The doctoral thesis is devoted to the question of research of transient stability of large power systems. A classical method of interconnected power system reduction by the mean of load nodes elimination, based on the linear transformation using the Jordan elimination method was analyzed and applied. Taking into account the inapplicability of method classic version for considering the real loads, a new methodic which complements the method referred above and allows eliminate nodes with nonlinear loads, dependent from voltage and frequency and represented by its static and dynamic characteristics is proposed. Approbation of new methodic was completed on the example of power system model of Baltic’s country “electrical power grid “БРЭЛЛ” ”, that is a content of power system IPS/UPS. On the example of initial and received reduced model of power system were modeled and examined transient processes of different kind of difficulty rate as well as completed comparative analyses of obtained results. The Doctoral thesis is written in Latvian language, it contains introduction, 5 chapters, conclusions and recomendations, list of references, and 1 appendix. Total number is 173 pages, which include 68 figures and 71 tables. The list of references includes 94 literature sources.