## 6 Power System Simulation of Desynchronization from the Synchronous Area  
In the numerical simulations, the sum of exchange with the synchronous area was about 325 MW and constant, HVDC1-HVDC4 schedules varied within \(\pm\)50 MW, and the load forecast was up to 50 MW. The first indication that fast desynchronization from the synchronous area has occurred in the EPS is switching off the overhead lines OHLS, OHL9, OHL10, and OHL11 (see Figure 1). Exchanging active power between a stable EPS and a regional EPS during transition to isolated operating mode changes from 100 MW to 0 MW. The frequency deviation of EPS is shown in Figure 10.  
Due to an unplanned decrease in the throughput between EPS parts from 100 MW to 0 MW, EPS part No.2 operates the G11 and G12 FCR of the generators. The FCR function of these generators is always on and has the lowest insensitivity limit, i.e., less than 10 mHz. The increase in EPS load (up to 100 MW) affects the frequency change, which is not very large. The frequency drops to 49.97 Hz. Intersystem bandwidths are also affected by the size of the load, i.e., the transmission lines between part No.2 and No.3 of the EPS. The voltage level increases at stabile EPS part No.3 because unloaded energized lines appear in the system. The voltage value increases to 363-364 kV, so it is necessary to react immediately and take measures to reduce the voltage value. Switching on shunt reactors in the area of voltage increase is perfect for this. Another effective method of voltage control is the disconnection of unloaded 330 kV overhead lines. After doing so, the voltage of 330 kV in part No.3 settles down to the permissible value of 359-360 kV. The second step is switching off the part of EPS (part No.1, part No.2, and part No.3) switching off isolated operating mode from the large and stabile EPS, this time in parallel, switching off overhead lines from OHL1 to OHL7 (see Figure 1). The exchange active power between the stabile EPS and the regional EPS in isolated operating mode changes from 225 MW to 0 MW. The frequency deviation of EPS is shown in Figure 11.  
After the reduction in throughput between EPS parts from 225 MW to 0 MW, EPS part No.1 operates the G3-G6 FCR of generators. The FCR function of these generators is always on and their insensitivity limit is close to 10 mHz. The increase in EPS load (upto 225 MW) affects the frequency change, which is already more pronounced this time. The frequency drops to 49.65 Hz. System bandwidths are also loaded with the size of the load, i.e., transmission lines between EPS part No.1 and No.2 and between No.2 and No.3. EPS parts No.1, No.2 and No.3 are in isolated operation. Actions in isolated operation are balancing the system and controlling the voltage. The voltage level increases at all isolated EPS parts, No.1, No.2, and No.3, because of unloaded energized lines in isolated parts of system. The voltage values increase to 364-365 kV, so it is necessary to react immediately and take actions to reduce the voltage values. Switching on shunt reactors is perfect for this. A very effective way to control the voltage is to disconnect the unloaded 330 kV overhead lines. After doing so, the 330 kV voltage in isolated EPS parts No.1, No.2, and No.3 stabilizes to permissible values. If these means to suppress voltages are not enough, it is necessary to use hydro aggregates that can work in SC mode [1]. _Isolated Power System Trips Simulation_  
Reaction of the isolated EPS to disturbances:  
1. Tripping of the generator G5 + FCR (generator 80 + 55 = 135 MW) at EPS part No.1. Activation of additional power (FCR of generators: G3, G4, and G6) at EPS part No.1 restored the frequency back to 50 Hz. Frequency deviation of isolated EPS shown in Figure 12.  
Figure 10: Frequency in system after region EPS transition to an isolated operating mode. After the reduction in active power generation in an isolated EPS from 135 MW to 0 MW, in the EPS part No.1 operates G3, G4, and G6 FCR generators.