## 7 EPS Synchronization with the Synchronous Area  
For an isolated EPS to be connected to a synchronous power system, the isolated EPS must maintain stable parameters, such as active power generation, frequency, import/export stable timing via HVDC, and low and steady load variation. In addition, the isolated EPS must maintain sufficient inertia magnitude, active and reactive power reserves, FCR, and FRR. The system needs to have the means to manage the voltage and the generation source must not exceed the allowed maximum value of 300 MW. Intraday trading must be limited to 0 MW throughout this period [37; 38; 39; 40]. In addition to the above requirements, for successful synchronization of an isolated EPS with a synchronous system, the following technical conditions for this synchronization must be fulfilled:  
1. Voltages of two EPSs must be equal (difference not exceeding \(\Delta U<0.05U_{\text{N}}\));
2. Phase sequence of two EPS voltages must be the same;
3. Frequency of two EPS voltages must be the same (difference not exceeding \(\Delta f<\)0.1 Hz);  
Figure 14: Tripping of the biggest active power HVDC1 unit of the isolated EPS. Voltages of two EPSs must be of the same phase at the moment of switching on (difference between the vectors of the same phase not exceeding \(\pm 15^{\circ}\)). To fulfil the synchronization conditions, the reactive power influence of the overhead/cable lines within the system on the EPS has to be calculated and assessed. Charging reactive power (MVar) per 100 km is shown in Table 5.  
To ensure the smoothest and most reliable synchronization of two EPSs, the second and additional transmission lines must be put into operation as soon as possible. The EPS synchronization process can be carried out as described below (see Figure 15). In a case where two EPSs with active power transfer and connected load have different modes, the synchronization of two EPSs requires maintaining the phase difference angles \(\varphi_{1}\) and \(\varphi_{2}\) of two EPSs at the same value and controlling the voltages \(U_{1}\) and \(U_{2}\). Reduce the phase difference angle \(\Delta\varphi\) and the voltage difference \(\Delta U\) of two EPSs using a generator. If the line is not loaded, the load is off and no active power is transmitted through the line, then the phase difference angle \(\Delta\varphi\) between two EPSs will be maintained at 0 and the load will not be switched on. However, this mode of operation has a high risk of significantly increasing the EPS voltage. If there is a second line, it reduces the load on the first line but increases the voltage on the first line. The synchronization conditions are implemented according to the requirements specified in Figure 16.  
Figure 15: EPS parts restoration—closing of loops. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Voltage, kV** & **Line, Mvar** & **Cable, Mvar** \\ \hline
20 & 0.1 & 30 \\
110 & 3.5 & 200 \\
220 & 14.5 & 500 \\
380 & 60 & 1000 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Reactive power (Mvar) values per 100 km. The voltage difference between two EPSs is found as follows:  
\[\Delta U=U_{1}-U_{2}, \tag{11}\]  
where \(U_{1}\) is the voltage of the first EPS (kV), and \(U_{2}\) is the voltage of the second EPS (kV). The phase difference angle between two EPS voltages is calculated as follows:  
\[\Delta\varphi=\omega t_{1}-\omega t_{2}+\varphi_{0}, \tag{12}\]  
where: \(\omega=2\pi f\) is angular frequency (s\({}^{-1}\)), \(t\) is time (s), the initial phase of the voltage (\({}^{\circ}\)). The difference between two EPS frequencies is found as follows:  
\[\Delta f=\frac{(\omega_{1}-\omega_{2})}{2\pi}, \tag{13}\]  
where \(\omega_{1}\) is the angular frequency of the first EPS (s\({}^{-1}\)), and \(\omega_{2}\) is the angular frequency of the second EPS (s\({}^{-1}\)). The frequency change after successful synchronization and the distribution of inter-system flows are presented in Figure 17. Voltage values in the EPS nodes range from 355 to 358 kV.