The main objective of this thesis is to model and control a high-voltage direct current (HVDC) power transmission system based on modular multilevel converters (MMCs). MMC-HVDC systems are crucial for the efficient transmission of energy over long distances and for the integration of renewable energies. Based on a suitable model of a five-level MMC-based terminal, control strategies have been developed to ensure stability, increase reliability and improve overall system performance. These strategies include predictive control and Direct Power Control (DPC) based on sliding mode control. Finally, numerical simulations are presented to validate the models and control strategies adopted. The results demonstrate the effectiveness of these approaches in a variety of scenarios.