## 5 Signs of an Abrupt and Unexpected EPS Disconnection  
Plausible indications that an EPS is being prepared for disconnection/isolation:  
1. Transferring stable EPS generation to new power plant units to ensure reliable frequency management;
2. Abnormally low power flow between different EPSs;
3. Abnormally low power flows between different EPS even with spare power capacity;
4. A large part of the inter-system lines are disconnected;
5. Sharing of SCADA data between different EPSs is discontinued;
6. Signs of a cyberattack. Signs of an abrupt disconnection of the EPS from the shared EPS:  
1. Information received in SCADA that the connecting devices of the inter-system lines to the shared EPS have been disabled (on the shared EPS side);
2. Flows in the inter-system lines equal 0 MW;
3. Frequency deviations greater than \(\pm\)0.2 Hz on the disconnected part of the EPS;
4. Significant voltage level deviations;
5. The effect of the disconnected EPS on the UFLS [34; 35; 36]. If the EPS remains in a stable islanding mode after an abrupt disconnection/isolation, it is important to assess factors such as the activation of the FCR for operating generators as soon as possible. Check that the n-1 reliability criterion is maintained in the isolated EPS. The largest source of generation must not exceed the set value of the calculated active power. A sufficient amount of EPS inertia must be ensured. The frequency of the EPS must not exceed the critical limit, as this is the only way to ensure a frequency drop reserve that prevents the triggering of the UFLS (see Figures 8 and 9). The situation after an unplanned disconnection of the EPS can be much worse if the isolated EPS has an insufficient level of inertia, if the frequency control (FC) function is unavailable or not activated via the HVDC interconnectors and the HVDC interconnectors are under maximum load, if there is insufficient primary active power reserve in an isolated EPS and not enough generators in operation, or if the active power reserve of the primary regulation of the generators in operation is too low [5; 18; 22; 29]. The unplanned or abrupt disconnection of the EPS may result in emergency disconnection of users/load due to the operation of the UFLS. A drop in frequency may also lead to the disconnection of some generation sources. A digital simulation of the frequency variation in an isolated EPS has been carried out where a generation source of 524 MW is disconnected and the UFLS is triggered, and the initial active power reserve is only 100 MW. The results of the simulation are shown in Figure 8.  
Figure 8: Tripping of the 524 MW active power at isolated EPS. Figure 9: Frequency of EPS after disturbances. In an isolated EPS, in the event of a 524 MW active power system disturbance and having only a 100 MW primary active power reserve, the system frequency drops to 48.10 Hz. When the system frequency reaches 49.2 Hz, the UFLS is triggered, causing the disconnection of electricity users. Once the system frequency has stabilized, it reaches 49.4 Hz. To restore the nominal 50 Hz frequency of the isolated EPS, aFRR or mFRR are required. Without sufficient active power reserve in an isolated EPS, frequency restoration becomes a systemic problem that can only be solved by limiting the supply of electricity to users. The digital simulation and analysis of frequency variation in an isolated EPS when there is a disconnection of a generation source with 100-400 MW of active power and only 100 MW of primary active power reserve was performed, and the impact of UFLS analyzed.