Integration of renewable energy sources and market circumstances have significantly changed the transmission network planning process which leads to the need to increase capacity of transmission network. It is necessary to better use of the existing transmission grids in purpose to transferee wind and solar power to where it’s needed. Lack of capacity on key points of the transmission grid is also postpone new renewable energy projects from being developed and build. To achieve integration of renewables it is necessary to build new overhead lines (OHL) or increase capacity of existing ones. Building new OHL is an expensive and time-consuming process that highly depends by landowner, lawsuits and unfavorable regulatory rulings although renewables developers and their offtakers can afford it. Process of building new OHL takes years and may deviate significantly from the initially planned deadlines. New approach to increase transmission capacity is needed which include new technologies that can help to solve the problem. Possible solution that can be integrated into the grid with low cost and can have significant amounts in congestion managment are power flow control, dynamic line ratings and topology optimization. Each of mentioned technology doesn’t provide long term solution and is not applicable in each case of solving transmission congestion and additional solution must be considered. One of the way to increase OHL capacity is to increase operating current or voltage level on existing OHL. Increasing current on existing OHL as a consequence have increase in conductor temperature, melt or fail conductor strength and increase in sag. In that purpose new conductor was developed that can withstand mentioned demands (High Temperature Low Sag Conductors HTLS). HTLS conductor can withstand permanent operating temperatures up to 210 °C carrying higher power compared to conventional conductors. These conductors have a wide range of application possibilities when there is need for increase transmission capacity, clearance problems and restrictions to the use of new and higher towers. Big advantage of HTLS conductors is fast installation comparing to building/reconstructing OHL and without need to modify most of the existing towers. Increasing voltage level on existing OHL is considered in cases when there is need to increase transmission capacity where building new OHL is not possible. Voltage uprating on existing OHL demands investigation on electrical parameters and mechanical parameters of existing infrastructure OHL. Main characteristics of voltage uprating is smaller clearance, composite insulators, coordination insulation, overvoltages and main electrical characteristics (natural power, electrical gradient, corona etc.). For that reason (smaller clearance) and designee solution such OHL is named compact OHL. In some cases voltage uprating with compact designee requires modification and foundation reinforcement of existing tower or new tower designee and foundation. Main advantage of compact OHL is higher power transfer, better stability in operation, smaller impact on the environment, smaller right-of-way compared to conventional solution. However, the cost of the voltage uprating is high and requires additional considerations comparing to conventional methods in process of reconstructing or building OHL. Motivation for dissertation came from desire to consider the implementation HTLS and compact designee technologies in the transmission network when the main aim is to increase the capacity of the existing OHL. The aim of the research is to provide better insights into the behavior of HTLS conductors and compacted transmission lines in the transmission network taking into account the transmission network development plans and the influence of chosen solutions. Conducted analyzes consider increase in transmission capacity and impact of losses in the transmission network for different types for selected technologies. Recently transmission system operator become responsible for amount and cost of losses and new investment must be optimized from prospective that the lifetime of these investments is longer than 40 years. For any type of investments it is necessary to determine possibilities of applicability taking account of other limitations. For compact solution in this dissertation will be consider dielectric properties and electric fields on individual components of the transmission line and in the transmission line corridor. The application of HTLS conductors allows to increase the temperature of conductors and consequently suspension equipment and insulators. To determine radial temperature distribution in the HTLS conductor and to establish how the temperature expands thermal model of HTLS conductor will be proposed. For both approaches additional verification of electric and thermal parameters will be verified using FEM and CFD numerical programs. Doctoral thesis analyzed the possibilities of increasing the capacity of existing transmission lines with the main goal that proposed solution are applicable in the Croatian transmission system. Dissertation can be divided in three main parts that represent process from the beginning of determine which OHL doesn’t have satisfactory capacity, consider possible solution and true the additional research determine technical parameters of proposed designee. The first part of doctoral thesis presents overview of the parameters that influence where and when is increase in transmission capacity is needed. The process of determining the limitation of existing OHL in transmission network using the procedure in which it is necessary identify input data, perform additional calculations and