A Submodule-Capacitor Voltage Balancing Strategy for Alternative-Arm Converters in HVDC Systems

The standard alternative-arm converters (AAC) with the sweet spot or short overlap operation have faced a challenge of balancing submodule (SM) capacitor voltages, where this issue has not been paid much attention so far. In this paper, a new control strategy for the SM-capacitor voltage balancing of the AAC applied in high-voltage dc transmission systems is proposed. The suggested control technique is composed of three regulators, where a control for the average voltage of all SM capacitors, the voltage balancing among the converter arms, and the voltage balancing of the individual SM capacitor are performed. Based on the operation principle of the standard AAC, which activates only three of six converter arms simultaneously, the energy balancing of the three activated arms is performed every one sixth of the fundamental cycle by the converter common offset voltage. Band-stop filters with the central frequencies at the fundamental, second-, and third-order components of the line frequency are applied to obtain the average values of the SM capacitor voltages for the feedback controllers. The simple proportional and integral controllers are employed for this scheme and the voltages of SM capacitors are mostly balanced during the steady states and kept closely to the rating with overshoot of less than 7% under the transient conditions. The proposed control method is verified by simulation results for a 33-level AAC under both normal and fault conditions.