A Novel Modular DC Chopper Based on Combination of Fully and Semicontrolled Devices for Offshore Wind VSC-HVDC Transmission System

Voltage source converter based high voltage DC (VSC-HVDC) transmission is widely used for grid integration of offshore wind farms. However, DC overvoltage caused by AC grid faults poses a serious threat to the system. Different DC choppers are commonly employed as energy dissipation devices to enhance the fault ride-through capabilities. This paper proposes a novel hybrid DC chopper with composite submodules that combines both fully-controlled and semi-controlled devices, specifically the IGBT and thyristor. This modular hybrid DC chopper incorporates both lumped and distributed resistors, inheriting the merits of both centralized and distributed DC choppers. Meanwhile, the use of thyristors in the submodule topology enables precise power dissipation at a lower cost compared to counterparts relying solely on fully-controlled devices. A submodule modulation scheme is applied to facilitate the forced commutation of thyristors, and the operation principle and control strategies of the chopper are thoroughly investigated. Finally, a full-scale simulation model and a scaled-down experimental prototype are established to validate the chopper's operation principle and performance.