This paper proposes a fast optimum-predictive controller to balance the dc capacitor voltages and to control the power flow in a long distance HVDC system using bipolar back-to-back connected NPCs. For both converter sides, the control strategy considers active and reactive power to establish ac grid currents on sending and receiving ends, while guaranteeing the balancing of both NPC dc bus capacitor voltages. Furthermore, the fast predictive controller minimizes the semiconductor switching frequency to reduce global switching losses.
The performance and robustness of the new fast predictive control strategy and the associated dc capacitors voltage balancing are evaluated.