Novel Three-Phase Three-Level-Stacked Neutral Point Clamped Grid-Tied Solar Inverter With a Split Phase Controller

Characterized by the low leakage current and high efficiency, a three-level neutral point clamped (3L-NPC) inverter becomes more popular for a transformerless photovoltaic grid connected system. The three-level-stacked neutral point clamped (3L-SNPC) structure is a derivative of 3L-NPC providing more advantages such as a double apparent switching frequency and parallel load current paths. In this paper, the power loss distribution and features of 3L-SNPC are analyzed first when applied to solar inverters. Based on the analysis, a novel 3L-SNPC leg structure is proposed for solar applications in order to reduce the power loss particularly for the low power range, given the fact that solar inverters generally operate in the low power range during most of the daytime. Then, a two-stage solar inverter topology, applying the proposed structure to the phase leg, is described. Further, a split phase controller is applied for the two-stage solar inverter, consisting of the maximum power point tracking control, optimized dc-link voltage control, and grid current control. Finally, a new dead-time elimination pulsewidth modulation strategy is proposed and conveniently implemented to each phase based on the split phase controller. Experimental results are illustrated to demonstrate the applicability of the proposed topology and controller.