## 5 Calculations for Different Test Networks
The second network on which the influence of different types of line lengths, different ages and the influence of the number of damages to system transformers and transmission lines was shown is a large real network of over 3000 nodes, consisting of over 40 thousand kilometers of transmission lines consisting of over 3000 lines and 230 system transformers consisting of 30 400/220 kV transformers, 65 400/110 kV transformers and 135 220/110 kV transformers. The network contains about 3000 110 kV lines, 171 220 kV lines, and 130 400 kV lines [132; 133; 134; 130]. Similarly to the IEEE 118 test network, in the case of the actual network under study, the authors assumed that the average duration of a month would be 30 days. For calculation purposes, the authors also determined the number of failures of a particular line in the period under study from the normal distribution. The average line lengths were assumed from the actual network model. Then, the average annual failure rate for a 110 kV line according to the relationship (5) is 1.64 failures per 100 km (data for 9 years) and 2.94 failures per 100 km of line in the case of data for a period of 5 years. The transmission network for the large model is also composed of 171 lines with a voltage of 220 kV. In the case of lines with this voltage level, the reliability coefficient according to the relationship (5) is 0.69 (data for 9 years), and there are 1.25 emergency shutdowns in the case of data for a period of 5 years. In the case of 400 kV lines, the coefficient is 0.21 for data from 9 years and 0.36 for data from 5 years. In the case of transformer reliability index calculations, the same relationship was used as in the case of the IEEE 118 test network. The reliability index of transmission transformers is then 0.41 emergency disruptions per 100 transformers per year. As can be seen, the smaller the calculation network, the higher the reliability index of the transformer. In practice, such a situation does not occur, and emergency shutdowns are at a much lower level, and according to the literature [125], one can expect one emergency shutdown per 3 years. Averaging the shutdown rates from all voltage levels for the Polish Power System, it can be assumed that the average shutdown rate in overhead lines in Poland is between 1.3 and 1.8 shutdowns per 100 km of line. Therefore, an average indicator of 1.5 outages per 100 km of line can be assumed without dividing into voltages. In the case of transformers, on the other hand, an indicator of 1 emergency outage per year can be assumed in calculations for the Polish Power System. It should be noted, however, that actual outages occur even less frequently, and it can be assumed that such an element is removed from the system approximately once every three years. The approach presented in this article, based on the determination of non-watertightness indicators with available data, provides important conclusions about the technical condition of the transmission infrastructure. However, the method has some limitations that need to be borne in mind when interpreting the results. Firstly, the analysis is based on historical data collected statistically, which limits the ability to take into account dynamic states of system operation and short-term load changes. Secondly, the current methodology does not take into account the full impact of RES generation on network operation. Thirdly, due to the lack of comparable contemporary studies for Polish conditions (the last such study for the transmission grid was produced more than 30 years ago), no comparison of the results with those of other contemporary studies was made. In future work, the authors plan to extend the approach to include models using machine learning techniques. They also plan to take into account factors such as temperature, humidity, and weather anomalies. This approach will enable a more comprehensive estimation of power system reliability in the energy transition in the future.