## 1 Introduction  
An electric power system (EPS) can be described as a network consisting of generation, transmission, and distribution systems. The main purpose of the EPS is to ensure efficient and reliable power supply for consumers. To achieve this goal, it is important to maintain the integrity and compatibility of the EPS elements, as well as to ensure the management, operation, and coordinated expansion of the generation capabilities, transmission network, and interconnections with other EPSs [1; 2; 3; 4]. The sustainable functioning of an EPS requires the continuous development of the security and reliability of energy supply, availability and adequacy of energy resources, and efficient use of energy resources and energy consumption. There must be a coherent and sustainable development of the energy sector, along with a strategy to reduce the negative environmental impacts of energy activities as well as the protection of consumer rights and legitimate interests. A strategy for the use and development of local and renewable energy resources must also be included [1; 2; 3; 4; 5; 6]. The EPS operator is responsible for planning and coordinating the topology of the electric power transmission network, and preventing and dealing with disturbances and accidents. The operator must also provide additional services necessary for the power system's safe, reliable, and efficient operation, and also assure that the quality of transmitted electrical energy meets EU standards. In accordance with the EU Commission Regulation 2017/1485, all European EPSs collectively follow a methodology for coordinating the operational security analysis of the electrical grid. This methodology aims to standardize the analysis of the operation and security of the electrical grid in the synchronous area. Alongside this methodology, the capacity calculation provisions for each individual region must be followed by the regional security coordinators and the EPS capacity engineers [7]. Reliability is the most important property of an EPS. It is ensured in two ways: by uninterrupted operation of the power system through real-time planning and control, and by a retrospective analysis of the equipment, electrical grid parameters and regimes, and corresponding long-term planning [4; 5]. The stability of the EPS requires real-time management of the power balance and the quality of the power in the transmission network [4; 5; 6]. To ensure the reliability of the EPS in real time, it is necessary to monitor and control the key parameters of the power system, coordinate operational switching in the transmission network, remotely control the network elements, coordinate actions with neighboring transmission-system operators, and implement preventive measures necessary for emergency response [1; 2; 3; 4; 5; 6]. Long-term planning, related to EPS stability, as well as real-time management are very important for its reliability. This requires research into its capability withstand disturbances in particular operation modes. This paper focuses on the research into the stability of EPS operation during its desynchronization, its operation in islanded mode and its synchronization. The operation of the 400-330-110 kV electricity transmission network, which consists of power plants, transformer substations, switchgears, overhead transmission, and cable lines, was analyzed. This was performed to determine capability of the transmission network to maintain reliability in the event of a system disturbance [5; 6]. Numerical simulation software was used to analyze the behavior of the three EPSs in the event of a sudden and unexpected disconnection from the integrated and stable EPS. Guidelines are presented on the measures and actions to be taken to maintain stable parameters of an isolated EPS. The steps to be taken to stabilize the frequency of the EPS to a value of 50 \(\pm\) 0.1 Hz are specified. Analysis was performed to determine in what range electrical parameters need to be maintained to prevent under-frequency load shedding (UFLS) in an isolated EPS.