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HVDC systems can transmit more power from electricity-rich areas over longer distances than alternating-current lines of equivalent cost. It is pointed out that one advantage of direct-current transmission is its inherent immunity to the inductive reactance problems that make it difficult to use alternating current when long lines and large loads are involved. Direct-current cables can also carry more power than AC cables of the same size, because there are no charging currents and dielectric losses. This makes direct current the obvious choice for submarine cables longer than about 70 or 80 km. Another advantage is that HVDC transmission systems can link neighboring asynchronous networks economically and reliably. Attention is given to the health issues that these lines have raised. The export and import arrangements between countries that involve HVDC lines are also discussed.