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This work presents an analysis and a new control design of a voltage-source converter (VSC) transmission system operating under unbalanced network conditions. The system is analyzed in the positive and negative synchronous reference frames. The proposed control strategy contains a main controller and an auxiliary controller. The main controller is implemented in the positive d-q frame using decoupling control without involving positive/negative-sequence decomposition. The auxiliary controller is implemented in the negative-sequence d-q frame using cross-coupling control of negative-sequence current. Simulation results using the SIMULINK power system blockset show good performance of the proposed control strategy for a 300-MW 300-kV dc VSC transmission system during both balanced conditions and unbalanced conditions as may be caused by a solid single-phase-to-ground fault.