## 9 Discussion  
This research aimed to investigate the behavior of electrical power systems (EPSs) during desynchronization from a synchronous area and their ability to operate and withstand disturbances in islanded mode. Synchronization of the system was considered as well. The main factors related to EPS's desynchronization/synchronization and its operation in islanded mode were indicated, researched, and discussed. The rapid identification of desynchronization is critical for EPS operators. The main signs of a sudden EPS disconnection from a shared EPS are SCADA control system data which indicate that connecting devices of the inter-system lines to the shared EPS (on the side of the shared EPS) are disabled are that the active power flows through the inter-system transmission lines equal 0 MW and sustained deviations in frequency (greater than \(\pm\)0.2 Hz) and voltage (above 362 kV in 330 kV network) are observed. The most important key factors affecting the response of an EPS to a sudden disconnection are the power balance of the disconnected EPS, including the active power flows from neighboring EPSs; the number of power plants operating in the isolated EPS and their FCR; the amount of inertia remaining in the disconnected EPS; the remaining active power reserve through HVDC interconnectors; and the EPC of the interconnectors and loads of the insulated EPS, i.e., UFLS automatic self-regulation. Figure 18: Algorithm of the sequence of actions after a sudden disconnection of the EPS. The more generators operate in the islanded EPS, the more generators are available with the FCR function, enabling lower frequency variation in the system. Another very important aspect is the \(k\) factor of the EPS. Its calculation is useful when the system frequency varies in the range of 0.2-0.25 Hz and the EPS itself is in a quasi-static state, i.e., when the frequency variation (\(\Delta f\)) is in the range of 0.2-0.3 Hz. The situation after an unscheduled disconnection in an isolated EPS is untoward if it has small inertia, if FC and FCR functions remain inactivated in the HVDC interconnections, and if the HVDC interconnections are at maximum load, the isolated EPS has insufficient FCR and not enough generators in operation, or the power reserve of the operating generators is too low for primary regulation. To determine the operation and reliability of the EPS, consisting of three parts with known parameters, during its desynchronization, in islanded mode, and in synchronization, numerical simulations were caried out with a widely used power-system simulation package. The analysis revealed that the most effective control of generators is observed when their generation range varies from 50 to 300 MW and the FCR of the generators varies from 50 to 170 MW. The simulations showed that the frequency gradient of the EPS restoration with a single generator is too low. The frequency restoration with several generators determines the higher gradient of generators (sum of involved generators) and lower frequency deviation. It was defined that none of the generators operating in the islanded EPS should be driven to the maximum (nominal) value and they could exceed the power limit of 300 MW. The amount of electrical power generation and its management must be distributed evenly across all three parts of the EPS. Frequency deviation in the system does not exceed 200 mHz for disturbances of lower than 100 MW in the islanded EPC, when the FC function of the HVDC converters is disabled. The maximum permissible disturbance that can occur in the analyzed isolated EPS amounts to 300 MW. This causes the frequency to drop to 49.25 Hz in the EPS, which is close to the critical limit of 49.2 Hz where the UFLS is triggered. Analysis revealed that voltage increases of up to 365 kV are observed in the isolated EPS when the HVDC converters are operating without the FC function. In response to the voltage increase and control, it is recommended to maintain the voltage below 357 kV.