This final thesis aims to show how, by carrying out technical and economic analyses, multi-year complex investments in the power industry can be successfully and efficiently controlled and managed. The aim of the thesis is to show the advantages resulting from the reconstruction and transition of transmission lines from 10 kV to 20 kV voltage level using a techno-economic analysis. In the paper, a techno-economic analysis of a concrete example of the reconstruction and transition of the 10 kV Trakošćan transmission line from TS 110/20/10 kV Ivanec to 20 kV was carried out. All this in order to obtain the highest quality information that would serve as a basis for further planning and management of capital investments of this type. The thesis consists of six units. The first one is an introduction in which the subject, objectives and structure of the thesis are presented. It introduces the topic of the entire final thesis. The second unit presents the strategy for the development of the electric distribution network with an emphasis on the strategy for the transition to the 20 kV voltage level and provides an overview of the theoretical part of investments in the power industry. The unit ends with an overview of general characteristics and a historical overview of the transition to the 20 kV of Elektra Varaždin. The third unit of the thesis presents a technical analysis consisting of a general and technical description and an analysis of the technical indicators of the observed investment through a comparison of technical criteria before and after the reconstruction and transition of the observed transmission line to a voltage level of 20 kV. The fourth unit provides an economic analysis of the investment in question. It consists of an analysis of costs, the determination of income, and, finally, the calculation of profitability indicators, the breakeven point and the safety coefficient. In the fifth part, the financial indicators of the investment in question are observed through the analysis of the financial criteria of net present value, payback time and internal rate of return. Finally, the sensitivity of the mentioned financial criteria to changes in key variables is analysed. The sixth unit is the conclusion.