Voltage Source Converter High Voltage Direct Current (VSC-HVDC) systems have the ability to control rapidly the transmitted active power and independently exchange a reactive power with transmissions systems. The present work investigates the modeling and control design of a high-voltage direct current (HVDC) transmission system based on three-level NPC multilevel converters with Subharmonic PWM modulation technique (SH-PWM), and a feed-forward decoupled current control strategy. By using this control strategy, not only the active and reactive power of HVDC can be controlled independently but also the dynamic responded time can be shortened. Simulation studies of a 200 MW/±100 kV back to back VSC-HVDC system connecting two asynchronous ac networks are presented to confirm the satisfactory performance of the proposed system under active and reactive power variations from single phase to ground and three phases to ground fault conditions.

Keywords: Neutral-point clamped (NPC) multilevel converter, Voltage source converter (VSC) based heavy voltage direct current (HVDC), Subharmonic pulse width modulation (SH-PWM).