This thesis is composed of 7 chapters. The first chapter is the introduction that deals with transfer capacity insufficiency and the importance of forecasting congestions on cross-border power lines so could the said congestions be avoided or removed. The second chapter describes electric energy system, it is division and function of each part and the importance of balance between energy production and consumption is described, as well as power flow through the grid dependent on Kirchhoff’s laws on a simple example. Third chapter mentions models of overground power lines and cables, transformers, generators, and loads. It also shows their substitute models which are used in power flow and short circuit calculations. Fourth chapter concerns electrical grid models. Fifth chapter explains two most used power flow calculation methods, which are Gauss-Seidel with y matrix method and Newthon-Rapshon method. Knowing these two methods allows for better understanding of the calculation executed in in the PSS/E program during the analysis of DACF file. Sixth chapter describes basics of congestions management and four phases of management. Seventh chapter describes day ahead congestion forecasting procedure itself and daily overview of Croatian electric energy system. The calculations made in two DACF files acquired from HOPS are described. Changing the sum of power production and power loads as well as engaging and disengaging certain 400 kV and 220 kV lines shows the importance of accurate forecasting. Even small changes in production, consumption, or grid topology can cause overload on cross-border power lines. Furthermore, both examples showed that the Croatian electric energy system is robust enough so even greater changes do not cause congestion. The reason lies in large power capacity in transfer power lines. With day ahead congestion forecasting, more effort is given on developing an hour ahead congestion forecasting methods.