Planning of the national energy system by looking at alternative investment decisions through annual simulations (using the EnergyPlan tool) due to its (combinatorial) complexity leaves little room for explicitness regarding the network's impact on planning decisions and therefore there is a need to look at the power flows in the network. The investment decisions that are made remain with a degree of vagueness in the technical aspect, which can be removed with this connection. The CASE tool is used for numerous system calculations of power flows, shortcircuit currents, series faults, stability to small disturbances and transient stability. Calculations based on the calculation of power flows are also implemented in the program: network reduction, security (N-1 and N-X), NTC and OTDF/PTDF. The coupling of the CASE program and EnergyPlan was achieved through the output text file of EnergyPlan, which contains the total production of power plants by type, the total consumption of the system divided into several subtypes, export and import from the regulatory area. CASE loads the output file of EnergyPlan and distributes the total production by power plant types to each individual power plant in the system according to predefined distribution coefficients, while the total consumption is distributed proportionally to the existing loads by network nodes. Thus, a system model for power flow calculation is defined for each hour of the year. Calculation results for each hour are stored in the working memory of the CASE program and can be sorted by the most loaded element of the system, the largest exchange of the observed control area, losses in the control area, production by a certain type of power plant, consumption, etc. In this way, it is possible to efficiently analyse the operation of the system according to different criteria. First of all, the goal of this work is to couple the two software tools, which has not been realized until now, as well as the presentation of some of the most significant results selected from among the numerous options of state sorting. The result of the work is the integration of software tools that serve the benefit of planning activities in terms of their technical feasibility, that is, to examine more precisely whether a prospective scenario is realistic from the aspect of power flows in transmission grid.
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