Constant Overlap-Time Based Sms Capacitor Voltage Balancing Scheme For Alternate Arm Converter

The alternate arm converter (AAC) is being considered as a potential alternative for MMC in HVDC applications because of lower energy storage requirement and dc-fault tolerant capability. However, to achieve energy balancing over wide range of operating modulation indices, it requires a dedicated controller that needs to exchange the unbalanced energies between the upper and lower arms of each phase-leg to achieve the balancing of its SMs capacitor voltages. In this paper, a new control scheme is proposed for AAC to balance its SMs capacitor voltages. The proposed scheme is based on the constant overlap-time and the magnitude of current during overlap-time is varied depending on the operating modulation index and power-factor. As the overlap-time is kept constant in the proposed control scheme, the operation of AAC can be extended to higher modulation range without increasing the number of SMs per arm. A detailed analysis of arm energy in AAC is presented to determine the required magnitude of current to be injected during the overlap for energy balancing. The efficacy of the analysis and the control scheme is verified using simulation and experimental studies on a scaled down laboratory prototype.