Cascaded Multilevel Converter Topology for Large-Scale Photovoltaic System With Balanced Operation

Cascaded H-bridge (CHB) multilevel converters are promising candidates for large-scale grid-connected photovoltaic systems (megawatt to gigawatt). However, the conventional CHB multilevel converter cannot ensure the inter-module and inter-phase power balance, resulting in imbalanced grid currents. Therefore, in this paper, a new topology consisting of CHB multilevel converters and interleaved-boost full-bridge LLC (IB-FBLLC)-based three-port dc-dc isolation converters is proposed. In this proposed topology, a common dc bus made up of the low-voltage-side ports of IB-FBLLC-based three-port dc-dc converters can completely solve the inherent power imbalance issues. Moreover, the design parameters of dc-dc converters considering the minimization of the input current ripple and optimization of switching frequency range are discussed in detail. Simulation results of the three-phase system and experimental results of the single-phase system clearly verify the effectiveness and feasibility of the proposed topology and control strategies.