## 4 Discussion  
Since the TrafoGrade system is the most complex algorithm and incorporates much more data than the other two methods, it is assumed that it also provides the most reliable output for technical condition judgement. This means that the whole population of 96 transformers is free of excessive risk in further operation and only two units are in average condition. The health index I calculation method showed significantly different outcomes compared to the TrafoGrade method. This is due to the extended time of operation for some units, as they had already \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technical Condition Judgement** & **Health Index I** & **Health Index II** & **TrafoGrade System Classification** \\ \hline Good & 78 & 95 & 94 \\ Average & 16 & 1 & 2 \\ Risky & 2 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The results of transformer population analysis using three different algorithms. Figure 6: Values of health indexes of 96 transformers: (**a**) according to algorithm I; (**b**) based on the advanced oil diagnostics algorithm II. exceeded their predicted lifetime. The higher value of the age coefficient leads to the evaluation of older units as unable to undergo further operation. Nonetheless, operational and internal revision experiences [17] show that, depending on the quality of service, the technical condition of the active part of such a transformer is satisfactory considering their age, and after a minor renovation their operation period can be extended by another 10-15 years [18]. In a similar study [19], in an analyzed group of twelve fifty-year-old transformers, all of the transformers were ranked as bad or risky during the five years of the experiment, but none of them ended their lives despite the critical evaluation obtained with algorithm I. The method proposed by authors based on advanced oil diagnostics ranked almost all transformers as in good technical condition. This algorithm prefers the presence of such gases as hydrogen and acetylene and the presence of furfural. These are compounds generated mainly during the development of defects, thermal aging of the insulation, and the aging changes in the active part of the transformers. However, the algorithm takes into account the service life factor to a much lesser extent compared to health index I, which is possible under the conditions of good current service. This is also intended to allow more accurate observation of changes in health index value over time, as this increases the impact of the current technical condition on the final result. The comparison of Figure 6b and Table 7 indicates that the assessment of the technical condition in the TrafoGrade system was practically consistent with the conclusions drawn with the HI values determined with algorithm II. This correlation seems promising in terms of future work on effective simplification of transformer technical condition assessment based on the analysis of only selected oil and operation parameters. However, it should be remembered that the TrafoGrade system uses expert knowledge to a much greater extent because it also includes such parameters as analysis of deformation and loosening of the windings, quality of bushings insulation, and operation of the OLTC. It should also be noted that the TrafoGrade system demands the transformer to be de-energized for several hours, which usually requires a change in the configuration of the power grid and is both time-consuming and expensive. Some differences in the values of the health index for individual units were also observed. In the group of transformers operating for less than 5 years, it was found that some of them were in worse condition compared to others. On the other hand, the condition of some transformers operating for 35-40 years was comparable to units of devices working 15-25 years.