Choice of AC operating voltage in HV DC/AC/DC system

Demand for DC/DC conversion in HV applications is expected to rise because of the increasing number of HVDC links using different DC voltage levels. This paper presents a DC/AC/DC system consisting of two VSCs connected through an inductor. The two VSCs are Alternate Arm Converters (AAC). Since the two AACs share the same AC voltage level, they cannot be operated at their respective “sweet-spots” at the same time. This results in an energy drift in the valves which is tackled by additional balancing currents. However, the choice of the AC voltage level remains critical as it determines the required amount of balancing current, the number of devices and the cell topology, influencing greatly the total efficiency and volume of the obtained DC/DC converter. A study on a scale-down converter highlights the trade-offs affecting the AC voltage choice.