A Transformerless Three-Port Nonagonal MMC for the Grid Connection and Local Consumption of Distributed Generation

While large-scale distributed generation (DG) grid connection requires its grid-connected converter to be able to boost the output voltage and connect it to the host grid, its priority is still to consume the power locally so as to increase the energy utilization rate. Using a common ac bus to transmit the distributed power to local load requires two separate converters, resulting in large size and multistage power conversion. Therefore, to lose the transformer and increase the compactness, a three-port nonagonal modular multilevel converter (MMC) is proposed in this paper, which consists of nine bridge arms connected from end to end. From the vertexes, three 3-phase ac ports protrude, connecting DG, ac grid, and local load together. By reusing power modules, with only nine bridge arms, the proposed nonagonal MMC realized three-port direct ac/ac power conversion in a single converter. Furthermore, the topology can operate without a transformer or high-voltage dc link, and under low frequency. It is then compared with other topologies to demonstrate its advantage in applications that require high compactness and flexibility. Finally, the proposed topology and its corresponding control are verified in RTlab to be effective for DG grid connection and local consumption.