## 2 Method
### Power Grid Description  
The actual Baltic electricity network, which consists of the networks of three Baltic countries--Lithuania, Latvia, and Estonia--is used to test our method. The transmission network consists of 110 kV and 330 kV power lines. The network data for each country are presented in Table 3.  
The official data of the Baltic countries are provided in sources [30; 31; 32]. The Baltic transmission network is specific in that it operates in the BRELL ring with the electricity networks of Russia and Belarus. Commercial traffic between the Baltic and Belarus/Russia is currently limited. Thus, Baltic countries operate on a kind of \"isolated\" island, as it does not have/use AC connections to other electrical systems. The Baltic region has four DC system connections with continental Europe and the networks of the Nordic countries. Three more connections are planned: Estonia--Finland, Estonia--Germany, Lithuania--  
Figure 3: Algorithm for set creation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**State**} & **Length of 330 kV** & **Length of 110 kV** & **Number of** & **Number of** \\  & **Lines, km** & **Lines, km** & **Lines to Model** & **330 kV Nodes** \\ \hline LTU & 1896 & 4968 & 27 & 16 \\ \hline LV & 1742 & 3812 & 24 & 19 \\ \hline EST & 1634 & 3361 & 27 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The network data of the Baltic States. Poland. A map of the Baltic 330 kV power grid with the planned new 330 kV lines and future DC connections is presented in Figure 4. It should also be noted that the Baltic electricity system will be connected to continental European grids after 2025. In Figure 4, the red crosses indicate the point of disconnection with the BRELL system, the black circles indicate the 330 kV nodes/buses, and the solid green line indicates the existing DC links. During this study, only the 330 kV power transmission network is simulated, and the number of simulated lines is 78. For the case of the N-2 and full check performed, the number of possible combinations is 3003. When only 10% of the lines are selected, the number of possible combinations shrinks to 28, when 20%--120, when 30%--276. Generally, system operators, with historical data and expert experience, can reliably determine the periods of the heaviest load mode in the power system. We are considering the working mode of the normal scheme, before contingency, when capacities of power lines or transformers are practically reached, and these elements operate at their permissible static safety limits. By knowing the minimum and maximum modes, operators assess whether additional measures need to be taken, such as changing the scheme or limiting part of the load/power plants. Four operating modes of different loads were simulated:  
* Summer day (max). * Summer night (min). * Winter day (max). * Winter night (min). Different scenarios of the seasons and time of day allow us to reveal the distributions of different generation and load configurations in the electricity system. The system load was selected according to the data on the loads of the Baltic countries in recent years [34], and according to the maximum values of individual countries, which are usually experienced during the winter season in our region. The load data of the Baltic countries observed in 2022 are presented in Figure 5.  
Figure 4: Development map of the Baltic network [33]. The highest total load in the Baltic system of 2022 was observed in the second week of the year, then it reached about 4720 MW, in Lithuan--2050, in Latvia--1190, in Estonia--1480. These values do not differ much from the maximum historical values, which are for Lithuania--2231 MVA, Latvia--1240 MVA and Estonia--1591 MVA. In winter max mode, we use approximated data--5000 MW. Two load scenarios based on the actual (2022) and the planned (2030) electricity consumption were created. The historical and planned data of load values provided by the operator of the Lithuanian transmission system were used in this study (this data is provided in Table 4).