Limitations of overlap control for energy balancing of the alternate arm converter under imbalanced ac grid conditions

Energy balancing of the alternate arm converter (AAC) takes place during the overlap period, when a circulating current flows through both arms of the converter. In this paper, the limitations of the commonly used energy balancing methods of the AAC under grid imbalances are demonstrated. It is shown that the overlap control can only provide sufficient energy balancing for a relatively limited voltage imbalance degree of the ac voltages and larger imbalances greatly impact the energy balancing of the AAC. The limitations of the overlap control are demonstrated through real-time digital simulations of an 800-MVA benchmark AAC model under single-phase, phase-to-phase and three-phase ac imbalances with varying imbalance degrees.