A Five Level Space Vector Modulation Scheme for Parallel Operated Three Level Inverters with Reduced Line Current Distortion

Parallel operation of voltage source inverters is useful to achieve larger power capacity and improve the system reliability, which has become the trend of power system design. However, the current harmonic of the parallel system is the same as that of every inverter when synchronized pulses are used for the paralleled inverters. The interleaved operation is helpful to reduce the current distortion, though the space left for improvement is limited, and high-frequency circulating current inevitably emerges. A novel parallel operation scheme based on five-level space vector modulation applied on two parallel three-level inverters is proposed in this article, which can provide reduced line current distortion, smaller zero-sequence circulating current, and less switching transitions. Theoretical analysis reveals that it is viable to operate the parallel three-level inverters as a single five-level system, and specific criteria of switching sequences design are presented to ensure the simple implementation. A neutral point balancing algorithm is also developed based on the specific switching sequences of the proposed modulation process, which is essential to the normal function of the system. The simulation and experimental results verify that the proposed scheme has superior performance compared with the synchronized and the interleaved operation.