The power supply systems of large cities are component parts of the power system of a country or its region. The creation of power supply systems of large cities (LCPSS) and the necessity for analyzing the parameter optimization is determined by the intensive dynamics of the number of cities, their territories, the number of population in different countries, the composition of electricity consumers and the installed power, along with continued electrification of everyday life, improvement of the services sector and ensuring of the life processes of population. The electric power networks of cities are characterized by a multifaceted composition of consumers. Various groups of consumers significantly differ in terms of consumer capacity, operating modes, requirements regarding the quality of electric power and the uninterrupted operation of power supply. In the layout of substations and construction of power transmission lines, it is necessary to consider architectural, urban construction and environmental limitations as well as the established tradition of urban planning. The performance of the task of designing electric power systems of cities is compounded by large amounts of normative acts or by lack of initial information. The power supply system has to meet a number of contradictory requirements: reliability, electric safety in operation, and required quality of electric power. In the creation and development of electric power networks, it is necessary to consider the possibilities of gradual development, both in terms of the capacities of the transformers of the substations as well as the transfer capability of the power transmission line and the voltages of the supplying networks. The above-mentioned factors determine continuous development of electric power networks in space and time as well as increase of the load of the power supply systems of large cities. Creation of new electric power networks as well as extension or reconstruction of the existing ones requires considerable material, financial and labour resources. These are required for electrical equipment and for performing works of construction, installation, bringing into order and others. It has to be taken into account that the funds invested into construction as well as reconstruction of power facilities do not ensure economic profit until the moment operation is started. Therefore, it is necessary to perform evaluator technical and economic calculations, determining the proportions of funds for the construction of new power facilities and the reconstruction of the existing ones. Optimum development of a complicated, dynamic LCPSS requires in-depth scientific research applying system analysis methods, considering the dynamics of the changes of the equipment parameters and an increase in the consumer loads for the whole city power supply system. The difficulty of solving the tasks of optimum development and rational construction of networks can be explained by the fact that these tasks are solved at the conditions of incomplete and uncertain information about the timeframes of putting into operation individual facilities as well as lack of reliable data regarding the loads of prospective consumers and detailed designs of buildings and structures in certain districts of a city. At these conditions, it turns out that it is difficult to find a solution to the complex task of optimum LCPSS development as a global system. In the search for a decision, a complex task is divided into a number of local optimization tasks, considering the requirements that appear on the part of the global system as well as other external sides regarding the local system. Given this condition, the optimum plan of an individual subsystem, with a certain relative error, is contained as a component part within the optimum development plan of the global system. The present Doctoral Thesis is dedicated to solving individual tasks of rational formation of power supply systems of large cities in the example of Riga city (the capital of Latvia). The presented and technically and economically substantiated choice of parameters is an important stage in rational designing of the electric power networks of cities. This issue is topical in the power industry at any stage of the development process. The obtained solutions can be used in the power supply systems of Riga, other cities of Latvia and other cities abroad for working out the optimum development strategy.