Integrated Modulation of Dual-Parallel NPC Inverters With Eliminated CMV

Abstract

An integrated zero common mode voltage (ZCMV) modulation algorithm is developed in this research for dual-parallel neutral-point-clamped (NPC) inverters treated as an integrated system. While the conventional methods achieve CMV elimination with the cost of worse harmonic performance, the proposed method guarantees zero CMV and better current quality simultaneously by taking advantage of the extra space vectors provided by paralleling. Specially designed switching sequences are developed to maintain a balanced neutral point of DC-bus and ensure the averaged circulating current being zero, which is critical to the parallel operation and normal function of NPC inverters. In addition, although the switching sequence design is complicated, a simple approach is proposed and implemented on a commonly used digital signal processor (DSP). A laboratory prototype is also developed and used to test the system performance under steady-state conditions and dynamic process in grid-connected operation. Compared with conventional ZCMV modulation, experimental results verify the effectiveness of the integrated modulation in eliminating CMV, in addition to the dramatically reduced output current distortion.