VSC Transmission System Using Flying Capacitor Multilevel Converters and Hybrid PWM Control

A high-voltage direct current (HVDC) transmission system based on three-level flying capacitor (FC) multilevel converters with hybrid pulse-width modulation (PWM) is presented in this paper. Selective harmonic elimination PWM (SHE-PWM) is used during normal operating conditions and is switched to phase-shifted sinusoidal PWM (PS-SPWM) during an asymmetric network fault. The generation of the switching patterns under SHE-PWM control for each power device is described taking into account the natural balancing of the FC voltages. A new and simple control method for balancing the FC voltages when using SHE-PWM is proposed which is based on the small change of the firing angle according to the polarity of the load current. The FC voltage ripple under SHE-PWM control is estimated and compared to that under PS-SPWM. A method to implement the proposed hybrid PWM with capacitor voltage balancing is also provided. Simulation studies on a 300-MW/plusmn150 kV voltage-source converter transmission system are presented to confirm the satisfactory performance of the proposed system under active and reactive power variations and single-phase fault conditions