In this thesis, high-voltage direct current networks and their growing application due to the progress of power electronics are explained. Historical development and comparison with high-voltage AC networks are generally explained. The parts and operation of a high-voltage direct current station for energy transmission, as well as the most used topologies of high-voltage direct current networks, are presented. Furthermore, as the most important part, high-voltage converter substations and their management strategies are described and explained in detail. Direct current distribution in the maritime industry is explained in general, followed by the application of high-voltage direct current networks in the power supply of vessels from the shore, and the direct application of high-voltage direct current networks as the main power supply system on board. At the end, the functional design of a high-performance ship system using a high-voltage level, which is considered medium voltage on vessels, is illustrated and explained in detail.