A Hybrid Three-Level and Modular Multilevel Converter With DC Fault Blocking Capability and Reduced Semiconductor Losses

This article proposes a hybrid three-level converter (H3LC) with AC-side cascaded full-bridge submodules (FBSM) for high voltage direct current (HVDC) transmission. The three-level converter operates at fundamental frequency switching to modulate square-wave output voltages. The AC outputs of the three-level converter are connected to cascaded FBSMs, which shape the square-wave voltages into multilevel sinusoidal voltages at the point of common coupling. Using third-order harmonic voltage injection, the AC-side chainlink voltage can be limited to within a quarter of the DC-side voltage which reduces the number of FBSMs and the associated semiconductor losses. The H3LC is capable of blocking DC pole-to-pole fault using the AC-side FBSMs and the proposed protection branches, each of which consists of a low-loss load-current carrying branch and a main breaker. Simulation studies demonstrate the effectiveness of the proposed converter for independent real and reactive power control as well as DC pole-to-pole fault blocking. Compared to other state-of-the-art HVDC converter topologies with DC fault blocking capability, i.e., the mixed half- and full-bridge modular multilevel converter and the hybrid two-level converter with AC-side cascaded FBSMs, the proposed H3LC provides reduced semiconductor losses due to fundamental frequency switching of the three-level converter and the reduced number of submodules.