [33; 34]. The results are presented in Appendix A Tables A1-A3.