Control of circulating current and neutral-point potential balancing in two parallel three-level NPC inverters

This paper adopts a parallel system where two three-level neutral-point-clamped (NPC) inverters connect both ac and dc sides directly considering compact size, low cost, simplicity and easy expandability. But there is a potential zero-sequence circulating current (ZSCC), when the space-vector modulation method is used in the non-isolated system, then a control strategy for the ZSCC is present. The balancing problem of the DC-link capacitor voltage is inherent in the NPC inverter, it is solved throug zero-sequence voltage injection. A coordinate control strategy is then proposed to solve the two problems in NPC inverter at the same time. Experimental results validate the developed model and the proposed control scheme.