Control of VSC transmission systems under unbalanced network conditions

This paper presents a new control design for VSC transmission systems with special attention to unbalanced ac network conditions. The proposed control strategy contains a main controller and an auxiliary controller. The main controller is implemented in the positive dq frame using decoupling control without involving positive/negative sequence decomposition. The auxiliary controller is implemented in the negative sequence dq frame using cross coupling control of negative sequence current. Simulation results using SIMULINK power system blockset (PSB) 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 which may be caused by solid single-phase to ground fault.