Cascaded Transformers-Based Multilevel Inverters With NPC

The basic structure and generalization of two cascaded transformer multilevel inverters (CTMIs) are proposed in this paper. Such configurations are based on cascaded symmetrical transformers coupled to two-level (2L) legs and three-level (3L) neutral-point clamped (NPC) legs. One configuration has a shared 2L leg, connected to all transformers. The other configuration consists of multiple shared legs, where each is connected between two transformers. A modulation technique is also presented that allows only one leg of the proposed inverters operates at high frequency, while all other legs operate at low frequency. Compared to three conventional CTMIs, the proposed structures can produce more output voltage levels using a smaller number of switches and transformers, offering a better power quality, less weight, volume, and cost of the system. Moreover, the proposed inverters offer a natural balancing property that can self-regulate the dc-link capacitor voltages. Simulation and experimental results are presented in order to demonstrate the performance and feasibility of the proposed configurations.