Capacitor Voltage Balancing for Alternate Arm Converter Based on Conduction Angle and Zero-Sequence Voltage

The alternate arm converter (AAC) owns additional advantages over the traditional modular multilevel converter, such as reduced number and capacitance of submodules (SMs), and dc fault ride-through capability. The capacitor voltage balance of the AAC has always been a hot and difficult research topic. The balance performance of the conventional short-overlap control is limited by the maximum circulating currents, which may also lead to additional power losses. In this article, the conduction angle of the direction switches is employed to balance the SM capacitor voltage and a modified SM energy balance condition is developed. As the upper and lower arms are strictly alternatively conducted, the circulating currents no longer exist, leading to an improvement in system efficiency. The operation range of the AAC is also expanded under this method, so the AAC can operate far away from the sweet spot, such as grid voltage sags. A three-layer closed-loop control method based on conduction angle, zero-sequence voltage, and nearest-level modulation is further proposed, which enables the voltage balance of all SMs even under unbalanced conditions. Finally, the effectiveness of the proposed methods is verified by the simulation and hardware-in-the-loop tests.