## 5 Conclusions  
In this paper, a time-dependent reliability assessment of the interconnected system by the renewal theory was provided. Calculations of the 5-year time evolution of unavailability were performed successively, first for the separate systems of the power grid and the communication network and then for the overall interconnected infrastructure. The presented numerical results show an unfavourably high impact of the power network unavailability (S1) on the overall interconnected infrastructure unavailability, despite the fact that S1 unavailability is lower than the S2 communication network unavailability (see Figure 12). Numerical results of the time-dependent reliability analysis indicate reduction in the unavailability of the improved interconnected system in comparison with the initial reliability design. It is evident that unavailability reduction is particularly caused by design changes (parallel duplication of Transformer T1). Figure 12 shows that the unavailability curve of the improved Power grid S1 is significantly lower than those of both S1 original and S2. As a result of this, a significant unavailability reduction in the Interconnected network SS can be observed. Thus, the proposed design brings a valuable reliability improvement, especially to the power grid network of the analyzed interconnected energy infrastructure. The innovative character of the proposed solution can be briefly described in the following paragraphs. * Description of the novel real critical energy infrastructure use case, which consists of interconnected urban power grid and communication network, was presented. Parameters of reliability and maintenance models of components were estimated from the literature review and expert knowledge. * The developed computational model assumes the ageing of components, which is simulated by the Weibull distribution. The use of the Weibull distribution is consistent with real failure datasets of power distribution components. Moreover, the interconnected model also exploits the observation that the time to repair of these components can be modeled by an exponential distribution. * Time-dependent reliability assessment of the interconnected use case was performed. The identification of the critical components of the interconnected network and their interdependencies was provided by the general directed AG. Highly reliable components and interconnected networks were properly modeled. The software tool leveraged exact reliability quantification of highly reliable events. * Results indicated that the original design has an unacceptable large unavailability S1. Figure 12: Comparison of the unavailability evolution within the mission time of 5 years with original and improved parameters for Systems S1 and S2 (a) andSystem SS (b). * Slightly modified design to improve the reliability of the interconnected system was proposed, in which only a limited number of components in the system were modified to keep the additional costs of the improved design limited. * Numerical results indicated reduction in the unavailability of the improved interconnected system in comparison with the initial reliability design. * The proposed unavailability exploration strategy is general and can bring a valuable reliability improvement in interconnected systems including the energy and communication sectors. The aim of further research is to investigate the factors that contribute to communication equipment failures in electricity distribution systems and to quantify their relative importance. Specifically, the influence of environmental conditions, hardware and software failures, and communication disruptions that affect the reliability of the RTU will be investigated. A mathematical model will be developed to estimate the probability of failure of the RTU and assess the importance of each factor. The results of this study will provide valuable insights into the design and maintenance of distribution systems, which will contribute to increased reliability and reduced downtime. Conceptualization, M.V. and R.F.