Analysis of Hybrid Modular Multilevel Rectifier Operated at Nonunity Power Factor for HVDC Applications

Abstract

Hybrid modular multilevel rectifier (HMMR) is a type of unidirectional modular voltage source converter (VSC) that has been proposed for the high-voltage (HV) dc transmission applications. Compared to the modular multilevel converter (MMC), up to 50% of the submodules (SMs) and capacitors can be replaced by the simple HV line-frequency diode stacks at unity power factor (PF). The nonunity PF operation is also favored for the rectifier station to support the grid. Therefore, this article proposes two new current commutation methods for HMMR to reduce the SM number at nonunity PF. In addition, an average model is given to reveal the inherent power flow relationship of HMMR. It is found that part of the active power is delivered from ac to dc side through the diode directly, which reduces the energy storage requirement. The performance of HMMR is also evaluated and compared with MMC. Both methods can successfully decrease the 30% low voltage devices. Method 1 can reduce 32% capacitor and method 2 achieves 52% reduction. The feasibility of the proposed converter is verified by the simulation results from a 200-kV 200 MVA HVdc model. Finally, a scale-down 400-V HMMR prototype is developed to validate the effectiveness of the proposed control scheme.