Expansion double-electrode direct current transmission system

The invention discloses an expansion double-electrode direct current transmission system which comprises a rectification convertor station, a rectification side current conversion unit, a three-electrode power transmission circuit, an inversion side current converting unit and an inversion convertor station. When the expansion double-electrode direct current transmission system normally works, the rectification convertor station controls the magnitude of direct currents by controlling the triggering angle of a thyristor and converts the three-phase alternating currents into double-electrode direct currents in a rectification mode; afterwards, the double-electrode direct currents are converted into three-electrode direct currents through current conversion processing, flow through three transmission lines respectively and undergo current conversion reverse processing. The three-electrode currents are converted into the double-electrode direct currents and the direct currents are converted into three-phase alternating currents through the inversion action of an inversion side thyristor converter and a received end alternating current system is filled with the three-phase alternating currents. Thus, power is transmitted from a sending end alternating current system to the received end alternating current system. Thus, the investment cost is much less than that of a three-electrode direct current transmission system, the transmission capacitance is 1.37 times that of a double-electrode system, consumption loss of extra power will not be introduced into a direct current side, good economical efficiency is achieved and engineering application is facilitated.