Extra high voltage (EHV) power transmission systems have been traditionally constructed by using overhead lines (OHL) to transfer power over long distances. During the last decade, the opposition against the construction of new OHLs has significantly increased due to societal and environmental concerns, which has caused major obstacles for the grid development. Under this circumstance, system operators have been urged to find solutions and alternatives for the future grid developments. A promising solution of this challenge is to underground the transmission grids (fully or partially) by means of EHV AC underground cables. In this regard, the future transmission grids will be composed of OHLs in non-sensitive areas and underground cables in sensitive areas like populated neighbourhoods and environmentally sensitive locations, which implies on a large-scale utilization of long cables in future EHV grids. These grids are known as hybrid OHL-Cable grids. Although this is very encouraging from the societal and environmental points of view, new challenges arise mainly from the technical perspective and high capital cost. Regarding the technical perspective, the large-scale application of long cables in transmission grids is not yet a well-practiced technique for system operators. In fact, cables have been widely used in low and medium voltage distribution grids, but not in EHV transmission grids. The electrical, thermal, and mechanical characteristics of cables and OHLs are significantly different. These differences can cause various technical problems in the grid, which in return may increase the chance of damage to system components and reduce the reliability of the power supply. Therefore, a decision for the large-scale utilization of EHV cables will be very risky without gaining the complete knowledge and insight of the expected hazards and their countermeasures. This was the main driving force for system operators and manufacturers to carry out research and investigation on the technical performance of EHV grids with long cables. So far, lots of researches have been performed to investigate the design and operation of long EHV cables in transmission grids. These studies have answered many questions and unknowns, but there are still several important scientific gaps that have to be tackled. As a result, the Dutch transmission system operator, TenneT, began an extensive ten-year cable research program together with the Technical Universities of Delft and Eindhoven to investigate the technical possibilities of utilizing long EHV underground cables in the future transmission projects. This thesis, as the last part of the Dutch cable research program, provides robust and comprehensive answers to the most crucial scientific gaps and addresses the required techniques for the reliable operation of cable projects. These techniques can be used in practice by system operators since they are based on realistic assumptions and reliable simulations on an accurate model of an actual power transmission system. This thesis focuses on crucial phenomena related to the steady-state operation, harmonic behaviour, and transient operation of hybrid OHL-Cable systems. A hypothetical future project in the Dutch 380 kV grid with 80 km transmission length was selected as the case study, for which all phenomena were studied according to the most recent standards and grid code. The main scientific contribution of this thesis is the rigorous and comprehensive analysis of a hybrid OHL-Cable system to identify the impact of long cables, system parameters and topology on the system operation. The thesis proposes a methodology for optimal compensation of the cable reactive power in order to enhance the system performance. Moreover, the significance of energization overvoltages is investigated by a robust statistical analysis, which is the first of its kind for hybrid OHL-Cable grids. Last but not the least, two new countermeasures for the zero-missing phenomenon have been developed and several other countermeasures have been also investigated. The main conclusion of the thesis is that the large-scale application of underground cables in transmission systems is technically possible under the condition that all technical phenomena and issues are properly addressed in the planning and designing phases of each project. A case-by-case study for cable projects is a “must” as each project has its own electrical and geographical characteristics. System parameters and topology are different in different areas and consequently the severity of phenomena and challenges will be different. Several countermeasures are available for each technical issue, where the most optimal one should be selected by conducting an in-depth technical analysis. The decision to choose the right countermeasure is highly dependent on the project specifications. Finally, for each cable project, it is always recommended to perform a step-by-step study similar to the presented approach in this thesis, in which all the relevant phenomena from the steady-state operation to the electromagnetic transient behaviour are investigated. The study should follow the guidelines, grid code, manufacturer requirements, and standards in order to guarantee that all requirements for a reliable system operation are met accordingly.