analyzes considering future plans and in the end interpret obtained results. Presented process is algorithm that consider implementation compact designee and HTLS conductor with the aim to increase transmission network capacity. The algorithm presents steps in terms of a systematic procedure that involve the collection and data processing, calculations and decision-making criteria. The computational parts of the algorithm are based on power flow calculations for chosen scenarios conditions that consider development plans of transmission network. After determining the critical transmission OHL, the sensitivity analysis is carried to determine electrical parameters of OHL which need to be reconstructed in order to prevent possible congestion in the network. Conducted sensitivity analysis is perform considering increase capacity of each OHL and capacity of transmission network considering influence on losses. Determining the capacity of the transmission network are the main issues in the planning and management of the transmission network and at the same time an extremely important basis for planning the development of the transmission network. Due the several calculations of transmission network capacity methods Net Transfer Capacity and Total Transfer Capacity calculation are considered dominant and used in conducted considerations. Model for monitoring losses on each transmission lines is developed and presents an improvement of the existing process of determining Joule and corona losses on high-voltage transmission lines. From the available measurements and available input data, prediction of losses can be made for each OHL considering predicted power flows. The results of the losses assessment model have significant importance and is used in purpose of planning and procurement of losses leading to reduced costs for proposed solution. Furthermore, ability to manage electricity losses in the transmission system has practical significance, since they are an important technical and economic indicator of transmission system management. As was mentioned before compact designee have a smaller clearance and to achieve that applications of composite insulators as insulating cross-arms is needed. Due the smaller clearance temporary overvoltages can occur and their amounts need to be limited to acceptable level. For the proposed solution of 400 kV compacted transmission line calculation of insulation coordination is conducted according to IEC 60071-2. According to the results installation of surge arrester is needed where optimum installation locations is determine based on amount of temporary overvoltages and techno economical method that was presented. The third part of doctoral thesis presents calculations using FEM to evaluate electromagnetic behavior of individual transmission line components and thermal behavior of HTLS conductor comparing to existing conventional methods. Compact designee implies smaller clearance which causes increase in electrical filed on components in OHL tower. Although the design principles are similar to those for classic OHL designee, the margins are additional reduced, so special attention is made to understand how geometrical configuration influence on spreading electrical field on each elements of OHL. In that purpose 2D and 3D FEM model is developed to better understand difference between OHL designee. Conducted analysis consider distribution of electrical potential in tower OHL, electrical field on insulators, suspension equipment and potential gradient on conductors. In order to better understand the influence of conductor geometry on electric field different type of conductor was analyzed. The influence of compact designee on electrical and magnetic field in for OHL right of way OHL was also demonstrated. Conducted research gives overview of technical aspects that need to be consider before making final compact designee from dielectrically point of view. For HTLS conductors and insulator thermal analysis was performed using the FEM method to determine the temperature behavior compered to ACSR conductors. To determine conductor temperature IEEE 738 standard was applied for ACSR, ACCR and ACCC Drake conductors. For all three conductor influence of speed and angle of wind, insolation and air temperature on conductor current was performed. The results are used as input data for modeling 2D conductors in FEM analysis. For each of the three mentioned conductors temperature distribution was calculated using FEM Ansys. In addition to fully understand the process of generating and dissipating heat, an analysis was made using computational fluid dynamics (CFD) simulations. CFD simulation through the application of a multi-physical model examines in more detail the temperature distribution of conductor for processes of dissipating heat by velocity of air and heat radiation. Using the numerical procedures FEM and CFD it is possible to determine the surface and internal temperature of the observed elements and consider thermal limitation of conductors and insulator. Research in doctoral is relevant for transmission network development and revitalization power system. Scientific contributions of the dissertation are: algorithm for determining electrical parameters of compacted transmission lines and HTLS conductors with respect to the increase of transmission capacity and reduction of losses in the transmission system; model for selection conductor characteristics and configuration based on performed analyzes of electromechanical and thermal parameters for compacted transmission lines; multicriteria model for compact configuration selection with the aim of determining the availability of compacted transmission line.