## 6 Conclusions  
The issue related to the failure rate of transmission lines and network transformers is particularly important in the area of these networks where the connection of RES is planned. In this article, the authors have proposed an extensive literature review presented in point 2. Based on it, as well as on the authors' own calculations, it can be stated that the line reliability indicator will differ significantly depending on the locations on the continent. Based on the literature review and the authors' own calculations, Figure 6 presents a summary of the averaged reliability factors for transmission lines by continent. For this purpose, the authors reached for data available not only in scientific works by other authors but also in data provided by operator companies operating on a given continent. The presented graph does not include data for Australia and Oceania due to the unavailability of reliable information on this subject in these regions. Operators operating in Australia are reluctant to provide such data, and despite reviewing many works on this subject, the authors were unable to find reliable data. Figure 7 presents a graph comparing the reliability coefficients of lines with individual voltage values for Poland, Europe, and those resulting from the authors' own calculations. It can be seen that the results obtained as a result of the authors' calculations correspond to the values for Poland and Europe. Figure 6: Comparison of transmission line reliability indicators across continents. Figure 7: Comparison of reliability indicators of transmission lines with different voltage levels. Based on the graphs presented below, the authors recommend using the reliability coefficient of the 110 kV line at the level of 1.7 for calculation purposes, the 220 kV line at the level of 1.45, and the 400 kV line at the level of 0.6. Without dividing into voltages, the authors recommend using the reliability coefficient at the level of 1.5. In the case of the reliability indicator (Figure 8), the authors recommend using the statistical value of 0.11 disturbance per year per 100 system transformers for calculation purposes. Regarding the failure removal time (Figure 9) for transmission lines, the authors recommend adopting a repair duration of 14 h per line. These data are consistent with the values for Europe, where the time needed to repair a single transmission line is 15 h. In the case of system transformers (Figure 10) located in the Polish Power System, the authors suggest considering the time needed to repair this element as 11 h without dividing it into the voltage levels at which they operate. The presented analysis is a first step toward a broader and more complex approach to reliability issues in the transmission system, especially in the context of RES development. In their engineering practice, the authors, as authors of numerous connection studies for renewable sources, observe that the main limitation for the connection of new generation units at the voltage levels of 110 kV, 220 kV, and 400 kV is the outdated and overloaded infrastructure components. The determination of reliability indices for these system components enables the identification of key technical constraints and will form the basis for the development of a tool to support the process of connecting distributed renewable energy sources to the system. In further stages of their research, the authors plan to develop a methodology based on artificial intelligence to support investment decisions. Figure 8: Comparison of HV/HV transformer reliability indices. Figure 9: Summary of times needed to repair a transmission line by region. In the presented methodology, an N-1 analysis approach will be adopted with a random selection of both the number of lines subjected to outages and the hours of these outages. The number of lines and the time of fault clearing will be determined based on the reliability indices presented in the article.