## 1 Introduction
Forced interruptions are mainly due to automatic switching operations performed by protection systems [66; 67; 68]. A transformer goes through several cycles in its life phase [67]. Subsequently, it goes through the phases of \"infant mortality\", \"useful life\", and \"wearout\". In each phase of the life cycle, there are different chances of failure. The reliability coefficient for transformers depending on age is presented by [PERSON] and his team [69]. The results obtained by the authors show that the risk of damage to the transformer, which would exclude it from further use in the first 20 years of its service life, is very small but increases with age. Also in [70], the authors focus on examining the reliability index of the transformer in relation to its age. In [71], the author analyses the reliability aspects of 220/110 kV transformers in Egypt. In his work, he focuses on examining factors such as the annual number of failures, the average annual interruption, and the average annual repair time. The author obtained data from the Egyptian transmission operator from 2002 to 2009, so his research is based on specific real data. It is estimated that the average service life of transformers is between 25 and 30 years [72], while according to [73], it is reasonable to extend their service life to 40-50 years. The final service life of a transformer is greatly influenced by the way it is used, the damage it has sustained, and the care taken during maintenance. For example, block transformers operating in large power plants are loaded for a significant part of their service life between 90% and 100% of their rated power, which makes the care taken in maintenance particularly important. In [74], the authors focus on analyzing the reliability of transformers used in South Africa. [PERSON], in collaboration with [PERSON] [75], analyzes the reliability of transformers in Australia and New Zealand. Also in [76], the authors once again rated the reliability indices of transformers located in Australia and New Zealand. In [77] the team led by [PERSON] focuses on the reliability of high-power transformers located in Egypt. By reading [78] the reader has the opportunity to learn about the reliability of transformers located on the Greek island of Crete. The issue of transformer reliability is also discussed in [79; 80; 81]. For example, in [80], [PERSON] and his team provide a reliability analysis of individual transformer components that are most frequently damaged. Also, in work [79], the authors address the issue of high-power transformer reliability. An interesting supplement to the reliability of transmission lines and transformers may be found in [82; 83], where the authors focus on presenting the effect of short-circuit and overload currents on the reliability of power system components. In [84] [PERSON] focuses on the reliability of switching operations of transmission lines. The article proposes a reliability assessment method based on the normal distribution. The authors test their method on a closed 118-node IEEE test network. [PERSON] and [PERSON] [85] present an original method in their paper for calculating reliability indices for an extended transmission system. For this purpose, they use Monte Carlo simulations as well as check the effectiveness of their method on the IEEE test system. In [86; 87; 88; 89], the authors focus on the reliability parameters of transformer distribution stations. A more detailed discussion of the formulas and relationships used in the above articles is presented in the next chapter. The authors of the present paper conducted an analysis of the unreliability of 110 kV lines over a 9-year period based on the obtained data. The damage rate per 100 km of the 110 kV overhead line was, respectively, 0.67, 0.52, 0.53, 0.28, 0.42, 0.28, 0.43, 0.87, and 0.49 in each of the nine years, which gives an average result of 0.69 damage per 100 km. However, it should be remembered that the data obtained by the authors constitutes only a small part of the power system.