## 3 Results
In the next stage, to increase the survivability of the EPS, we determined the maximum value of the total active power of the DG facilities, necessary to maintain the stability of the EPS. This value for the normal network scheme was 40 MW (DG facilities were connected to nodes 26, 30, 32, 38, and 39). The connection of DG facilities caused overload in branch 18-23, but this shutdown did not lead to further cascading process development, and the voltage remained in the range of admissible values. The EPS nodes for connecting distributed generation facilities were selected based on the results of mode calculations. The EPS nodes were selected where in the normal mode the load of the branches approaching them was close to the maximum permissible value or the voltage was below the nominal value and approached the limit of \(-10\%\)\(U_{\text{rated}}\). In the network repair scheme, the connection of DG facilities with a total active power output of 50 MW caused overload in branch 18-23. After its disconnection, a loss of voltage stability occurred at node 34. When the active power output from DG facilities rose to 55 MW, an overload occurred in branch 18-23, but after its disconnection, voltage stability was maintained. Consequently, DG facilities contributed to the non-homogeneity of the EPS parameters. They can foster cascading failure processes, leading to the disruption of power supply to consumers. However, they can also have the potential to boost the survivability of EPSs and improve the reliability of power supply to consumers [80]. The capacity of DG facilities and their operating conditions should be determined in the planning stage of their construction. The appropriate choice of parameters and operating conditions of DG facilities when integrated into EPSs can provide substantial benefits. However, their operating parameters (output power) should be planned for various topologies and operating conditions. Figure 7 shows a test scheme of a 500 kV EPS (36 nodes) in which the topology and operating conditions were modeled to ensure an active power flow of 2868 MW between the EPS1 subsystem (the left three columns of network nodes) and the EPS2 subsystem (the right three columns of network nodes). We will present the results of the steady-state calculation following an emergency disturbance resulting in the disconnection of branch 8-9 under the repair scheme (branch 3-4 was disconnected). This scenario involved an inadmissible overload of the 500 kV intersystem overhead PTL (branch 13-14), resulting in a cascading failure process and its development. It is accompanied by the EPS islanding and the loss of voltage stability. The trajectory along which the cascading process developed in this case and ended in the region of inadmissible power flows with the disconnection of the branches due to the inadmissible overload was as follows: Step 1--8-9; Step 2--9-14, 13-14; Step 3--10-15, 18-19, 33-34, and 34-35. Changes in currents in the branches of the EPS scheme during the development of the cascading failure process are shown in Figure 8.  
To increase survivability and prevent cascading failure development, we explored the installation of various FACTS devices designed to enhance the transfer capability of overhead PTLs [81-83]. A series of simplified calculations were conducted as a part of the feasibility study on the implementation of FACTS devices. The calculated value of the rated power of the FACTS device for the case under consideration, where an overload resulted from a short circuit in the overhead PTLs 8-9, followed by the disconnection of overhead the PTLs 9-14 and 13-14, was 1065 MVA. To unload the specified overhead PTLs, the FACTS device had to be installed in the overhead PTLs 13-14, which had \(X=112.8\) Ohm. Figure 7: Test scheme of 500 kV power system (36 nodes).