Analysis on the asymmetrical operation ability of Y-connected CMIs

When the system made up of cascaded multilevel inverters (CMI) is operating under the three-phase unbalanced loads or the system is implementing the fault-tolerant strategy based on the neutral point shift, the three-phase Y-connected CMIs must output asymmetric phase voltages. In order to meet the demand of the asymmetrical operation condition and output the three-phase asymmetric phase voltages, the phase voltage design must consider sufficient margin. Obviously, the larger the phase voltage margin is, the stronger the asymmetrical operation ability. But when the system is operating in the normal condition, the capacity utilization of the CMIs will be lower and the cost will be higher. Thence, the phase voltage margin design must consider the cost and the asymmetrical operation ability simultaneously. By modeling the relationship of the phase voltages and the unbalanced degree, the minimum number of power units when the system is operating asymmetrically is deduced and the assessment method of the asymmetrical operation ability is presented, which has provided the theoretical basis for the design and assessment of the Y-connected CMIs.