The book describes the mathematical models of the electrical system regimes and their basic components. The steady conditions of the (possible) tasks of the electric power system management are considered, as well as the calculation methods of equations characterising these conditions. To calculate the steady regimes of the electric power systems, the diacoptic method is offered which observes the intersystem exchange regarding both in the current and power balance . A separate chapter is devoted to the criteria of analysis of the steady stability of of the power system regime. The theoretical assumptions are illustrated by examples. The book may serve as a textbook for the power engineering students and doctoral students, as well as engineers.