A monopolar conductor is a key component used in high-voltage direct current (HVDC) transmission systems that operate in monopolar configuration. In this mode, electric power is transmitted using a single high-voltage conductor, while the return path for the current is provided either by the earth (ground return) or by a separate metallic return conductor operating at a much lower potential. This approach allows efficient long-distance transmission of large amounts of electrical power, especially over challenging terrains, underwater routes, or between asynchronous grids.The monopolar conductor itself is typically a high-capacity cable or overhead line designed to carry direct current at voltages that can range from several hundred kilovolts to higher levels, depending on the system design. For overhead systems, the conductor is usually made of aluminum or aluminum alloy reinforced with steel or composite materials to provide both electrical conductivity and mechanical strength. For submarine or underground cables, the conductor is often made of copper or aluminum and is surrounded by multiple layers of insulation, metallic screens, and protective sheaths to ensure reliability and resistance to environmental stresses.In an earth-return monopolar system, the conductor carries current in one direction, while the earth serves as the return path. This reduces the number of physical conductors required and can significantly lower construction costs for very long transmission links. However, the use of earth return must be carefully evaluated due to potential environmental and technical impacts, such as corrosion of buried metallic structures, interference with communication systems, and possible effects on nearby infrastructure. To address these issues, dedicated ground electrodes are installed at each end of the link. These electrodes are specially designed to disperse current into the earth over a large area, minimizing local current density and associated impacts.In some cases, a metallic return conductor is used instead of earth return. This configuration still belongs to the monopolar category because only one high-voltage pole is in service for power transfer, while the metallic return operates close to ground potential. This arrangement increases control over the current path and reduces the environmental concerns associated with earth return, at the expense of higher material and installation costs.Monopolar conductors are usually designed with the possibility of future system expansion in mind. Many HVDC projects initially operate in monopolar mode and are later upgraded to bipolar operation by adding a second conductor of opposite polarity. In the bipolar stage, each conductor carries current in opposite directions, and the earth or metallic return is used only under contingency conditions. This staged development allows optimization of initial investment while keeping open a path for capacity increase.The design of a monopolar conductor must consider electrical performance, thermal limits, mechanical strength, corona effects for overhead lines, and reliability under various environmental conditions. Proper selection of conductor material, cross-sectional area, insulation system, and protective structures ensures that the monopolar conductor can safely and efficiently transmit power over the intended lifetime of the HVDC installation.